Chapter  155:  Meditation Practices for Health: State of the Research

Literature Sources

Comprehensive searches were conducted in 17 relevant electronic databases up to September 2005. Other sources of potentially relevant studies included hand searches, reference tracking, contact with experts, and gray literature searches.

Study Selection

A set of strict eligibility criteria was used to include potentially relevant studies. They had to be comparative, be on any meditation practice, have more than 10 adult participants, provide quantitative data on health-related outcomes, and be published in English. The criteria of study methodology were modified to address each of the research topics of the review. Sources of secondary data (e.g., systematic reviews, narrative reviews, and book chapters) were used for topic I. Topics II to V included studies with a comparison/control group or control period: randomized controlled clinical trials (RCTs), nonrandomized controlled clinical trials (NRCTs) (topics III to V), prospective and retrospective observational studies with controls (topic II), case-control studies (topic II), uncontrolled before-and-after studies (topics II and V), and cross-sectional studies with controls (topic II).

Data extraction and Assessment of Study Quality

Trained research assistants extracted the data using a comprehensive and pretested data extraction form. One reviewer verified the accuracy and completeness of the data.

Studies included in the descriptive overview on the practice of meditation (topic I) were not assessed for methodological quality. For topics II to V, the methodological quality of RCTs and NRCTs was assessed using the criteria for concealment of allocation and the Jadad Scale. The quality of observational analytical studies (e.g., prospective and retrospective observational studies, case-control studies, and cross-sectional studies with controls) was assessed using the Newcastle-Ottawa Scales (NOS). The quality of the before-and-after studies was evaluated against four criteria adapted from the NOS.

Two independent reviewers assessed study relevance, extracted the data and assessed the methodological quality of the studies. Disagreements among reviewers were adjudicated by a third reviewer.

Synthesis of the Evidence

Data for topic I on the practice of meditation were synthesized qualitatively. A combination of qualitative and quantitative approaches was used to synthesize the data in Topics II to V. Details of individual studies were summarized in evidence tables including information on the article source, study design, study population (e.g., sample size, age, and gender), treatment groups, and outcomes. Meta-analyses using the standard inverse variance and random effects model were planned to derive pooled estimates from individual studies to support inferences regarding the magnitude and direction of the effect of meditation practices. Forest plots were used to display the individual and pooled results. An analysis of publication bias was also planned.

Topic I. The Practice of Meditation

Five broad categories of meditation practices were identified in the included studies: Mantra meditation (comprising the Transcendental Meditation^® technique [TM^®], Relaxation Response [RR], and Clinically Standardized Meditation [CSM]), Mindfulness meditation (comprising Vipassana, Zen Buddhist meditation, Mindfulness-based Stress Reduction [MBSR], and Mindfulness-based Cognitive Therapy [MBCT]), Yoga, Tai Chi, and Qi Gong. Given the variety of the practices and the fact that some are single entities (TM^®, RR, and CSM, Vipassana, MBSR, and MBCT) while others are broad categories that encompass a variety of different techniques (Yoga, Tai Chi, Qi Gong), it is impossible to select components that might be considered universal or supplemental across practices. Though some statement about the use of breathing is universal among practices, this is not a reflection of a common approach toward breathing. The control of attention is putatively universal; however, there are at least two aspects of attention that might be employed and a wide variety of techniques for anchoring the attention. The spiritual or belief component of meditation practices is poorly described in the literature and it is unclear in what way and to what extent spirituality and belief play a role in the successful practice of meditation. The amount of variation in the described frequency and duration of practice make it difficult to draw generalizations about the training requirements for meditation techniques. The criteria for successful meditation practice have also not been described well in the literature.

Topic II. State of Research on the Therapeutic Use of Meditation Practices in Healthcare

Eight hundred and thirteen studies provided evidence regarding the state of research on the therapeutic use of meditation practices. The studies were published between 1956 and 2005, with half of the studies published after 1994. Most of the studies were published as journal articles. Studies were conducted mainly in North America (61 percent). Of the 813 studies included, 67 percent were intervention studies (286 RCTs, 114 NRCTs and 147 before-and-after studies), and 33 percent were observational analytical studies (149 cohort and 117 cross-sectional studies).

Quality of studies. Overall, we found the methodological quality of meditation research to be poor, with significant threats to validity in every major category of quality measured, regardless of study design. The majority of RCTs did not adequately report the methods of randomization, blinding, withdrawals, and concealment of treatment allocation. Observational studies were subject to bias arising from uncertain representativeness of the target population, inadequate methods for ascertaining exposure and outcome, insufficient followup period, and high or inadequately described losses to followup.

Meditation practices. Mantra meditation practices such as the TM® technique and the RR were the most frequently studied meditation practices. Other mantra practices such as CSM, Acem meditation, Ananda Marga, concentrative prayer, and Cayce's meditation have been examined less frequently. The second category of meditation practices most frequently examined is Yoga. It includes a heterogeneous group of techniques such as Hatha yoga, Kundalini yoga, and Sahaja yoga. Mindfulness meditation, which includes MBSR, MBCT, and Zen Buddhist meditation, constitutes the third most studied group of meditation practices, Tai Chi the fourth, and Qi Gong the fifth. Finally, less than 5 percent of the studies on meditation have failed to explicitly describe the meditation practice.

Control groups. The number of control groups used in the 668 controlled studies ranged from one to four. The majority of the studies utilized an active, concurrent control. Among the RCTs and NRCTs, the practice of exercise and other physical activities constituted the most frequent active comparator followed by conditions involving states of rest and relaxation, health education, and progressive muscle relaxation. Almost half of the RCTs and NRCTs included comparison groups consisting of participants assigned to waiting lists, or participants that did not receive any intervention. The vast majority of observational studies used comparison groups consisting of individuals that had not been exposed to any type of meditation practice.

Study population. The majority of studies on meditation practices have been conducted in healthy populations. The three most studied clinical conditions are hypertension, other cardiovascular diseases, and substance abuse. Other diseases that have been frequently examined include anxiety disorders, depression, cancer, asthma, chronic pain, type II diabetes mellitus, and fibromyalgia.

Outcome measures. Physiological functions, particularly cardiovascular outcomes, were the most frequently reported outcome of interest in meditation research. Psychosocial outcomes, outcomes related to clinical events and health status, cognitive and neuropsychological functions, and healthcare utilization outcomes have also been evaluated in studies of meditation practices.

Topic III. Evidence on the Efficacy and Effectiveness of Meditation Practices

We summarized the evidence from RCTs and NRCTs on the effects of meditation practices for the three most studied clinical conditions identified in the scientific literature: hypertension (27 trials), other cardiovascular diseases (21 trials), and substance abuse disorders (17 trials).

A few studies of overall poor methodological quality were available for each comparison in the meta-analyses, most of which reported nonsignificant results. TM^® had no advantage over health education to improve measures of systolic blood pressure and diastolic blood pressure, body weight, heart rate, stress, anger, self-efficacy, cholesterol, dietary intake, and level of physical activity in hypertensive patients; RR was not superior to biofeedback in reducing blood pressure in hypertensive patients; Yoga did not produce clinical or statistically significant effects in blood pressure when compared to nontreatment; Zen Buddhist meditation was no better than blood pressure checks to reduce systolic blood pressure in hypertensive patients. Yoga was no better than physical exercise to reduce body weight in patients with cardiovascular disorders. When the relative effectiveness of a variety of meditation practices was assessed using indirect meta-analysis, we found that there were no significant differences between MBSR and Yoga to control anxiety symptoms in cardiovascular patients. Meta-analysis of the effects of meditation practices for substance abuse was not possible due to the diversity of practices, comparison groups, and outcome measures reported in each of the studies reviewed.

The results of the three highest quality trials (Jadad score = 3/5) examining, respectively, Mindfulness meditation, RR, and Yoga are inconclusive with respect to the effectiveness of meditation pratices.

The study comparing Mindfulness meditation with usual care (NS) for alcohol and cocaine abuse found little indication that Mindfulness meditation enhanced treatment outcomes for substance abuse patients. The study comparing RR with PMR and rest groups for alcohol abuse found generalized effects for BP, but not for the other outcome measures (anxiety, HR, and GSR). The RR and PMR groups did not exhibit increased BP as observed in control subjects. RR and PMR produced significant changes in tension. The study comparing Yoga with exercise for alcohol abuse found a significantly greater recovery rate for the Yoga group.

Statistical and clinical heterogeneity among the trials constituted a frequent and considerable problem when pooling the results, and in some cases, it precluded summarizing data across the studies. The poor methodological quality of the trials limits the strength of inference regarding the observed treatment effects reported in this review. The lack of description of the methods of allocation concealment, randomization, description of withdrawals and dropouts per treatment group, the absence of appropriate blinding, and the use of incompatible or inappropriate control groups undermine the validity of the results of many clinical studies.

Topic IV. Evidence on the Role of Effect Modifiers for the Practice of Meditation

The role of patient or meditation characteristics as effect modifiers in the practice of meditation is a topic that has so far been neglected in the scientific literature. Few studies have systematically examined factors such as dose, duration, or other specific features of meditation as moderators of the effects on outcomes. Evidence from RCTs and NRCTs regarding the interaction of meditation with other variables in populations of patients with hypertension, cardiovascular disorders, or substance abuse is scarce. A few studies conducted exploratory post hoc analyses (i.e., a subgroup analysis, multiple regression, or analysis of variance) that were intended to be hypothesis generating. No conclusions on the role of effect modifiers can be drawn from the analysis of the individual studies. Individual patient data is required to appropriately examine this issue.

TOPIC V. Evidence on the Physiological and Neuropsychological Effects of Meditation Practices

The physiological and neuropsychological effects of meditation practices were evaluated in 311 studies. The majority of studies have been conducted in healthy participants. Meta-analysis revealed that the most consistent and strongest physiological effects of meditation practices in healthy populations occur in the reduction of heart rate, blood pressure, and cholesterol. The strongest neuropsychological effect is in the increase of verbal creativity. There is also some evidence from before-and-after studies to support the hypothesis that certain meditation practices decrease visual reaction time, intraocular pressure, and increase breath holding time. As found in studies included for topic III, the overall low methodological quality of the studies indicates that most of the studies suffered from methodological problems that may result in overestimations of the treatment effects or compromise the generalizability of the study results. Particularly, the lack of a concurrent control group in the before-and-after studies results in an inability to control for temporal trends, regression to the mean, and sensitivity to methodological features. Therefore, results from meta-analyses of the physiological and neuropsychological effects of meditation practices should be interpreted cautiously.

The very small number of trials available for each comparison precluded testing for publication bias.

Topic I. The Practice of Meditation

The following questions pertain to the description of the practice of meditation and meditation techniques:

• 1

  What is known about the practice of meditation?

  • a

    What are the main components of the various meditation practices (e.g., breathing, chanting, mantras, and relaxation)? Which components are universal and which ones are supplemental?

  • b

    How is breathing incorporated in these practices? Are there specific breathing patterns that are integral elements of meditation? Is breathing passive or directed?

  • c

    For each type of meditation practice, where is the attention directed during meditation (e.g., mantra, breath, image, nothing)?

  • d

    To what extent is spirituality a part of meditation? To what extent is belief a part of meditation?

  • e

    What are the training requirements for the various meditation practices (e.g., the range of training periods, frequency of training, individual and group approaches)

  • f

    How is the success of the meditation practice determined (i.e., was it practiced properly)? What criteria are used to determine successful meditation practice?

Topic II. State of Research on the Therapeutic Use of Meditation Practices in Healthcare

The following key questions pertain to the scope of research on meditation in healthcare:

• 2

  What meditation practices have been examined in clinical trials and observational studies? What control groups are used?

• 3

  Can these practices be separated by the diseases, conditions, and populations for which they have been examined?

• 4

  What outcome measures are used? Are psychosocial outcomes included in these studies? If so, what types?

Topic III. Evidence on the Efficacy and Effectiveness of Meditation Practices

The following key questions pertain to the potential benefits and harms of meditation:

• 5

  What is the evidence that meditation practices are efficacious for the three most studied diseases or conditions identified in question 2 above?

• 6

  If more than one form of meditation has been studied for a particular disease or condition (as identified in question 2 above), does the efficacy of these practices differ?

• 7

  For specific disease populations, are meditation practices that are used as a complement to conventional therapy more effective than either the conventional therapy or meditation therapy alone?

Topic IV. Evidence on the Role of Effect Modifiers for Meditation Practices

The following key questions pertain to specific elements of the meditation practice, population and practitioner that may influence the outcomes:

• 8

  What dose of meditation is necessary before successful health outcomes are realized? That is, is the duration of meditation important for outcomes?

• 9

  Does the direction of attention during meditation affect outcomes?

• 10

  To what extent is a rhythmic aspect (i.e., mantra, controlled breathing, or other ordered, recurrent sound or motion) critical to the practice of meditation and to health outcomes? Do such approaches to meditation that rely on these rhythmic behaviors demonstrate consistent effectiveness versus nonrhythmic approaches to meditation? More broadly, do the number and types of components that make up the various meditation practices influence the outcomes?

• 11

  Do individual difference variables (age, gender, race, education, income, other) predict success in the process of meditation (i.e., adherence, acceptance), as well as predicting health outcomes?

Topic V. Evidence on the Physiological and Neuropsychological Effects of Meditation Practices

The following key questions pertain to the physiological and neuropsychological effects of meditation practices:

• 12

  What is known regarding cardiopulmonary, endocrine, immunologic, metabolic, and autonomic changes seen during meditation practices?

• 13

  What is known regarding the effects of meditation practices on brain function (e.g., brain imaging, electroencephalogram (EEG), neuropsychological and cognitive functions)?

Figure 1. Analytic framework for evidence report on (more...)

   Figure 1. Analytic framework for evidence report on the state of research on meditation practices in healthcare

Figure 1

For topic I, the practice of meditation, the steps involved in the development of the descriptive overview included:

• development of an operational definition of meditation

• literature search

• study selection

• data extraction

• qualitative synthesis of information

For topic II on the state of research for the therapeutic use of meditation practices, topic III on the efficacy and effectiveness of meditation practices, topic IV on the role of effect modifiers of meditation practices, and topic V on the physiological and neuropsychological effects of meditation practices, a number of steps were involved in conducting the literature review and synthesis of the evidence:

• literature search and retrieval

• study selection

• assessment of study quality

• data extraction

• data analysis and synthesis

Development of Operational Parameters to Define Meditation Practices

There is no consensus on a definition of meditation in the scientific literature. For the purposes of this report, a set of parameters to describe meditation practices was developed using a modified Delphi methodology.^36,37 The systematic process used to reach consensus on the operational definition of meditation was documented and is described briefly below (Appendix B).¹

A first-round questionnaire was distributed to TEP members to solicit their opinion on a set of parameters extracted from the scientific literature to describe meditation. Participants independently rated the importance of each parameter to characterize a practice as meditation. They were also asked to suggest other parameters not included in the questionnaire that they considered important. A feedback summary from the first-round responses was sent to TEP members along with a second-round questionnaire. In light of round-one group responses, participants were asked if they would reconsider their first-round responses. The process stopped when consensus among participants was reached. Responses to questions were analyzed and categorized by frequency of endorsement. Consensus was defined as agreement on a value or category by 80 percent of the Delphi participants.³⁷ If consensus was not reached by the Delphi technique, the TEP convened and group consensus techniques were used in a teleconference.

Literature Search and Retrieval

Table 1

Databases searched for relevant studies

Table 1

Databases searched for relevant studies

Database	Date of search	Years/issue searched
Cochrane Central Register of Controlled Trials	August 4, 2005	3rd Quarter 2005
CSA Neurosciences Abstracts	August 4, 2005	1982-2005
MEDLINE® and PreMedline®	September 8, 2005	1966 to August, 2005; Week 5
Old Medline	February 21, 2006	1950-1965
EMBASE	September 8, 2005	1988 to 2005; Week 36
Cochrane Database of Systematic Reviews	September 9, 2005	3rd Quarter 2005
PsycINFO®	September 9, 2005	1872 to August, 2005; Week 4
Web of Science®	September 21, 2005	1900-2005
OCLC FirstSearch (Articles and Proceedings)	September 22, 2005	1993-2005
AMED	September 30, 2005	1985 to September, 2005
CINAHL®	October 4, 2005	1982 to September Week 5, 2005
Cochrane Complementary Medicine Trials Register	October 25, 2005	1943-2003
CAMPAIN (Complementary and Alternative Medicine and Pain Database)	October 25, 2005	1983-2003
NLM® Gateway	October 25, 2005	1950-2005
Current Controlled Trials - BioMed Central	October 24, 2005	1998-2005
National Research Register	October 24, 2005	2000-2005
CRISP	February 21, 2006	2005-2006

In addition to the electronic databases, the following journals and collections were hand searched: International Journal of Behavioral Medicine (1994-2005), Scientific Research on The Transcendental Meditation^® Program: Collected Papers (Volumes 1 to 4), Journal of Bodywork & Movement Therapies (1996-2005), Journal for Meditation and Meditation Research (2001-2003), International Journal of Yoga Therapy (1997-2005), and Explore: The Journal of Science and Healing (2005).

The reference lists of relevant studies (e.g., included studies, other systematic or narrative reviews) were reviewed to identify potentially relevant studies. Gray literature was searched to identify unpublished studies and works in progress. Scientific abstracts from the Society of Behavioral Medicine (2005) and the American Psychosomatic Society (1999-2005) annual scientific meetings were reviewed. The National Research Register from the National Health Service was searched for ongoing trials. Primary authors of potentially eligible ongoing studies were contacted if this was necessary to clarify whether those studies did indeed meet the inclusion criteria. TEP members were also requested to provide additional information about potentially relevant studies.

Criteria for Selection of Studies

Table 2

Inclusion criteria for topic I

Table 2

Inclusion criteria for topic I

Category	Criteria
Source	English-language scientific literature
Population	Adults (i.e., individuals aged ≥ 18 years)
Intervention	Empirical description of meditation practice according to the parameters defined by the TEP in the Delphi process
Study design	Systematic reviews, narrative reviews, book chapters and other sources of secondary data
Outcomes of interest	Components of meditation practices (e.g., breathing, chanting, mantras, relaxation)
	Role of breathing
	Role of attention
	Role of belief/spirituality
	Training conditions
	Criteria for success

Information from primary studies and other original research identified for topics II to V was considered for topic I if it provided a detailed description of the meditation practice under study according to the parameters defined by the Delphi process.

Table 3

Inclusion criteria for topics II to V

Table 3

Inclusion criteria for topics II to V

Category	Criteria
Source	Primary research report published in English
Population	Adults (i.e., individuals aged ≥ 18 years)
	Normal (topics II and V only) and clinical populations (topics II to V)
	No previous meditation practice
Intervention	Any meditation practice according to the parameters provided by the TEP in the Delphi study
Sample size	N greater than 10
Study design	Studies including a comparison/control group or control period in the methodological design: RCTs, NRCTs (topics III to V), prospective and retrospective observational studies with controls (topic II), case-control studies (topic II), uncontrolled before-and-after studies (topics II and V), and cross-sectional studies with controls (topics II)
Outcomes of interest	Measurable data for health related outcomes

Study Selection Process

Screening of titles and abstracts. We developed a predefined set of broad criteria to apply to the results of the literature searches to ensure that potentially relevant articles were not excluded early in the selection process (Appendix D).² Four independent reviewers evaluated the title and abstract of each study to select references potentially relevant to the topics of the report. The full-text of studies meeting the criteria was retrieved as was the full-text of those that reported insufficient information to determine eligibility.

Identification of studies eligible for the review. Two independent reviewers appraised the full-text of potentially relevant articles using a standard form that outlined the inclusion and exclusion criteria for each research topic (Appendix D).* Decisions regarding inclusion and exclusion and the reasons for exclusion were documented.

The level of agreement among reviewers at all stages of the selection process was evaluated using the Kappa (κ) statistic.³⁸ A κ score in the range from 0.0 to 0.40 was considered poor agreement; 0.41 to 0.60 moderate agreement; and 0.61 to 0.80 substantial agreement.³⁹ Disagreements about the inclusion or exclusion of studies were resolved by consensus. When consensus was not reached, a decision was made in consultation with the TEP.

Evaluating the Methodological Quality of Studies

Rating the quality of individual articles. Studies included in the descriptive overview on the practice of meditation (topic I) were not assessed for methodological quality; therefore, the following methods for quality assessment apply to studies meeting eligibility criteria for topics II to V only.

Quality of intervention studies (RCTs, NRCTs, and before-and-after studies). The methodological quality of RCTs was assessed using the criteria for concealment of allocation^40,41 and the Jadad scale.⁴² The former is based on the evidence of a strong relationship between the potential for bias in the results and allocation concealment: failure to conceal the process of treatment allocation can undermine randomization and, consequently, a selection bias may occur.⁴⁰ The Jadad scale is a validated scale that includes questions related to bias reduction: randomization, double-blinding and description of dropouts and withdrawals. This tool scores quality from 0 to 5. Studies scoring less than 3 points are usually considered to be of low quality.⁴² The psychometric properties of the Jadad scale have been thoroughly tested, providing rigorous evidence to support its use.^42,43 We used individual components of the Jadad scale to create a 3-point scale based on blinding and participant attrition to assess the methodological quality of NRCTs.

No completely or partially validated instruments are available to assess the methodological quality of uncontrolled before-and-after studies. Quality of reporting of uncontrolled before-and-after studies included in topics II and V was evaluated with four questions assessing whether the study participants were representative of the target population, the method of outcome assessment was the same for the pre- and postintervention periods for all participants, outcome assessors were blind to the intervention and the purpose of study, and the number of and reasons for study withdrawals were reported.

Quality of observational analytical studies. The methodological quality of observational analytical studies (i.e., prospective and retrospective observational studies, case-control studies, and cross-sectional studies with controls) was assessed using the Newcastle-Ottawa Scales (NOS).⁴⁴ These are eight-item instruments that use a star system to assess methodological quality across three categories: the methods of selecting the study groups, their comparability, and the ascertainment of the outcome of interest. Scores range from 0 to 9 stars. The NOS scales have been recommended by the Cochrane Nonrandomized Studies Methods Working Group, and studies on their psychometric properties are in progress.⁴⁴

The assessment of quality of observational studies is more difficult than the assessment of RCTs and NRCTs. Empirical research has shown that numeric scores based on arbitrary weights given to each item in a scale are unreliable and difficult to interpret.⁴⁵ Therefore, we decided to describe the methodological quality of observational analytical studies using the individual components of the NOS scales.

Finally, information regarding the source of funding was collected for all the included studies.⁴⁶

Two reviewers assessed the methodological quality of studies independently. Disagreements were resolved by consensus or, when no consensus could be reached, a senior methods expert adjudicated (Appendix D).^*

Data Collection

For topic I on the practice of meditation, a single reviewer extracted information that was organized according to narrative categories (e.g., components of the meditation practices, role of breathing and spirituality, training requirements, and criteria for success) to allow for a systematic description of the meditation practices considered in this report.

For topics II to V, trained research staff at the UAEPC extracted the information. A comprehensive and pretested data extraction form and guidelines explaining the extraction criteria were developed (Appendix D)^*. Information regarding the study design and methods, the characteristics of participants, interventions, comparison groups, and outcomes of interest were extracted. Data collection on study design and methods included information on the country and year of publication, type of publication, objective of the study, study design, duration, number of centers, and source of funding. Data on characteristics of the participants included setting of the study, type of primary health problem or health condition of study participants, and diagnostic criteria (as reported by the authors of the studies). Data on characteristics of the intervention (i.e., meditation practices) included a description of the practice in terms of components, content and format, frequency, and intensity. Likewise, data on the characteristics of the control group included a description of the components, content, and format. Finally, information was extracted on the type of outcomes and on the units or instruments of measurement for each outcome. A single reviewer extracted the data from the primary studies and another independent reviewer verified the accuracy and completeness of the data. Any discrepancies in data extraction were solved by consensus between the data extractor and the data verifier. During this process, the reviewers consulted with TEP members both for content and methodological advice as needed.

Study selection, methodological quality assessment, and data extraction were managed with the Systematic Review Software™ (SRS), version 3.0 (TrialStat!; Ottawa, ON). Graph extraction was performed using Corel Draw^®, version 9.0 (Vector Capital, San Francisco, CA). Extracted data were exported into Microsoft Excel™ (Microsoft Corporation, Redmond, WA) spreadsheets.

Data Analysis and Synthesis

Classification of the meditation practices. The first step in synthesizing the data for topics I to V was to create categories of analysis for the meditation practices described in the scientific literature. Based on data from the Delphi study, input from the TEP members, and a review of the literature, a set of seven categories was constructed to classify the meditation practices. Two independent researchers coded each study according to this classification scheme. Coding was discussed between researchers on a study-by-study basis. Coding discrepancies were resolved by consulting the original research study.

The following seven categories were used for data synthesis for topics II to V:

Mantra meditation. This category comprises meditation practices in which a main element of practice is mantra: the Relaxation Response technique (Relaxation Response or RR), the Transcendental Meditation^® technique (hereafter, simply “Transcendental Meditation^®” or “TM^®”), Clinically Standardized Meditation (CSM), Acem meditation, Ananda Marga, and other concentrative practices that involve the use of a mantra such as Rosary prayer, and the Cayce method.

Mindfulness meditation. Though described slightly differently by Eastern and Western interpreters, this category refers generally to meditation practices that cultivate awareness, acceptance, nonjudgment, and require paying attention to the present moment.⁴⁷–⁴⁹ This category includes Mindfulness-Based Stress Reduction (MBSR), Mindfulness-Based Cognitive Therapy (MBCT), Vipassana meditation, Zen Buddhist meditation, and other mindfulness meditation practices not further described.

Qi Gong. This category refers to an ancient practice from traditional Chinese medicine that combines the coordination of different breathing patterns with various physical postures, bodily movements, and meditation. External Qi Gong, in which a trained practitioner directs his or her own qi outward, with the intention of helping patients clear blockages, remove negative qi and balance the flow of qi in the body, to help the body rid itself of certain diseases is not a form of meditation according to the working definition developed for this report.

Tai Chi. This category describes a Chinese martial art characterized by soft, slow, flowing movements that emphasize force and complete relaxation. It has been also called “meditation in motion.”

Yoga. This category includes a broad group of techniques rooted in yogic tradition that incorporate postures, breath control, and meditation. It includes practices such as Hatha yoga, Kundalini yoga, and individual components of Yoga such as pranayama (breath control exercises).

Miscellaneous meditation practices. This category describes techniques that combine different approaches to meditation in a single intervention, without giving prominence to one. It includes combined practices such as Yoga plus RR, TM^® and Buddhist Meditation, and RR plus Mindfulness meditation. The category was also used to describe meditation practices that do not fall within any of the other categories (e.g., coloring mandalas).

Meditation practices (not described). This category refers to meditation practices that were not described in sufficient detail to allow them to be assigned to a more specific category, including techniques that were described by vague terminology such as “meditation,” “movement meditation,” and “concentrative meditation.”

Topic I. Data for topic I on the practice of meditation were synthesized qualitatively. Information was presented in a structured format, with narrative categories of interest for the different practices of meditation identified in the scientific literature. Once categorized, the similarities and differences among the various meditation practices could be appraised. Categories of analysis include the main components of the meditation practices, the role of breathing, attention, and spirituality, the training requirements, and the criteria of success for the various meditation practices.

Topic II. Data collected for topic II on the state of research for the therapeutic use of meditation practices were summarized using descriptive statistics (e.g., proportions and percentages for categorical data, means with standard deviations [SD], or medians with interquartile ranges [IQR], for continuous data). Evidence and summary tables were constructed to summarize relevant characteristics of the included studies. Data from the included studies were synthesized qualitatively. We used the systematic approach of the Cochrane Collaboration for the synthesis of the evidence.⁵⁰ The basic conceptual framework of the qualitative synthesis for topic II focused on the types of meditation practices that have been examined in intervention studies (RCTs, NRCTs, and uncontrolled before-and-after studies) and observational analytical studies (cohort studies, case-control studies, and cross-sectional studies), the types of control groups, the populations, and the types of outcome measures that have been examined in the included studies.

Topics III and IV. Based on the results of topic II describing the populations that have been examined, RCTs and NRCTs assessing the effects of meditation practices for the three most studied clinical conditions were included in the analyses of efficacy and effectiveness of meditation practices (topic III) and the role of effect modifiers for meditation practices (topic IV). The first step in synthesizing the data for topics III and IV was to construct evidence tables that included information on each article's source, study design, study population (e.g., sample size, age, and gender), treatment groups, and outcomes. The evidence tables also included summaries of study quality and comments to help interpret the outcomes.

Meta-analyses were planned as part of the data analysis to derive pooled estimates from individual studies to support inferences regarding the magnitude and direction of the effect of the meditation practices. If studies evaluating specific meditation practices were sufficiently similar, effect sizes were combined and weighted using the standard inverse variance method⁵¹ to produce an overall effect size for a given outcome. Meta-analyses used a random effects model. In this method, study means are averaged, weighting by a combination of inverse variance augmented by heterogeneity.

The types of summary statistics considered were risk ratios (RR) or odds ratios (OR) with 95 percent confidence intervals (95% CI) for dichotomous outcomes and weighted or standardized mean differences (WMD and SMD, respectively) with 95% CI for continuous outcomes.⁵² WMD was chosen as the default method, with SMD being used only when units for the outcome were different among the studies being compared (i.e., stress measured on different scales).^50,53

Hedges adjusted g was used as the SD estimate when the SMD was used.⁵⁴ If the means were not reported, they were either imputed from medians or discarded from meta-analysis if neither mean nor median was available. Occasionally studies did not report SDs of their estimates. In these cases, we determined the SD exactly from confidence intervals or exact p-values; estimated the SD from upper-bound p-values, interquartile ranges, ranges, or exact nonparametric p-values; or imputed from other studies reporting similar outcomes in a similar population. All the meta-analyses used endpoint data or change from baseline to endpoint data instead of using the average of separate mean changes calculated at different intervals of time. Forest plots were used to display the individual and pooled results.

Since some common outcomes were reported for many interventions, indirect comparisons⁵⁵ were made of these active interventions. This type of comparison involves taking the differences between the differences derived from separate meta-analyses. For example, by taking the difference between the derived meta-analysis of A versus B, and the derived meta-analysis of A versus C an estimate of the comparison of B versus C can be obtained. For some outcomes, when more than four interventions could be compared indirectly, a mixed treatment comparison was conducted. Indirect comparisons are a valid approach to meta-analysis when there is insufficient direct evidence from randomized trials reporting head-to-head comparisons between interventions.^55,56

In this method, a Bayesian formulation of the data is employed. The differences between each intervention and a baseline intervention (in this case, “no treatment” was chosen as the baseline) are modeled by choosing a prior distribution for the effect and combining this prior value with the data from the studies to arrive at a posterior estimate and 95% credible interval. Such an estimate was obtained for all pairwise comparisons of interventions as well as the comparisons to the baseline intervention. Since the resulting posterior distributions are too complex for direct computation, a Markov Chain Monte Carlo simulation⁵⁷ was used to obtain the posterior estimates. This procedure involved simulating the unconditional, unknown posterior distribution by sampling many times from the conditional distribution and averaging the results. We used a sample of 20,000 burn-in iterations followed by 200,000 samples and noninformative normal (point estimate) and uniform (variance estimate) priors to obtain the distributions. We also computed a statistic to estimate the probability that each intervention was the best (e.g., lowered blood pressure the most) by recording the best intervention at each iteration. This simulation was performed using the WinBUGS software, version 1.4 (MRC Biostatistics Unit, Cambridge, United Kingdom).

We tested for statistical heterogeneity using the chi-square test⁵¹ and quantified it using the I² statistic.⁵⁸ When there was evidence of clinical or statistical heterogeneity among studies, effect size estimates with corresponding 95% CI were presented separately for each study.^59,60 Sources of heterogeneity were explored qualitatively. They may be methodological (differences in design or quality), or clinical (differences in key characteristics of participants, interventions, or outcome measures).⁶¹ Where appropriate, subgroup analysis based on patient, intervention, and study characteristics were conducted and sensitivity analysis based on study quality (Jadad score of greater than and equal to 3 points or less than 3 points) were conducted to assess the effect of quality on precision of the pooled estimates if the number of studies per comparison allowed it.⁶²

Two analytic strategies were considered for topic IV on the effect modifiers of meditation practices. First, a meta-regression analysis using RCT-level covariates was planned to explore whether certain characteristics of the participants (e.g., age, gender, ethnicity, education, and income) or the interventions (e.g., dose, frequency, and duration) were associated with increased benefits of meditation practices. The outcome (or dependent) variable in the meta-regression analyses would be the pooled effect size (log OR for binary outcomes, or WMD or SMD for continuous outcomes). If a meta-regression was not feasible due to a small number of trials, or limited data from primary studies, subgroup analyses would be conducted based on participant or intervention characteristics.

Topic V. Based on the types of outcomes identified in topic II, RCTs, NRCTs, and uncontrolled before-and-after studies (i.e., without a parallel control group) were included in the analysis of the physiological and cognitive/neuropsychological effects of meditation practices. Evidence tables were constructed to summarize each article's source, study design, study population (e.g., setting, sample size, age, and gender), treatment groups, and outcomes. The evidence tables also included summaries of the strength of the evidence, study quality, and comments to help interpret the outcomes.

Meta-analyses of RCTs and NRCTs using the methods described above for topic III were also planned for topic V. For studies with pre- and post-measures, data on change from baseline were used if available; otherwise, endpoint data were used. If meta-analytic methods were not feasible, effect size estimates with corresponding 95% CIs were presented separately for each study.^59,60 Data from uncontrolled before-and-after studies were analyzed separately, and, if appropriate, the individual estimates of the treatment effect were pooled using the generic inverse variance method. Sensitivity analyses were conducted to assess the robustness of the findings when necessary. Data were displayed using forest plots.

Publication bias. Publication bias, or the selective publication of research depending on the results, was assessed using funnel plots, and the trim and fill method⁶³ if enough data were available from the meta-analyses. Funnel plots of effect sizes (axis X) against the SD (axis Y) for each meta-analysis were examined to identify gaps suggesting publication bias. Finally, the trim and fill method provided estimates of the number of studies potentially missing from a meta-analysis and the effect these omissions might have had on its outcome.

All analyses were performed using SAS/STAT^® software version 9.1 (SAS Institute Inc., Cary, NC), Statistical Package for the Social Sciences^® for Windows^® (SPSS^® version 14.1, SPSS Inc., Chicago, IL), and RevMan version 4.1 (Cochrane Collaboration, Oxford, UK).

Potential limitations, conclusions, and implications for future practice and research were discussed. The results were interpreted in light of the heterogeneity of the individual studies (e.g., differences in design, study populations, interventions or exposures, and outcome measures) and any evidence of publication bias, if present. Recommendations for practitioners and researchers were based solely on the evidence available.

Main Components

The main components of any meditation practice or technique refer to its most general features. These may include specific postures (including the position of the eyes and tongue), the use of a mantra, breathing, a focus of attention, and an accompanying belief system. Posture refers to the position of the body assumed for the purpose of meditation. Though traditional meditation practices prescribe particular postures (e.g., the lotus position), postures vary between practices with the only limitation being that the posture does not encourage sleep.⁶⁴ Because accounts of most meditation practices describe explicitly the use and role of breathing, mantra, attention, spirituality and belief, training, and criteria for successful meditation practice, these topics are described individually.

Breathing. Breathing in meditation can be incorporated passively or actively. In passive breathing, no conscious control is exerted over inhalation and exhalation and breathing is “natural.” In contrast, active breathing involves the conscious control over inhalation and exhalation. This may involve controlling the way in which air is drawn in (e.g., through the mouth or nostrils), the rate (e.g., drawn in quickly or over a specified length of time), the depth (e.g., shallow or deep), and the control of other body parts (e.g., relaxation of the abdomen).

Mantra. A distinctive feature of some meditation practices is the use of a mantra. A mantra is a sound, word, or phrase that is recited repetitively, usually in an unvarying tone, and used as an object of concentration. The mantra may be chanted aloud, or recited silently. Mantras can be associated with particular historical or archetypal figures from spiritual or religious systems, or they may have no such associations.⁶⁵

Relaxation. Relaxation is often considered to be one of the defining characteristics of meditation practices and meditation itself is often considered to be a relaxation technique.⁶⁶–⁶⁸ Indeed, it has been suggested that the popularity of meditation practices in the West is due, at least in part, to the widely accepted plausibility of their alleged effects with respect to arousal reduction.⁶⁹ Some researchers have attempted to draw a distinction between relaxation and meditation practices on the basis of intention.⁷⁰

Attention and its object. The intentional self-regulation of attention is considered crucial to the practice of meditation, as is the development of an awareness in which thoughts do not necessarily disappear, but are simply not encouraged by dwelling on them, a state of so-called “thoughtless awareness.”^71,72 Some meditation practices focus attention on a singular external object (e.g., mandala, candle, flame), sound (e.g., breath), word or phrase (i.e., mantra), or body part (e.g., the tip of the nose, the space between the eyebrows).⁷¹ In contrast, “mindfulness” meditation techniques aim to cultivate an objective openness to whatever comes into awareness (e.g., by paying attention to simple and repetitive activities or to the sensations of the body). In doing so, the breath may be used as an anchor (but not a focal point) to keep the meditator engaged with the present moment.^65,73 Each of these techniques serves, in a different way, to discourage logical and conceptual thinking.⁶⁵

Spirituality and belief. This component refers to the extent to which spirituality and belief systems are a part of meditation practices. Spirituality and belief systems are composed of metaphysical concepts and the rules or guidelines for behavior (e.g., devotional practices or interpersonal relations) that are based on these concepts.

Training. Training refers to the recommended frequency and duration of periods of practice, and how long a practitioner is expected to train before being considered proficient in a given technique.

Criteria of successful meditation practice. The criteria of successful meditation practice are understood both in terms of the successful practice of a specific technique (i.e., is the technique being practiced properly) and in terms of achieving the aim of the meditation practice (e.g., has practice led to reduced stress, calmness of mind, or spiritual enlightenment).

Five broad categories of meditation practices were identified in the scientific literature: mantra meditation (comprising Transcendental Meditation^® [TM^®], Relaxation Response [RR], and Clinically Stadardized Meditation [CSM]), mindfulness meditation (comprising Vipassana, Zen Buddhist meditation, Mindfulness-based Stress Reduction [MBSR], and Mindfulness-based Cognitive Therapy [MBCT]), Yoga, Tai Chi, and Qi Gong. These broad categories were used for descriptive purposes throughout the report to address the key research questions.

Transcendental Meditation^®

TM^® is a technique derived from the Vedic tradition of India by Maharishi Mahesh Yogi.⁷⁶ In TM^®, a meditative state is purportedly achieved in which the repetition of the mantra no longer consciously occurs and instead the mind is quiet and without thought.⁷⁷ During the practice of TM^®, the ordinary thinking process is said to be “transcended” (or gone beyond) as the awareness gradually settles down and is eventually freed of all mental content, remaining silently awake within itself, and producing a psychophysiological state of “restful alertness.”^78,79 These periods, referred to as pure consciousness or transcendental consciousness, are said to be characterized by the experience of perfect stillness, rest, stability, order, and by a complete absence of mental boundaries.⁸⁰

Main components. In the TM^® technique, the meditation state is achieved by the repetition of a mantra. The mantra is a meaningless sound from the ancient Vedic tradition and is given to the meditator by an instructor in the TM^® technique.^81,82 TM^® practitioners sit in a comfortable posture, with eyes closed, and silently repeat the mantra.⁸³ Though there are reports of the components of the mantras and how they are assigned, it is difficult to confirm these reports as many of the details of practice, including mantras, are revealed only to those who have formal instruction in TM^®. Instruction in the TM^® technique is a systematic, but individualized process. It is believed that keeping the techniques confidential prevents students from having preconceptions about the technique (making the learning process simpler) and that it maintains the integrity of the technique across generations.

Breathing. TM^® involves passive breathing; no breath control procedures are employed and no specific pattern is prescribed.⁸⁰

Attention and its object. TM^® is described as not requiring any strenuous effort, concentration, or contemplation.^80,84 However, meditators are instructed to direct their attention to the mantra.⁸³

Spirituality and belief. The TM^® technique has a theoretical framework that is described in Maharishi Mahesh Yogi's writings on the nature of transcendental consciousness and the principles underlying the TM^® technique.⁸¹ However, it is unclear to what extent this theoretical framework, including any of its implications for spirituality, is a part of the practice. Sources that discuss this issue contend that the practice of the technique requires no changes in beliefs, philosophy, religion, or lifestyle,^78,80,85 implying that the theoretical framework plays no role in its practice.

Training. TM^® is usually taught in a course comprising five to six hours of instruction over four days.⁸⁴ General information about the technique and its effects is presented in a 1.5-hour lecture. More specific information is given in a second 1-hour lecture. Those interested in learning the technique meet with the teacher for a 5- to 10-minute interview. The participant learns the technique on a separate day in a 1- to 1.5-hour session, following a short ceremony in which the mantra is given to the prospective practitioner. The next three sessions consist of 1.5-hour meetings held in the 3 days following, in which further aspects of the technique are explained. The teacher explains the practice of the technique in more detail, corrects practice if necessary, and explains practical arrangements (e.g., when to practice), the benefits of practice, and personal development through the technique. In addition, the technique is regularly checked by the teacher in the first months of practice to ensure correct practice, and the student is advised to continue with periodic checks thereafter.^86,87

Clinical reports indicate that this technique can be learned easily by individuals of any age, level of education, occupation, or cultural background.^78,80,85 The technique requires systematic instruction by a qualified teacher to ensure effortless and correct practice.^78,84,86 The technique is practiced twice daily for 15 to 20 minutes, usually once in the morning (before breakfast) and once in the afternoon (before dinner).^78,85,88

Criteria of successful meditation practice. The successful practice of the TM^® technique is determined by a qualified teacher. As many details of the TM^® technique are restricted to those who receive instruction, a description of the criteria used by the instructor for the assessment of the technique is not available in the scientific literature.

Relaxation Response

The “relaxation response” is a term coined by Harvard cardiologist Herbert Benson in the early 1970s to refer to the self-induced reduction in the activity of the sympathetic nervous system,^68,89 the opposite of the hyperactivity of the nervous system associated with the fight-or-flight response. Benson believed that this response was not unique to TM^® and that all ancient meditation practices involved common components that together are capable of producing such a response.⁶⁸ Basing his belief on his scientific research on hypertension and TM^®, he integrated these common factors into a single technique (RR) and found that it promoted a decrease in sympathetic nervous system similar to TM^®.⁹⁰ Many techniques for eliciting the relaxation response have been presented in a religious context in Judaism, Christianity, or Islamic mysticism (Sufism). These techniques employ both mental and physical methods, including the repetition of a word, sound, or phrase (often in the form of a prayer); and the adoption of a passive attitude.⁹¹ Benson emphasized that the relaxation response is not simply a state of relaxation (and should not be confused with it) or a sleep-like state, but a unique state brought about by adherence to specific instructions.⁸⁹

Main components. The individual is instructed to assume a comfortable posture (usually sitting, but kneeling or squatting may also be used), the eyes are closed, and the muscles are relaxed, beginning at the feet and progressing upward to the face. Once the practitioner is relaxed, the eyes may be open or remain closed. Then, breathing through the nose and focusing on the breath, the practitioner inhales and exhales, silently saying the word “one” with each exhalation.^89,90,92 Like TM^®, the repetition of a sound, word, or phrase is considered essential to the technique.⁸⁹ Benson recommends “one” as a neutral, one-syllable word.⁹³ When the practice is completed, the meditator sits quietly for several minutes with eyes closed and then with eyes open.⁸⁹

More recent versions of the technique include a body scan (similar to that employed in MBSR, described below) in which practitioners are asked to move their attention slowly over the body focusing on relaxing different regions, and information sessions on the stress response and its effects on health.⁹⁴

Breathing. Breathing is active. Practitioners breathe through the nose, cultivating an easy, natural rhythm.⁸⁹

Attention and its object. Attention is focused on the breath. In addition, should distracting thoughts occur, an attempt should be made to ignore them and focus on the mantra.⁹² The mantra is therefore “linked” with the breath.⁶⁸ It has been claimed that Benson's RR demands a greater degree of concentration than either TM^® or CSM (described below).⁶⁴

Spirituality and belief. Because it is believed that RR incorporates the essential components of a wide variety of meditation practices, it is conceptualized as a secular technique^89,95 and does not require adopting a specific spiritual orientation or belief system.

Training. RR is learned in approximately five minutes. Patients are typically instructed to elicit the relaxation response twice daily, for 15 to 20 minutes, but not within two hours after any meal, as the digestive processes may interfere with the subjective changes induced by the technique.^89,90,96

Criteria of successful meditation practice. Instructions for this technique are available in books and articles and there is no explicit recommendation that an experienced practitioner teach the technique or that individualized instruction is necessary. The criteria for successful meditation practice rest with the subjective evaluation of the meditator; the results of practice judged against the reported effects of RR. Instructions for this technique include the injunction not to worry about whether one is successful in achieving a deep level of relaxation, and instead to maintain a positive attitude and let relaxation occur at its own pace.⁸⁹

Clinically Standardized Meditation

CSM was developed by Patricia Carrington while she was conducting studies on meditation at Princeton University in the early-to-mid 1970s. Believing that TM^® was not flexible enough to be suitable for all clinical purposes and that the cost of its instruction put it beyond the reach of most individuals and institutions, Carrington modified a classical Indian form of mantra meditation and produced what she called CSM.⁶⁴

Main components. Trainees are instructed to choose a mantra from a list of 16 Sanskrit mantras, or choose their own. In choosing their own mantra, practitioners are told to select a word that has a “pleasant ringing sound” and to avoid using words that are emotionally loaded. The word should help imbue the practitioner with a sense of serenity.⁶⁴ In its original formulation, CSM used a secular ritual for transferring the mantra. CSM is practiced while sitting comfortably, with eyes open and focused on a pleasant object of some kind. The mantra is repeated aloud, slowly and rhythmically, at ever decreasing volume, until it is a whisper, at which point the mantra is no longer said aloud, but instead is only thought. The eyes are then closed as the meditator continues repeating the mantra in thought. Meditators allow the mantra to proceed at its own pace, getting faster or slower, louder or softer “as it wants.”^64,97

Breathing. Breathing is passive, proceeding at its own pace and is unconnected to the repetition of the mantra.

Attention and its object. Like TM^® and RR, CSM is a passive technique that requires little concentration or discipline. In contrast to RR, CSM instructs practitioners to flow with their thoughts rather than ignore them, returning periodically to the mantra.⁶⁴

Spirituality and belief. CSM is designed as a secular, clinical form of meditation practice, so no specific system of spirituality or belief is required.

Training. CSM is taught in two lessons: a 1-hour individual lesson and a group meeting. CSM is practiced twice daily for 20 minutes.⁶⁴ As with RR, the contemporary version of CSM differs slightly from its original form, with perhaps the most important difference being that trainees are given a manual and an audio recording of instructions rather than individual instruction.⁶⁴

Criteria of successful meditation practice. The criteria for successful meditation practice rest with the subjective evaluation of the meditator, the results of practice judged against the reported effects of CSM. Books and audiotapes for self-instruction in CSM are readily available, and there is no explicit statement that an experienced practitioner teach the technique or that individualized instruction is necessary.

Vipassana

Considered by some to be the form of meditation practiced by Gautama the Buddha more than 2,500 years ago,¹⁰⁰ Vipassana, or insight meditation, is practiced primarily in south and southeast Asia but is also a popular form of meditation in Western countries. Vipassana is the oldest of the Buddhist meditation techniques that include Zen (Soto and Rinzai schools) and Tibetan Tantra.^47,99 The Pali term “Vipassana”, though not directly translatable to English roughly means “looking into something with clarity and precision, seeing each component as distinct, and piercing all the way through so as to perceive the most fundamental reality of that thing.”⁴⁷ The goal of Vipassana is the understanding of the 3 characteristics of nature which are impermanence (anicca), sufferings (dhuka), and non-existence (anatta). Vipassana meditation helps practitioners to become more highly attuned to their emotional states.⁴⁷ Through the technique, meditators are trained to notice more and more of their flowing life experience, becoming sensitive and more receptive to their perceptions and thoughts without becoming caught up in them. Vipassana meditation teaches people how to scrutinize their perceptual processes, to watch thoughts arise, and to react with calm detachment and clarity, reducing compulsive reaction, and allowing one to act in a more deliberate way.⁴⁷

Main components. Vipassana meditation requires the cultivation of a particular attitude or approach: (1) don't expect anything, (2) don't strain; (3) don't rush, (4) observe experience mindfully, that is, don't cling to or reject anything, (5) loosen up and relax, (6) accept all experiences that you have, (7) be gentle with yourself and accept who you are, (8) question everything, (9) view all problems as challenges, (10) avoid deliberation, and (11) focus on similarities rather than differences.⁴⁷

Vipassana meditation is practiced in a seated position when focusing on the breath; otherwise, no posture is prescribed and the meditator may sit, stand, walk, or lie down. Traditionally, if a static position has been taken, it is not to be changed until the meditation session has ended. However, many Western teachers allow students to move, though mindfully, to avoid persistent pain caused by being in the same position for too long.⁴⁷ The time devoted to seated meditation should be no longer than one can sit without excruciating pain. The eyes should be closed.⁴⁷

Breathing. Air is inhaled and exhaled freely through the nose. There is a natural, brief pause after inhaling and again after exhaling.⁴⁷

Attention and its object. The focus of attention or awareness in Vipassana can be categorized into 4 groups: body, emotions and feelings, thoughts, and mental processes.¹⁰¹ In focusing attention on the breath, novice Vipassana meditators attain a degree of “shallow concentration.”⁴⁷ This is not the deep absorption or pure concentration of the mantra meditation techniques. Gradually, the focus of attention is shifted to the rims of the nostrils, to the feeling of the breath going in and out. When attention wanders from the breath, the meditator brings it back and anchors it there.^47,100 To help concentrate on the breath, a novice meditator may silently count breaths or count between breaths.⁴⁷ The meditator notices the feeling of inhaling and exhaling and ignores the details of the experience. The movement of the abdominal wall while inhaling and exhaling may also be used as a focus of attention.⁴⁷

The primary technique for focusing on bodily sensations is the body scan.¹⁰² Beginning with the top of the head, the practitioner observes the sensations as if for the first time, and then scans the scalp, the back of the head, and the face. When visualizations of the body distract the meditator, the thoughts are simply directed back to the sensations. The focus of attention is moved continuously over the body, moving down the neck, to the shoulders, arms, hands, trunk, legs and feet. Throughout the entire scan, an attitude of nonanticipation and acceptance is maintained.¹⁰²

Mindfulness can be practiced during any activity and practitioners are encouraged to practice being mindful and fully aware during other activities such as walking, stretching, and eating.¹⁰⁰

Spirituality and belief. Though often described as a profound religious practice, no particular spiritual or philosophical system is required to practice Vipassana meditation.⁴⁷

Training. Vipassana should be practiced twice daily, morning and evening, for about 5 to 10 minutes.¹⁰⁰ Western interpreters of Vipassana have recommended that novice meditators should be instructed to sit motionless for no longer than 20 minutes.⁴⁷ Ideally, a meditator works up to at least two 1-hour sessions per day, and does at least one 10-day retreat per year.¹⁰² Longer meditation sessions allow for deeper periods of meditation.¹⁰² The length of time required to become proficient in Vipassana meditation varies by individual, some students progress rapidly, others slowly.

Criteria of successful meditation practice. As instructions for this technique are available in books and articles and there is no explicit instruction in the literature that an experienced practitioner teach the technique or that individualized instruction is necessary, it is presumed that the criteria for successful meditation practice rests with the subjective evaluation of the meditator. However, instruction may be given and, if this is the case, presumably successful practice is judged by an experienced meditator.

Zen Buddhist Meditation

Zen Buddhist meditation, or Zazen, perhaps one of the most well-known forms of meditation, is a school of Mahayana Buddhism¹⁰³ that employs meditation techniques that originated in India several thousand years ago and were introduced to Japan from China in 1191 A.D.¹⁰⁴ Zen Buddhist meditation is typically divided into the Rinzai and Soto schools.

Main components. The harmony of the body, the breath, and the mind is considered essential to the practice of Zen. In the traditional forms of Zen meditation, physical preparation involves eating nutritious food in modest amounts.¹⁰⁴

Posture is of great importance in Zen meditation. In traditional forms, Zen meditation is performed while seated on a cushion in either the full-lotus or half-lotus position; however, many Western practitioners practice in a variety of ways from chair sitting to full lotus.¹⁰⁴ In the full-lotus position, the legs are crossed and the feet rest on top of the thighs. In the half-lotus position, only one foot is brought to rest on top of the thigh, the other remaining on the ground as in the regular cross-legged position.^104,105 The hands are held in one of two prescribed ways, either with the left hand placed palm up on the palm of the right hand with the tips of the thumbs touching, or with the right hand closed in a loose fist and enclosed in the left hand, the left thumb between the web of the thumb and the index finger of the right hand.¹⁰⁴ The spine is held straight and with the top of the head thrust upward, with the chin drawn in and the shoulders and abdomen remain relaxed. The body should be perpendicular and the ears, shoulders, nose, and navel should be in line. The tongue should touch the upper jaw and the molars should be in gentle contact with one another. The eyes should be half closed and the gaze focused on a point on the floor approximately 3 feet in front.^104,105

Breathing. Breathing in Zen meditation is active and many breathing patterns are used. One deep breathing pattern begins with exhaling completely through an open mouth and letting the lower abdomen relax. Air is then inhaled through the nose and allowed to fill the chest and then the abdomen. This breathing pattern is repeated 4 to 10 times. The mouth is then closed, and air is inhaled and exhaled through the nose only. By the use of abdominal and diaphragmatic pressures, air is drawn in and pushed out. Both inhalation and exhalation should be smooth, with long breaths.¹⁰⁴ After practitioners have learned to focus on their breath by counting, counting is omitted and meditators practice “shikantaza,” which means “nothing but precisely sitting.”¹⁰⁶ Shikantaza is the most advanced form of Zen meditation.¹⁰⁶ With practice, the frequency of breathing becomes about three to six breaths per minute.¹⁰⁴

Attention and its object. Attention is focused on counting breaths or on a koan, a specific riddle that is unsolvable by logical analysis.¹⁰⁶ The frequency of breathing is silently counted in one of three ways: counting the cycles of inhalation and exhalation, counting inhalations only, or counting exhalations only.¹⁰⁴ Though some koans have become famous in the West (e.g., what is the sound of one hand clapping?), in practice, beginners often silently repeat the sound “mu” while counting. As a student advances, there are many koans that may be worked on over a period of years.⁴⁷ This silent repetition allows the meditator to become fully absorbed in the koan. In both counting of breaths and focusing on a koan, it is essential that the concentration of the mind is on the counting or on the koan and not on respiration as such.¹⁰⁴ No attempt is made to focus the mind on a single idea or experience; the meditator sits, aware only of the present moment.⁴⁹

Spirituality and belief. It is generally accepted that Buddhist metaphysical beliefs are not essential to the practice of Zen. At a spiritual level, Zen is considered a recognition of or, more accurately, the constant participation of all beings in the reality of each being.⁴⁹ Sitting should be based on the compassionate desire to save all sentient beings by means of calming the mind; however, this belief is not essential to practice. Only the wish to save all sentient beings and the strength to be disciplined in practice is necessary.¹⁰⁴

Training. Depending on the purpose, Zen meditation may be practiced for a few minutes or for many hours.¹⁰³

Criteria of successful meditation practice. Successful meditation practice is judged in terms of the internal changes that are brought about by cultivating awareness. The practice of Zen meditation should not be done with the aim of accomplishing some purpose or acquiring something.¹⁰⁴ Examples of incorrect aims or approaches include (1) sitting in order to tranquilize the mind, (2) sitting to be empty in one's mind, (3) attempting to solve a koan as if playing a guessing game, and (4) being motivated by a wish to escape from everyday conflicts.¹⁰⁴ Some Zen masters believe that it is acceptable for prospective students to be motivated by desires for good health, composure, iron nerves, etc., because in time their attachment to these less important purposes will be recognized.¹⁰⁴ The successful practice of Zen meditation is often described in terms of an awareness of the “true nature” of reality, of discovering the extent to which ordinary experience is constructed and manipulated by our interests, fears, and purposes. Thus, successful practice results in the realization that a dreamlike absorption in personal intentions is actually the principal content of daily mental life,⁴⁹ freeing the practitioner from circumstance and emotion.¹⁰⁴

Mindfulness-Based Stress Reduction

The MBSR program emerged in 1979 as a way to integrate Buddhist mindfulness meditation into mainstream clinical medicine and psychology.¹⁰⁷ Originally designed by Dr. Jon Kabat-Zinn at the University of Massachusetts Medical Center, the MBSR program was a group-based program designed to treat patients with chronic pain. Since then, MBSR has also been used to reduce morbidities associated with chronic illnesses such as cancer and acquired immunodeficiency syndrome and to treat emotional and behavioral disorders.⁹⁸

Main components. The mindfulness component of the program incorporates three different practices: a sitting meditation, a body scan, and Hatha yoga. In addition to the mindfulness meditation practice that forms the basis of the intervention, patients are taught diaphragmatic breathing, coping strategies, assertiveness, and receive educational material about stress.⁹⁶ The foundation for the practice of MBSR is the cultivation of seven attitudes:

• 1

  nonjudgment, becoming an impartial witness to your own experience;

• 2

  patience, allowing your experiences to unfold in their own time;

• 3

  beginner's mind, a willingness to see everything as if for the first time;

• 4

  trust, in your own intuition and authority and being yourself;

• 5

  nonstriving, having no goal other than meditation itself;

• 6

  acceptance, of things as they actually are in the present moment; and

• 7

  not censoring one's thoughts and allowing them to come and go.⁴⁸

In addition to these attitudes, a strong motivation and perseverance are considered essential to developing a strong meditation practice and a high degree of mindfulness.⁴⁸ These attitudes are cultivated consciously during each meditation session.⁴⁸ As with other mindfulness practices, posture and breathing are essential.⁴⁸ The practitioner sits upright, either on a chair or cross-legged on the floor, and attempts to focus attention on a particular object, most commonly on the sensations of his or her own breath as it passes the opening of the nostrils or on the rising and falling of the abdomen or chest.⁴⁸ Whenever attention wanders from the breath, the practitioner will simply notice the distracting thought and then let it go as attention is returned to the breath. This process is repeated each time that attention wanders from the breath. The MBSR program incorporates formal meditation (i.e., seated, walking, Yoga) and informal meditation (i.e., the application of mindfulness to the activities of daily life). In informal practice, practitioners are reminded to become mindful of their breath to help induce a state of physical relaxation, emotional calm, and insight.⁴⁸

The seated meditation is done either on the floor or on a straight-backed chair.⁴⁸ When sitting on the floor, a cushion approximately 6 inches thick should be placed beneath the buttocks. The practitioner may use the “Burmese” posture in which one heel is drawn in close to the body and the other leg is draped in front, or a kneeling posture, placing the cushion between the feet.⁴⁸ The sincerity of effort matters more than how one is sitting.⁴⁸ Posture should be erect with the head, neck, and back aligned. The shoulders should be relaxed and the hands are usually rested on the knees or on the lap with the fingers of the left hand above the fingers of the right and the tips of the thumbs just touching each other.⁴⁸

The body scan is the first formal mindfulness technique that meditators do for a prolonged period and is practiced intensively for the first 4 weeks of the program. Body scanning involves lying on your back and moving the mind through the different regions of the body, starting with the toes of the left foot and moving slowly upwards to the top of the head. Scanning is done in silence and stillness.

The third formal meditation technique used in the MBSR program is mindful Hatha yoga. It consists of slow and gentle stretching and strengthening exercises along with mindfulness of breathing and of the sensations that arise as the practitioner assumes various postures⁴⁸

Breathing. Breathing is passive and without any specific pattern.⁴⁸

Attention and its object. During sitting meditation, the attention is focused on the inhalation and exhalation of the breath or on the rising and falling of the abdomen. When the mind becomes distracted with other thoughts, the attention is gently, but firmly returned to the breath or abdomen. During the body scan, attention is focused on the bodily sensations. When the mind wanders, attention is brought back to the part of the body that was the focus of awareness.⁴⁸ In contrast to other Yoga practices, mindful Hatha yoga is focused less on what the body is doing and more on maintaining moment-to-moment awareness. As in the seated meditation and body scan, the attention is focused on the breath and on the sensations that arise as the various postures are assumed.

Spirituality and belief. MBSR was designed as a secular, clinical practice and its practice does not require adopting any specific spiritual orientation or belief.

Training. The program consists of an 8-week intervention with weekly classes that last 2 to 3 hours. There is a day-long intensive meditation session between the sixth and seventh sessions.^48,96 Participants also complete 45-minute sessions at home, at least 6 days a week for 8 weeks.⁴⁸ During the 2-hour weekly sessions, participants are instructed in the informal and formal practice of mindfulness meditation. Participants must commit to a daily, 45-minute home practice of the skills taught during the weekly meetings.⁴⁸ The components of practice change as participants become more adept in sitting meditation, body scan, and Yoga. Body scan is initially practiced at least once per day for 45 minutes for about 4 weeks. It is then practiced every other day, alternating with Yoga.⁴⁸

Criteria of successful meditation practice. The proper practice is determined by an experienced teacher. In the absence of any religious or spiritual component, the measure of success is the achievement of successful outcomes, whether subjective (reduced perceived stress, reduced anxiety, etc.) or objective (reduced blood pressure, reduction in medication usage, etc.).

Mindfulness-Based Cognitive Therapy

Developed by Zindel Segal, Mark Williams, and John Teasdale in the 1990s as a method for preventing relapse in patients with clinical depression, MBCT combines the principles of cognitive therapy with a framework of mindfulness to improve emotional well-being and mental health.^98,108 Based on the MBSR program developed by Jon Kabat-Zinn, the original aim of the MBCT program was to help individuals alter their relationship with the thoughts, feelings, and bodily sensations that contribute to depressive relapse, and to do so through changes in understanding at a deep level.¹⁰⁸

Main components. Like MBSR, the MBCT program incorporates seated meditation and body scan. The practice teaches patients decentering (the ability to distance oneself from one's mental contents), how to recognize when their mood is deteriorating, and techniques to help reduce the information channels available for sustained ruminative thought-affect cycles and negative reactions to emotions and bodily sensations.¹⁰⁸ The core skill that the MBCT program aims to teach is the ability, at times of potential relapse, to recognize and disengage from mind states characterized by self-perpetuating patterns of ruminative, negative thought

Breathing. Breathing is passive and without any specific pattern.¹⁰⁸

Attention and its object. During seated meditation, the attention is focused on the inhalation and exhalation of the breath or on the rising and falling of the abdomen. When the mind becomes distracted, the attention is gently, but firmly, returned to the breath or abdomen. During the body scan, attention is focused on the bodily. When the mind wanders, attention is brought back to the part of the body that was the focus of attention.

Spirituality and belief. Like MBSR, MBCT was developed as a secular, clinical intervention and does not require adopting any specific spiritual orientation or belief system.

Training. The program consists of an 8-week program, with one 2-hour session per week. Classes contain approximately 12 students. The program is divided into two main components: in sessions one to four, participants are taught to become aware of the constant shifting of the mind and how to bring the mind to a single focus using a body scan technique and breathing. Participants also learn how the wandering mind can give rise to negative thoughts and feelings. In sessions five to eight, participants learn how to handle mood shifts, either immediately or at a future time.

Like the MBSR program, participants must continue the sessions at home for 6 or 7 days and complete various homework exercises that teach and reinforce mindfulness skills and help participants to reflect on their mindfulness practice.¹⁰⁸

Criteria of successful meditation practice. The presence of an instructor who is adept in the practice of mindfulness is crucial to the success of the program. It is generally believed that if instructors are not mindful as they teach, the extent to which class members can learn mindfulness will be limited.¹⁰⁸ The proper technique is determined by an experienced practitioner. The measure of success is the achievement of successful prevention of relapse based on clinical criteria.

Main Components

What are the main components of the various meditation practices? Which components are universal and which ones are supplemental?.

The variety of meditation practices is an indication of the diversity of the combination of main components and the way in which a given component may be emphasized in a practice. Given the multitude of practices and the many variations or techniques within these practices, it is impossible to select components that might be considered universal or supplemental across practices. Some practices prescribe specific postures (e.g., Zen Buddhist meditation, Tai Chi, Yoga) while others are less concerned with the exact position of the body (e.g., TM^®, RR, CSM). Some practices (e.g., Vipassana, Zen Buddhist meditation, Yoga, Tai Chi, and Qi Gong) incorporate moving meditation, while others are strictly seated meditations (e.g., TM®, RR, and CSM). Some clinically-based practices (e.g., MBSR, MBCT), though guided by the underlying practice of mindfulness, combine several techniques. In this, however, they are not substantially different from older multifaceted meditation practices such as Yoga.

Table 4

Characteristics of included meditation practices

Table 4

Characteristics of included meditation practices

Meditation practice	Main components	Breathing	Attention	Spiritually/belief	Training	Criteria for success
Mantra meditation
TM®	Sitting (no prescribed posture)	Passive, unconnected to repetition of mantra	Attention directed to prescribed mantra	No specific spiritual or religious beliefs required	Taught in 4 consecutive days (preceded by two 1-hour lectures and a 5–10 minute interview) in a 1-hour training session and three 1.5 hour group sessions.	Proper technique as judged by experienced TM® teacher; no specific criteria
	Personalized Sanskrit mantra	No description of breathing	Mantra repeated silently		Individual instruction Practiced twice daily, 15–20 min/session
	Eyes closed				Instruction by qualified TM® teacher
Relaxation Response	Comfortable posture (sitting, kneeling, squatting)	Passive, but mantra is “linked” to exhalation	Attention focused on the breath	No specific spiritual or religious beliefs required	Taught in 5-min training session	Proper technique according to subjective evaluation and measured against reported effects of RR
	Eyes open or closed	Nasal	Mantra repeated silently		Individual instruction
	Can also include body scan and information sessions		Thoughts are ignored		Practiced twice daily, 15–20 min/session and not before 2hrs after a meal
Clinically Standardized Meditation	Comfortable seated posture	Passive, unconnected to repetition of mantra	Attention directed to individually chosen mantra (1 of 16)	No specific spiritual or religious beliefs required	Taught in 2 1-hr lessons	Proper technique according to subjective evaluation and measured against reports of effects of CSM
	Sanskrit mantra or individually chosen mantra		Mantra repeated aloud and then at decreasing volume until it is repeated silently		Individual instruction or training manual and audio tapes
	Eyes open initially and focused on pleasant object, then closed for repetition of mantra		Thoughts recognized, but not focused on		Practiced twice daily for 20 min/session
Mindfulness meditation
Vipassana	Cultivation of a “mindful” attitude	Passive	Attention is focused on the breath (first on the inhalation and exhalation, then shifted to rims of the nostrils) or on bodily sensations	No specific spiritual or religious beliefs required	No specific training period given	Proper technique determined by experienced meditator or by self-evaluation
	Seated posture	Nasal			Session should last no longer than one can comfortably sit
					Novice meditators no longer than 20 min
Zen	Specific seated postures (lotus or half-lotus), positioning of hands, mouth and tongue	Active	Attention focused on counting of breath, on a koan or “just sitting.”	No specific spiritual or religious beliefs required; however, attitude of nonpurposefulness is essential	No specific training period given	Successful practice determined by experienced teacher; specific personal experience of the true nature of reality
	Eyes half closed and focused on point on floor	Inhale through nose, exhale through mouth and nasal only	Breath counted by 1 of 3 methods		Sessions may last from several minutes to several hours
		Many breathing patterns	No attempt to focus on single idea or experience
MBSR	Cultivation of a “mindful” attitude	Active (diaphragmatic breathing) and passive	Seated meditation: attention focused on breath as it passes edge of nostrils or on rising and falling of abdomen	No specific spiritual or religious beliefs required; however, strong commitment and self-discipline are essential	Taught in an 8-week course involving weekly 2–3 hr classes and 45-min sessions at home 6 days a week with homework exercises	Successful meditation requires the technique be taught by an teacher experienced in mindfulness meditation; achievement of successful health outcomes
	Prescribed postures		Body scan: attention focused on somatic sensations in the part of the body being “scanned.”		After course, practiced daily for 45 min
	Seated meditation		Hatha yoga: attention focused on breath and the sensations that arise as different postures are assumed		Group instruction by an experienced MBSR practitioner
	Body scan (supine posture)
	Hath yoga postures
MBCT	Based on MBSR program	Passive	Seated meditation: attention focused on breath as it passes edge of nostrils or on rising and falling of abdomen	No specific spiritual or religious beliefs required	Taught in an 8-week course involving weekly 2-hr classes and 45-min sessions at home 6 days a week with homework exercises	Successful meditation requires the technique be taught by an teacher experienced in mindfulness meditation; successful prevention of depressive relapse as determined by clinical evaluation
	Cultivation of “decentered” or “mindful” perspective		Body scan: attention focused on somatic sensations in the part of the body being "scanned		Program taught in 2 main components: (1) teaching of mindfulness, (2) learning to handle mood shifts
	Seated meditation				Group instruction by an experienced practitioner of mindfulness meditation
	Body scan
Yoga
Kundalini yoga, Sahaja yoga, and Hatha yoga (many styles)	Emphasis of components vary among “schools” but can include ethical observances, physical postures, breathing techniques, concentrative and mindfulness meditation	Active and passive	Awareness for all techniques is centered on the breath	No specific spiritual or religious beliefs required unless the ethical component is included	Regular daily practice from 15 min to several hours; instruction by an experienced Yogi or Guru; may take several years or longer to properly execute asanas and pranayama	Successful technique is judged by the individual or Guru against the standards for posture and breathing and against reported benefits of successful practice
		Techniques vary	Some techniques also focus on posture
Tai Chi
Yang, Chen, Sun, Wu (Jian Qian), and Wu (He Qin) styles	A routine of slow, deliberate movements (movements and postures vary among schools)	Active	Attention is focused on movement and on one's internal energy (qi)	No specific spiritual or religious beliefs required	Routines vary in number of postures and duration Classical Yang-style Tai Chi includes 108 postures and takes approximately 20–25 min to complete; practice also includes a 20-min warm-up and 10-min cooldown	Proper movement and posture as judged by experienced Tai Chi teacher
	Body relaxed, upper body erect, not bending	Nasal			Should practice everyday
	Mouth closed, teeth not clenched
Qi Gong
Many techniques	Meditation	Active	Attention is focused on the “elixir field” and on the inhalation and exhalation of the breath	No specific spiritual or religious beliefs required	Practiced twice daily for 20–30 min with no single session exceeding 3 hr	Proper movement and posture as judged by experienced Qi Gong teacher
	Prescribed posture for seated meditation	Techniques vary				Safe practice requires instruction by experienced Qi Gong teacher
	Movements practiced in a relaxed stationary position
	Breathing exercises

Breathing

How is breathing incorporated in these practices? Are there specific breathing patterns that are integral elements of meditation? Is breathing passive or directed?

The use of the breath is ubiquitous in all practices; however, the importance and attention given to it vary from practice to practice. Each meditation practice and technique has a breathing pattern or element that can be considered integral to that technique, whether the breath is actively controlled in terms of its timing and depth (e.g., Zen Buddhist meditation, Yoga, Tai Chi), or passive and “natural” (e.g., TM^®, RR, CSM, Vipassana, MBCT). The practice of Yoga, which covers thousands of techniques, uses both active and passive breathing. Though the direction for active breathing may be relatively uniform across the techniques in a given practice (e.g., Zen Buddhist meditation), other practices use a wide array of breathing techniques that change according to the outcome desired (e.g., Kundalini yoga). For those practices that utilize passive breathing, there is no consistent pattern or rhythm as “breathing naturally” will vary from practitioner to practitioner.

Attention and Its Object

For each type of meditation practice, where is the attention directed during meditation (e.g., mantra, breath, image, nothing)?

The purposeful focusing of attention is considered crucial in all meditation practices. However, like breathing, the techniques for anchoring attention vary and there is no single method shared by all practices. For those practices that use a mantra (e.g., TM^®, RR, CSM), in some the mantra may be repeated silently, and in some aloud. The factors surrounding the choice of the mantra vary and the nature of the mantra chosen will influence the number of associations brought forth by the word and the vibrations caused by the vocalization of the mantra. Some mantras will have no meaning to Western practitioners unfamiliar with Sanskrit (e.g., TM^®, CSM, Yoga), while others will (e.g., RR).

Other forms of meditation practice focus attention on bodily sensations (e.g., Vipassana, MBSR, MBCT) or a body part (e.g., Tai Chi) to the exclusion of other thoughts. The so-called mindfulness techniques focus on the breath and cultivate an objective openness to whatever comes into awareness.⁷² Though this may be interpreted as not focusing attention, or, as it is sometimes paradoxically phrased, as focusing on nothing, the attention is controlled and directed with the aim of achieving a distance from one's emotional and cognitive responses to the objects in the field of attention. The difference between mindfulness meditation and other practices lies in the acceptance of these other thoughts into the field of awareness.

Though the distinction between concentrative and mindfulness meditation has prima facie validity, the reality is somewhat more complicated because some practices, such as Zen and Vipassana, have phases where concentration is used, and for which certain techniques such as counting or concentrating on a mantra are employed, while at other stages broad spaced mindful attention is encouraged.

Spirituality and Belief

To what extent is spirituality a part of meditation? To what extent is belief a part of meditation?

The one common feature of all meditation practices examined in this review is the apparent ability to practice meditation without adopting a specific system of spiritual or religious belief. However, the extent to which spirituality and belief are part of any given meditation practice is poorly described. Furthermore, if the Taoist metaphysical assumptions of Qi Gong are crucial to successfully understand, visualize, and guide qi, then at least this practice requires adoping a specific belief system

The extent to which spirituality or belief play a role in any meditation practice appears to depend in large part on the individual practitioner. Though the traditional practices were developed within specific spiritual or religious contexts (Vipassana, Zen Buddhist meditation, Yoga, Tai Chi, Qi Gong), and therefore have spiritual or religious aspects, this does not mean that a practitioner must adopt the belief systems upon which they were based. In addition, some practices developed for purposes other than spiritual enlightenment; for example, Tai Chi and Qi Gong were developed within a system martial exercise and Traditional Chinese Medicine, respectively. Though Yoga, too, has spiritual and religious components, it is often considered more properly a system of metaphysics and psychology, especially when the ethical instructions are ignored. In summary, it appears that all meditation practices can be performed, to some degree, without adopting a specific system of spirituality or belief.

Training

What are the training requirements for the various meditation practices (e.g., the range of training periods, frequency of training, individual and group approaches)?

Training refers to the specific periods of practice, the frequency and duration of practice, and how long a practitioner is expected to train before becoming proficient in a given technique. The training for meditation varies with periods of practice, ranging from 5 minutes (RR, Vipassana) to several hours (Yoga). The frequency of practice ranges from daily (MBSR, MBCT, Tai Chi, Vipassana, Yoga) to twice daily (TM^®, RR, CSM, Qi Gong). Zen meditation does not specify a frequency of practice. Few practices give a required duration of practice; however, some (Yoga, Zen Buddhist meditation) give an indication of the time required to master a given technique.

Criteria of Successful Meditation Practice

How is the success of the meditation practice determined (i.e., was it practiced properly)? What criteria are used to determine successful meditation practice?

The criteria of successful meditation practice is understood both in terms of the successful practice of a specific technique (i.e., the technique is practiced properly) and in terms of achieving the aim of the meditation practice (e.g., leading to reduced stress, calmness of mind, or spiritual enlightenment).

The successful practice of a specific technique is sometimes judged by an experienced or master practitioner (TM^®, MBSR, Yoga, Tai Chi, Qi Gong), and in some cases it can be judged by the individual (RR, CSM). However, the proliferation of self-instruction books and videos for some of the practices that also recommend an experienced teacher implies that individuals may judge, to some degree, the success of a practice.

General Characteristics

The studies were published between 1956 and 2005, with 51 percent of the studies (n = 417) published after 1994. Most of the studies (86 percent, n = 701) were published as journal articles. Seventy-nine (10 percent) were theses or dissertations, 25 (3 percent) were abstracts from scientific conferences, and 5 (0.5 percent) were book chapters or letters. Three unpublished studies (0.5 percent) were identified by contacting investigators. Studies were conducted in North America (61 percent), Asia (24 percent), Europe (11 percent), Australasia (3 percent) and other regions (1 percent).

Of the 813 studies included, 67 percent (n = 547) were intervention studies (286 RCTs, 114 NRCTs and 147 before-and-after studies), and 33 percent (n = 266) were observational analytical studies (149 cohort and 117 cross-sectional studies).

Methodological Quality

Intervention studies. Overall, the methodological quality of the 286 RCTs was poor (median Jadad score = 2/5; IQR, 1 to 2). Only 14 percent (n = 40) of the RCTs were considered of high quality (i.e., Jadad scores greater than or equal to 3 points). Three studies¹⁶⁷– ¹⁶⁹ obtained 4 points on the Jadad scale, and none obtained a perfect score (5 points). The remaining 246 RCTs had a high risk of bias.

The methodological quality of the RCTs was analyzed by the individual components of the Jadad scale. We found that 21 percent (n = 60) described how the randomization was carried out. Among these 60 trials, 75 percent (n = 45) reported adequate methods to randomize study participants to treatment groups, whereas 25 percent (n = 15) used inappropriate and unreliable methods (i.e., alternation or methods based on patient characteristics) that might have introduced imbalances and jeopardized the estimates of the overall treatment effect.

The vast majority of RCTs (97 percent, n = 278) did not use double blinding to hide the identity of the assigned interventions from the participant and assessor, or hide the hypothesis from the instructor and participant or participant and assessor. One of them¹⁷⁰ described an inadequate method of double blinding while the others did not provide any description about the double-blinding procedures. Finally, 51 percent (n = 145) of the RCTs provided a description of withdrawals and dropouts from the study.

Table 5

Methodological quality of RCTs

Table 5

Methodological quality of RCTs

Quality components	N studies (%)
Randomization	286 (100)
Double blinding	8 (2.8)
Appropriate randomization	45 (15.6)
Appropriate double blinding	...
Inappropriate randomization	15 (5.2)
Inappropriate double blinding	1 (0.3)
Description withdrawals	145 (50.7)
Total Jaded score (max 5); median (IQR)	2 (1, 2)
Number of high quality RCTs (Jaded score ≥3)	40 (13.9)
Appropriate concealment of allocation	12 (4.1)
Funding reported	118 (41.3)

IQR = interquartile range; RCT = randomized controlled trial

Table 6

Methodological quality of NRCTs

Table 6

Methodological quality of NRCTs

Quality components	N studies (%)
Double blinding	...
Appropriate double blinding	...
Inappropriate double blinding	...
Description withdrawals	52 (45.6)
Total modified Jadad score (max 3), median (IQR)	0 (0, 1)
Funding reported	30 (26.3)

NRCT = nonrandomized controlled trials

Table 7

Methodological quality of before-and-after studies

Table 7

Methodological quality of before-and-after studies

Quality components	N studies (%)
Study population representative of the target population	23 (15.6)
The method of outcome assessment was the same for the pre and post intervention periods for all participants	140 (95.2)
Outcome assessors were blind to intervention and assessment period	3 (2)
Description of the number of study withdrawals	45 (30.6)
Description of the reasons for study withdrawal	20 (13.6)
Funding reported	41 (27.9)

Table 8

Methodological quality of cohort studies (NOS scale) (more...)

Table 8

Methodological quality of cohort studies (NOS scale)

	Quality components	N studies (%)
Selection of the cohorts	Representativeness of the exposed cohort
	Truly representative of the average group in the community*	12 (8.1)
	Somewhat representative of the average group in the community*	43 (28.9)
	Selected group of participants	88 (59.1)
	No description of the derivation of the cohort	6 (4.0)
	Selection of the nonexposed cohort
	Drawn from the same community as the exposed cohort*	56 (37.6)
	Drawn from a different source	79 (53.0)
	No description of the derivation of the nonexposed cohort	14 (9.4)
	Ascertainment of exposure
	Secure record*	10 (6.7)
	Structured interview	4 (2.7)
	Written self-report	21 (14.1)
	No description of exposure ascertainment	114 (76.5)
	Ascertainment of outcome
	Demonstration that the outcome(s) of interest was not present at the start of the study*	10 (6.7)
Comparability of the cohorts	Study controls for two or more important confounding factors*	48 (32.2)
	Study controls for at least one important confounding factor*	51 (34.2)
	No adjustment for important confounding factors in the design or analysis of the study	50 (33.6)
Outcome assessment	Assessment of outcome
	Independent blind assessment*	24 (16.1)
	Record linkage*	85 (57.0)
	Self-report	38 (25.5)
	No description of outcomes assessment	2 (1.3)
	Length of followup
	Followup long enough for outcomes to occur*	44 (29.5)
	Adequacy of followup of cohorts
	Complete followup (all subjects accounted for)*	17 (11.4)
	Subjects lost to followup unlikely to introduce bias*	12 (8.1)
	Lost to followup likely to introduce bias	8 (5.4)
	No description of losses to followup	112 (75.2)
NOS total score (max 9); median (IQR)		3 (2, 4)

*

Positive responses earn stars for the final score.

IQR = interquartile range; NOS = Newcastle-Ottawa Scale

In general, the cohort studies failed to protect against selection bias when assembling the exposed and nonexposed cohorts. Participants in 60 percent (n = 94) of the studies were not representative of the target population about which conclusions were to be drawn. The selection of the nonexposed cohort was equally compromised (62 percent, n = 93).

Detection bias affecting the ascertainment of both exposure and outcome was introduced in 139 (93 percent) studies. These studies did not use reliable methods to ensure that no differences in accuracy of exposure data between the cohorts existed. A similar proportion was found for studies that failed to demonstrate that the outcomes of interest were not present at the start of the study. Similarly, 105 (71 percent) cohort studies did not provide enough information to assess whether the length of the followup period was sufficient for outcomes to occur.

Attrition bias was substantial; only 20 percent (n = 29) of the studies reported followup rates unlikely to introduce differences between the comparison groups. The only methodological component that did not appear to be severely jeopardized was the control of confounders in the design or analysis. Sixty-six percent (n = 99) of the cohort studies adjusted for potential confounders either in the design or analysis. Finally, 28 percent (n = 41) of the cohort studies reported the source of funding.

Table 9

Methodological quality of cross-sectional studies (NOS (more...)

Table 9

Methodological quality of cross-sectional studies (NOS scale)

	Quality components	N studies (%)
Selection of the comparison groups	Representativeness of the study group
	Truly representative of the average group in the community*	1 (0.9)
	Somewhat representative of the average group in the community*	61 (52.1)
	Selected group of participants	12 (10.3)
	No description of the derivation of the study group	43 (36.8)
	Selection of the comparison group
	Drawn from the same community as the study group*	24 (20.5)
	Drawn from a different source	56 (47.9)
	No description of the derivation of the comparison group	37 (31.6)
	Ascertainment of exposure
	Secure record*	...
	Structured interview	...
	Written self-report	2 (1.7)
	No description of exposure ascertainment	115 (98.3)
Comparability of groups	Study controls for two or more important confounding factors*	49 (41.8)
	Study controls for at least one important confounding factor*	14 (12)
	No adjustment for important confounding factors in the design or analysis of the study	54 (46.2)
Outcome assessment	Assessment of outcome
	Independent blind assessment*	...
	Record linkage*	62 (53.0)
	Self-report	52 (44.4)
	No description of outcomes assessment	3 (2.6)
NOS total score (max 9); median (IQR)		2 (1, 3)

*

Positive responses earn stars for the final score.

IQR = interquartile range; NOS = Newcastle-Ottawa Scale

Over half of the cross-sectional studies (53 percent, n = 62) chose study groups that were at least somewhat representative of the target population. However, only 21 percent of the studies (n = 24) drew the comparison groups from the same population as the study group. None of the studies used secure methods for ascertainment of exposure. Half of the cross-sectional studies (54 percent, n = 63) adjusted for potential confounders either in the design or analysis and used relatively reliable methods for assessing the outcomes (53 percent, n = 62). Finally, only 27 (23 percent) cross-sectional studies disclosed their source of funding.

Meditation Practices Examined in Clinical Trials and Observational Studies

Eight hundred and thirteen studies described meditation practices examined in intervention studies (RCTs, NRCTs, and before-and-after studies) and observational analytical studies (cohort and cross-sectional studies with control groups).

Table 10

Meditation practices examined in intervention and observational (more...)

Table 10

Meditation practices examined in intervention and observational analytical studies

Meditation practice	Intervention studies (n)			Observational analytical studies (n)		Total (n)
	RCT	NRCT	Before-and-after	Cohort	Cross- sectional
Mantra meditation	111	30	31	105	60	337
Mindfulness meditation	50	25	28	12	12	127
Meditation (ND)	11	6	2	1	1	21
Miscellaneous meditation practices	3	...	3	2	3	11
Qi Gong	13	...	9	7	8	37
Tai Chi	29	17	20	4	18	88
Yoga	69	36	54	18	15	192
Total	286	114	147	149	117	813

ND = not described; NRCT = nonrandomized controlled trials; RCT = randomized controlled trials

Mantra meditation. Forty-one percent (n = 337) of the included studies reported on interventions involving the use of a mantra as a pivotal component for the practice of meditation. The studies were published from 1972 to 2005, with 1986 the median year of publication (IQR, 1978 to 1991). Study sample sizes ranged from 10 to 602,000 participants with a median of 40 participants per study (IQR, 24 to 68).

A variety of mantra meditation techniques were assessed in the studies. The majority of the studies (68 percent, n = 230) focused on TM^® or the TM^®-Sidhi program. Fifteen percent (n = 51) reported on Benson's RR, and nine percent (n = 31) assessed practices in which words or phrases (mantra) were chanted aloud or silently and used as objects of attention. Mantra meditation techniques such as CSM, and SRELAX that are similar to TM^®, but developed specifically for clinical purposes, were assessed in four percent (n = 12) of the studies”. Acem meditation, an amalgam of traditional meditation techniques and Western psychological theory and practices, was evaluated in two percent (n = 7) of the studies. Finally, three percent of the studies focused on other mantra techniques such as Ananda Marga (n = 3), concentrative prayer (n = 2), and Cayce's meditation (n = 1)

Design and methodology. Thirty-three percent (n = 111) of the studies on mantra meditation were RCTs, 31 percent (n = 105) were cohort studies, 18 percent (n = 60) cross-sectional studies, and 9 percent for each of before-and-after studies (n = 31) and NRCTs (n = 30). The methodological quality of intervention studies on mantra meditation was poor: The median Jadad score for RCTs was 1/5 (IQR, 1 to 2). Only 13 out of 111 RCTs (12 percent) scored 3 points or more on the Jadad scale and thus could be considered high quality. The median modified Jadad score for NRCTs was 0.5/3 (IQR, 0 to 1). The quality of before-and-after studies was poor. The methodological quality of observational studies was also low, with a median NOS total score for cohort studies of 3/9 stars (IQR, 2 to 4) and a median NOS total score for cross-sectional studies of 2/6 stars (IQR, 1 to 3). There were major deficiencies in the selection and comparability of the study groups.

Mindfulness meditation. Sixteen percent of the studies (n = 127) described the use of mindfulness meditation techniques, such as MBSR (n = 49), mindfulness meditation techniques not further described (n = 37), Zen Buddhist meditation (n = 28), MBCT (n = 7), and Vipassana meditation (n = 6). The studies were published from 1964 to 2005, with a median year of publication of 2001 (IQR, 1992 to 2003). Study sample sizes ranged from 10 to 719 with a median number of 39 participants per study (IQR, 23 to 73).

Design and methodology. Thirty-nine percent (n = 50) of the studies on mindfulness meditation were RCTs, 22 percent (n = 28) were before-and-after studies, 20 percent (n = 25) NRCTs, and 9 percent for each of cohort (n = 12) and cross-sectional studies (n = 12). The methodological quality of intervention studies on mindfulness meditation was low (RCTs median Jadad score = 2/5; IQR, 1 to 2; NRCTs median modified Jadad score: 0.5/3; IQR, 0 to 1). The quality of before-and-after studies was poor. Only 7 of 50 RCTs (14 percent) scored 3 or more points in the Jadad scale and were thus considered high quality. The observational studies also exhibited major methodological shortcomings (cohort studies median NOS total score: = 3/9 stars; IQR, 2 to 4; cross-sectional studies median NOS total score: 3/6; IQR, 1 to 3), particularly in the areas of selection and comparability of the study groups.

Meditation practices not described. Three percent of the included studies (n = 21) reported on meditation practices that were not described. The studies were published from 1974 to 2004, with a median year of publication of 1998 (IQR, 1990 to 2002). Study sample sizes ranged from 10 to 230 with a median number of 46 participants per study (IQR, 27 to 97).

Design and methodology. Almost half (n = 11) of the studies were RCTs, six were NRCTs, two before-and-after studies, one cohort and one cross-sectional study. The methodological quality of the intervention studies was low (RCTs median Jadad score = 1.5/5; IQR, 1 to 2; NRCTs median modified Jadad score = 0/3; IQR, 0 to 0). Only 1 out of 11 RCTs scored 3 or more points on the Jadad scale and thus was considered high quality. The quality of before-and-after studies was poor. The cohort and cross-sectional studies obtained three and two stars on the NOS scales, respectively. Both studies failed to select unbiased study samples, thus compromising the comparability of the groups.

Miscellaneous meditation practices. One percent of the included studies (n = 11) reported on interventions that combined different meditation techniques in a single intervention. The studies were published from 1980 to 2005, with a median year of publication of 1985 (IQR, 1981 to 1993). Sample sizes ranged from 11 to 340 with a median number of participants per study of 84 (IQR, 20 to 181).

Design and methodology. Three out of 11 studies were RCTs, 3 were before-and-after studies, 2 were cohort studies, and 3 were cross-sectional studies. The methodological quality of studies on miscellaneous meditation practices showed important flaws. All RCTs scored 2 points on the Jadad scale and were considered low quality. The quality of before-and-after studies was also poor. The observational studies exhibited the same methodological flaws as the studies of other interventions described above (cohort studies median NOS total score = 3/9 stars; IQR, 1 to 3; cross-sectional studies median NOS total score = 2/6; IQR, 2 to 3).

Qi Gong. Five percent of the included studies (n = 37) reported on Qi Gong interventions. The studies were published between 1956 and 2005, with a median year of publication of 2000 (IQR, 1996 to 2004). Study sample sizes varied from 10 to 254 with a median number of 36 participants per study (IQR, 22 to 73).

Design and methodology. Thirty-five percent (n = 13) of the studies on Qi Gong were RCTs, 24 percent (n = 9) were before-and-after studies, 19 percent cohort (n = 7), and 22 percent cross-sectional studies (n = 8). The methodological quality of studies on Qi Gong was poor (RCTs median Jadad score = 1/5; IQR, 1 to 2), all scoring less than 3 points on the Jadad scale. The quality of before-and-after studies was also poor. The quality of observational studies was low (cohort studies median NOS total score = 2/9 stars; IQR, 2 to 4; cross-sectional studies median NOS total score = 2.5/6; IQR, 2 to 3). Major deficiencies were found in the selection and comparability of the study groups.

Tai Chi. Eleven percent of the included studies (n = 88) reported on Tai Chi interventions. The studies were published from 1977 to 2005, with a median year of publication of 2002 (IQR, 1998 to 2004). Study sample sizes ranged from 10 to 311 with a median number of participants per study of 39 (IQR, 25 to 65).

Design and methodology. Thirty-three percent (n = 29) of the studies on Tai Chi were RCTs, 23 percent (n = 20) were before-and-after studies, 19 percent (n = 17) NRCTs, 20 percent (n = 18) cross-sectional studies, and 4.5 percent (n = 4) were cohort studies. The methodological quality of studies on Tai Chi was poor (RCTs median Jadad score = 2/5; IQR, 1 to 3; NRCTs median modified Jadad score = 1/3; IQR, 0 to 1). Nine out of 29 RCTs scored 3 or more points on the Jadad scale and thus were considered high quality. The quality of before-and-after studies was also low. The observational studies exhibited major flaws and were likely to be affected by bias (cohort studies median NOS total score = 2/9 stars; IQR, 2 to 4; cross-sectional studies median NOS total score = 2/6; IQR, 2 to 4).

Yoga. Twenty-four percent of the included studies (n = 192) reported on interventions involving Yoga practices. The studies were published between 1968 and 2005, with a median year of publication of 1998 (IQR, 1991 to 2002). Study sample sizes ranged from 10 to 335 with a median of 40 participants (IQR, 23 to 70).

Design and methodology. Thirty-six percent (n = 69) of the studies on Yoga interventions were RCTs, 28 percent (n = 54) were before-and-after studies, 19 percent (n = 36) NRCTs, 9 percent (n = 18) cohort studies, and 8 percent (n = 15) were cross-sectional studies. The methodological quality of studies on Yoga was low (RCTs median Jadad score = 1/5; IQR, 1 to 2; NRCTs median modified Jadad score = 0/3; IQR, 0 to 1). Fourteen percent (n = 10) of the RCTs on Yoga scored 3 points or more on the Jadad scale and were considered high quality. The quality of before-and-after studies was also poor. The methodological quality of observational studies was low (cohort studies median NOS total score: = 3.5/9 stars; IQR, 2.5 to 5; cross-sectional studies median NOS total score = 3/6; IQR, 1 to 3).

Table 11

Methodological quality of RCTs by meditation practice* (more...)

Table 11

Methodological quality of RCTs by meditation practice*

Quality criteria	Mantra meditation (n = 111)	Mindfulness meditation (n = 50)	Meditation practices (ND) (n = 11)	Miscellaneous meditation practices (n = 3)	Qi Gong (n = 13)	Tai Chi (n = 29)	Yoga (n = 69)
Randomization; n (%)	All	All	All	All	All	All	All
Double blinding; n (%)	2 (1.2)	1 (2.0)	...	...	...	1 (3.4)	4 (5.8)
Appropriate randomization; n (%)	15 (13.3)	8 (16.0)	2	...	...	9 (31.0)	11 (15.9)
Appropriate double blinding; n (%)	...	...	...	...	...	...	...
Inappropriate randomization; n (%)	3 (2.7)	1 (2.0)	1	...	2	1 (3.4)	7 (10.1)
Inappropriate double blinding; n (%)	...	...	...	...	...	...	1 (1.4)
Description withdrawals; n (%)	50 (45.0)	27 (54.0)	5	3	8	19 (65.5)	33 (47.8)
Total Jadad score (max 5); Median (IQR)	1 (1, 2)	2 (1, 2)	1 (1, 2)	2 (2, 2)	1 (1, 2)	2 (1, 3)	1 (1, 2)
Number of high quality RCTs (Jadad scores ≥3); n (%)	13 (11.6)	7 (14.0)	1	...	...	9 (31)	10 (14.4)
Appropriate concealment of allocation; n (%)	3 (2.7)	1 (2.0)	...	...	...	3 (10.3)	5 (7.2)
Funding reported; n (%)	49 (44.1)	20 (40.0)	3	...	7	10 (34.4)	28 (40.6)

*

Percentages are reported for N ≥ 20 only

IQR = interquartile range; ND = not described

Table 12

Methodological quality of NRCTs by meditation practice* (more...)

Table 12

Methodological quality of NRCTs by meditation practice*

Quality criteria	Mantra meditation (n = 30)	Mindfulness meditation (n = 25)	Meditation practices (ND) (n = 6)	Tai Chi (n = 17)	Yoga (n = 36)
Double blinding; n (%)	...	...	...	...	...
Appropriate double blinding; n (%)	...	...	...	...	...
Inappropriate double blinding; n (%)	...	...	...	...	...
Description withdrawals; n (%)	15 (50.0)	14 (56.0)	-	9 (52.9)	14 (38.9)
Total modified Jadad score, (max 3); Median (IQR)	0.5 (0, 1)	1 (0, 1)	0 (0, 0)	1 (0, 1)	0 (0, 1)
Funding reported; n (%)	5 (16.7)	7 (28.0)	2	7 (41.2)	9 (25)

*

Percentages are reported for N ≥ 20 only

IQR = interquartile range; ND = not described

Table 13

Methodological quality of before-and-after studies (more...)

Table 13

Methodological quality of before-and-after studies by meditation practice*

Quality criteria	Mantra meditation (n = 31)	Mindfulness meditation (n = 28)	Meditation practices (ND) (n = 2)	Miscellaneous meditation practices (n = 3)	Qi Gong (n = 9)	Tai Chi (n = 20)	Yoga (n = 54)
Study population representative of the target population; n (%)	1 (3.2)	11 (39.3)	1	2	1	4 (20)	3 (5.5)
The method of outcome assessment was the same for the pre and postintervention periods for all participants; n (%)	29 (93.5)	25 (89.3)	2	3	8	20 (100)	53 (98.1)
Outcome assessors were blind to intervention and assessment period; n (%)	2 (6.4)	...	...	...	...	1 (5)	-
Description of the number of study withdrawals; n (%)	12 (38.7)	15 (53.6)	...	3	1	6 (30)	8 (14.8)
Description of the reasons for study withdrawal; n (%)	4 (12.9)	5 (17.8)	...	2	1	4 (20)	4 (2.1)
Funding reported; n (%)	4 (12.9)	7 (25)	...	2	5	3 (15)	20 (37)

*

Percentages are reported for N ≥ 20 only

ND = not described

Table 14

Methodological quality of cohort studies by meditation (more...)

Table 14

Methodological quality of cohort studies by meditation practice*

	Quality criteria	Mantra meditation (n = 105)	Mindfulness meditation (n = 12)	Meditation practices (ND) (n = 1)	Miscellaneous meditation practices (n = 2)	Qi Gong (n = 7)	Tai Chi (n = 4)	Yoga (n = 18)
Selection of the cohorts	Truly representative of the community; n (%)	10 (9.5)	2	...	...	...	...	...
	Somewhat representative of the community; n (%)	29 (27.6)	4	...	1	1	2	6
	Selected group of participants; n (%)	62 (59.0)	6	1	1	5	2	11
	No description of the derivation of the cohort; n (%)	4 (3.8)	...	...	...	1	...	1
	Drawn from the same community as the exposed cohort; n (%)	38 (36.2)	5	...	1	3	3	6
	Drawn from a different source; n (%)	56 (53.3)	7	...	1	4	1	10
	No description of the derivation of the nonexposed cohort	11 (10.5)	...	1	...	...	...	2
	Secure record; n (%)	7 (6.7)	1	...	...	...	...	2
	Structured interview; n (%)	...	2	...	...	...	1	1
	Written self-report; n (%)	11 (10.5)	6	...	...	1	2	1
	No description of exposure ascertainment; n (%)	87 (82.9)	3	1	2	6	1	14
	Demonstration that the outcome(s) of interest was not present at the start of the study; n (%)	7 (6.7)	1	1	...	...	...	1
Comparability	Study controls for two or more, confounding factors; n (%)	32 (30.5)	3	...	1	1	2	9
	Study controls for at least one confounding factor; n (%)	39 (37.1)	2	...	...	3	1	6
	No adjustment for confounding factors in the design or analysis of the study; n (%)	34 (32.4)	7	1	1	3	1	3
Outcome assessment	Independent blind assessment; n (%)	16 (15.2)	1	...	...	2	1	4
	Record linkage; n (%)	64 (61.0)	5	...	1	4	3	8
	Self-report; n (%)	23 (21.9)	6	1	1	1	-	6
	No description of outcomes assessment; n (%)	2 (1.9)	...	...	...	...	...	...
	Followup enough for outcomes to occur; n (%)	27 (25.7)	5	1	...	1	2	8
	Complete followup (all subjects accounted for); n (%)	7 (6.7)	2	1	...	1	1	5
	Subjects lost to followup unlikely to introduce bias; n (%)	9 (8.6)	2	...	...	...	1	...
	Lost to followup likely to introduce bias; n (%)	6 (5.7)	1	...	...	...	...	1
	No description of losses to followup; n (%)	83 (79.0)	6	...	2	6	2	12
NOS total score (max 9); Median (IQR)		3 (2,4)	3 (2,4)	3	3 (1,3)	2 (2,4)	2 (2,4)	3.5 (2.5,5)
Funding reported; n (%)		22 (21.0)	4	...	1	4	3	7

*

Percentages are reported for N ≥ 20 only

ND = not described; NOS = Newcastle-Ottawa Scale

Table 15

Methodological quality of cross-sectional studies by (more...)

Table 15

Methodological quality of cross-sectional studies by meditation practice*

Quality criteria		Mantra meditation (n = 60)	Mindfulness meditation (n = 12)	Meditation practices (ND) (n = 1)	Miscellaneous meditation practices (n = 3)	Qi Gong (n = 8)	Tai Chi (n = 18)	Yoga (n = 15)
Selection of the comparison groups	Truly representative of the community; n (%)	1 (1.7)	...	...	...	...	...	...
	Somewhat representative of the community; n (%)	27 (45.0)	6*	1	3	6	13	5
	Selected group of participants; n (%)	6 (10.0)	2	...	...	2	2	2
	No description of the derivation of the study group; n (%)	26 (46.3)	4	...	...	...	3	8
	Drawn from the same community as the study group; n (%)	14 (23.3)	2	1	2	1	2	2
	Drawn from a different source; n (%)	25 (41.7)	6	...	1	6	13	5
	No description of the derivation of the comparison group; n (%)	21 (35.0)	4	...	...	1	3	8
	Secure record; n (%)	...	...	...	...	...	...	...
	Structured interview; n (%)	...	...	...	...	...	...	...
	Written self-report; n (%)	1 (1.7)	1	...	...	...	...	...
	No description of exposure ascertainment; n (%)	59 (98.3)	11	1	3	8	18	15
Comparability	Study controls for two or more confounding factors; n (%)	23 (38.3)	6	...	2	3	8	8
	Study controls for at least one confounding factor; n (%)	7 (11.7)	2	...	...	3	1	1
	No adjustment for confounding factors in the design or analysis of the study; n (%)	30 (50)	4	1	1	2	9	6
Outcome assessment	Independent blind assessment; n (%)	...	...	...	...	...	...	...
	Record linkage; n (%)	28 (46.7)	6	...	...	4	16	8
	Self-report; n (%)	29 (48.3)	6	1	3	4	2	7
	No description of outcomes assessment; n (%)	3 (5.0)	...	...	...	...	...	...
NOS total score (max 9); Median (IQR)		2 (1,3)	3 (1,3)	2	2 (2,3)	2.5 (2,3)	2 (2,4)	3 (1,3)
Funding reported; n (%)		6 (10)	3	...	3	4	11	3

Meditation Practices Separated by the Diseases, Conditions, and Populations for Which They Have Been Examined

Eight hundred and thirteen studies contributed to the description of the diseases, conditions, and populations for which meditation practices have been examined.

Overall, 69 percent (n = 564) of the studies included healthy participants only, whereas 30 percent (n = 244) reported on clinical populations. Five studies (0.6 percent) included both healthy and clinical participants in the study populations. Overall, the median number of participants per study was 40 (IQR, 23 to 71), with a median age of 37 years (IQR, 26 to 50; n = 536). Both male and females were equally represented in the studies (median number of males per study, 19; IQR, 10 to 36; median number of females per study, 19; IQR, 7 to 39).

Table 21

Types of populations and conditions included in studies (more...)

Table 21

Types of populations and conditions included in studies on meditation

Category of interest	Study condition	Intervention studies	Observational analytical studies	Total	Total studies per category
Circulatory and cardiovascular	Hypertension	35	2	37	61
	Other cardiovascular diseases	24	...	24
Dental	Dental problems (NS)	1	...	1	2
	Periodontitis	...	1	1
Dermatology	Psoriasis	3	...	3	3
Endocrine	Obesity	1	...	1	11
	Type II diabetes mellitus	10	...	10
Gastrointestinal	Gastrointestinal disorders	1	...	1	3
	Irritable bowel syndrome	2	...	2
Gynecology	Infertility	1	...	1	10
	Menopause	2	...	2
	Postmenopause	1	3	4
	Pregnancy	1	1	2
	Premenstrual syndrome	1	...	1
Healthy	College and university students	123	65	189	553
	Elderly	34	26	60
	Healthy volunteers	90	160	250
	Army and military	8	...	8
	Prison inmates	7	3	10
	Workers	25	3	28
	Athletes	6	...	6
	Smokers	3	...	3
Immunologic	HIV	3	...	3	3
Sleep disorders	Insomnia	2	...	2	5
	Chronic insomnia	3	...	3
Mental health	Anger management	1	...	1	66
disorders	Anxiety disorders	14	...	14
	Binge eating disorder	3	...	3
	Burnout	1	...	1
	Depression	11	...	11
	Miscellaneous psychiatric conditions	6	1	7
	Mood disorders	3	...	3
	Neurosis	1	...	1
	Obsessive-compulsive disorder	1	...	1
	Parents of children with behavior problems	1	...	1
	Personality disorders	1	...	1
	Postraumatic stress disorders	1	...	1
	Psychosis	1	...	1
	Schizophrenia	1	...	1
	Schizophrenia AND antisocial personality disorders	1	...	1
	Substance abuse	18	...	18
Miscellaneous medical conditions	Heterogeneous patient population	10	...	10	11
	Chronic fatigue	1	...	1
Musculoskeletal	Balance disorders	1	...	1	42
	Carpal tunnel syndrome	1	...	1
	Multiple sclerosis	2	...	2
	Muscular dystrophy	1	...	1
	Chronic pain	10	1	11
	Chronic rheumatic diseases	1	...	1
	Fibromyalgia	10	...	10
	Regional pain syndrome	1	...	1
	Rheumatoid arthritis	6	...	6
	Hyperkyphosis	1	...	1
	Osteoarthritis	4	...	4
	Osteoporosis	1	...	1
	Postpolio syndrome	1	...	1
	Total hip and knee replacement	1	...	1
Neurological	Developmental disabilities	1	...	1	10
	Epilepsy	2	...	2
	Migraine and tension headaches	3	...	3
	Stroke	2	...	2
	Traumatic brain injuries	2	...	2
Oncology	Cancer	12	...	12	12
Organ transplant	Organ transplantation	1	...	1	1
Renal	End-stage renal disease	1	...	1	1
Respiratory and pulmonary	Asthma	11	...	11	16
	COPD	1	...	1
	Chronic airways obstruction	1	...	1
	Chronic bronchitis	1	...	1
	Pleural effusion	1	...	1
	Pulmonary tuberculosis	1	...	1
Vestibular	Tinnitus	2	...	2	3
	Vestibulopathy	1	...	1
Total		547	266	813	813

COPD = chronic obstructive pulmonary disease; HIV = human immunodeficiency virus; NS = not specified

In general, the majority of studies (68 percent) on meditation practices have been conducted in healthy populations such as college and university students, healthy elderly participants from the community, army and military personnel, prison inmates, workers, athletes, and smokers (553 studies comprising 196 intervention studies and 257 observational analytical studies). Individuals with mental health disorders constituted the second most studied population (and the most frequently studied category of clinical conditions) examined in studies on meditation practices (66 studies: 65 intervention studies, and 1 observational analytical study). Mental health conditions included substance abuse, anxiety disorders, depression, and binge eating disorders, among others.

People with cardiovascular and circulatory conditions were the third most studied population and the second most frequently studied clinical condition (61 studies comprising 59 intervention studies and 2 observational analytical studies). There were 37 studies on hypertensive participants (35 intervention studies and 2 observational analytical studies). Cardiovascular conditions (24 intervention studies) included hypertension and a group of heterogeneous cardiovascular diseases (diseases of the circulatory system—the heart, the blood vessels of the heart, and the veins and arteries throughout the body and within the brain) such as coronary artery disease, chronic heart failure, ischemic heart disease, and myocardial infarction.

Forty-two studies on meditation practices (41 intervention studies and 1 observational analytical study) have been conducted in musculoskeletal conditions including chronic pain, fibromyalgia, rheumatoid arthritis, and osteoarthritis. Respiratory conditions (e.g., asthma and chronic obstructive pulmonary disease) have been examined in 16 intervention studies. Twelve intervention studies in oncology have been conducted using different types of cancer populations, such as breast, prostate, skin and lymphoma. Endocrine diseases such as type II diabetes mellitus (DM) and obesity conditions have been examined in 11 intervention studies on meditation practices. Heterogeneous patient populations with a variety of medical conditions not specified have been examined in 11 intervention studies.

Gynecological conditions such as postmenopause, menopause, premenstrual syndrome, pregnancy, and infertility have been examined in 10 intervention studies. Populations with gastrointestinal disorders have been examined in three intervention studies. Three intervention studies have examined the effect of meditation practices in dermatological disorders, such as psoriasis, and on vestibular problems, such as tinnitus. Finally, patients with dental problems (one intervention study, one observational study), end-stage renal disease (one intervention study), and organ transplants (one intervention study) have been used as study populations for studies on meditation practices.

After excluding healthy populations, the distribution of conditions or disorders for which meditation practices have been examined was

• 1

  hypertension (35 intervention studies and 2 observational analytical studies);

• 2

  other cardiovascular diseases (24 intervention studies);

• 3

  substance abuse disorders (18 intervention studies);

• 4

  anxiety disorders (14 intervention studies);

• 5

  cancer (12 intervention studies);

• 6

  asthma (11 intervention studies);

• 7

  chronic pain (10 intervention studies and 1 observational analytical study);

• 8

  type II DM (10 intervention studies);

• 9

  fibromyalgia (10 intervention studies); and

• 10

  miscellaneous psychiatric conditions (six intervention studies and one observational analytical study)

Mantra meditation. Among the intervention studies on TM^®, the majority (72 percent, 57/80) have been conducted in healthy populations (college and university students [24 studies], healthy volunteers from the community [19 studies], prison inmates [4 studies], elderly [3 studies], smokers [2 studies], and athletes [1 study]). The second largest group of TM^® studies examined its effects on mental health disorders (nine studies) such as substance abuse (five studies), anxiety disorders (two studies), posttraumatic stress disorder (one study), and other miscellaneous psychiatric conditions (one study). Participants with circulatory or cardiovascular diseases such as hypertension (9 studies) and coronary artery disease (1 study) have been included in 10 studies on TM^®. Other conditions such as asthma (two studies), chronic insomnia (one study), and a miscellaneous group of cancer patients (one study) have also been included in intervention studies on TM^®.

The vast majority of observational analytical studies on TM^® (98 percent, 148/151) have been conducted in healthy populations (healthy volunteers from the community [91 studies], college and university students [48 studies], prison inmates [3 studies], and workers [1 study]). Conditions such as pregnancy (one study), postmenopause (one study), and dental problems (e.g., periodontitis, one study) have been also examined.

Intervention studies on RR have included mainly healthy populations (31 studies), in addition to circulatory and cardiovascular conditions (hypertension [4 studies], other cardiovascular conditions [5 studies] including chronic heart failure, congestive heart failure, ischemic heart disease, premature ventricular contractions, and peripheral vascular disease), mental health disorders (substance abuse [2 studies], anxiety disorders [1 study], schizophrenia or antisocial personality disorders [1 study]), gynecological conditions (menopause [1 study], premenstrual syndrome [1 study]), and other clinical conditions such as irritable bowel syndrome (1 study), total knee replacement (1 study), skin cancer (1 study), and a group of patients with heterogeneous clinical conditions (1 study). The only observational analytical study on RR has been conducted in a population of hypertensive patients

Nineteen intervention studies on mantra meditation techniques not further described have been conducted with healthy populations. Other populations included people with mental health disorders (anxiety disorders [three studies], substance abuse [two studies], miscellaneous psychiatric conditions [one study]), hypertension (one study), and epilepsy (one study). The six observational analytical studies conducted on mantra techniques not further described have included healthy volunteers from the community

Seven intervention studies on CSM have been conducted on healthy populations, three on mental disorders such as anxiety disorders (one study), schizophrenia (one study), and substance abuse (one study), and another study on chronic insomnia

All the intervention and observational analytical studies on Acem meditation, Ananda Marga, Cayce's meditation, and Rosary prayer have been conducted with healthy populations

Yoga. Among the intervention studies on Yoga, more than half (80/158) have been conducted with healthy populations (healthy volunteers from the community [34 studies], college and university students [26 studies], army and military personnel [7 studies], workers [5 studies], prison inmates [4 studies], and athletes [1 study]). The second largest group of conditions studied is constituted by circulatory and cardiovascular diseases (21 studies) such as hypertension (13 studies), and other cardiovascular conditions (8 studies). Studies on Yoga have also included participants with mental health disorders (16 studies) such as depression (7 studies), anxiety disorders (3 studies), substance abuse (3 studies), other miscellaneous psychiatric conditions (2 studies), and obsessive-compulsive disorders (1 study). Respiratory and pulmonary conditions such as asthma (nine studies), chronic airways obstruction, chronic bronchitis, pleural effusion, and pulmonary tuberculosis (one study each) have been also examined. Participants with musculoskeletal conditions such as chronic pain, rheumatoid arthritis (two studies each), carpal tunnel syndrome, chronic rheumatic diseases, fibromyalgia, hyperkyphosis, multiple sclerosis, osteoarthritis, and postpolio syndrome (one study each) have been included in intervention studies on Yoga. Other conditions examined in Yoga studies were gastrointestinal disorders (two studies), epilepsy, migraine, pregnancy, human immunodeficiency virus (HIV), lymphoma, chronic insomnia, tinnitus, and heterogeneous patient populations (one study each). All the observational analytic studies on Yoga (33 studies) have been conducted with healthy populations

Mindfulness meditation. Among the 49 intervention studies on MBSR, 12 were conducted with healthy populations and 12 with populations with mental health disorders. Mental health disorders included anxiety disorders (three studies), mood disorders (two studies), substance abuse (two studies), binge eating disorders, burnout, personality disorders, miscellaneous psychiatric conditions, and stress-related conditions of parents of children with behavioral problems (one study each)

Participants with musculoskeletal conditions such as chronic pain (four studies) and fibromyalgia (two studies) have been also included. Cancer patients have been included in four intervention studies on MBSR. Other conditions such as psoriasis (two studies), cardiovascular diseases (two studies), traumatic brain injuries (two studies), obesity, HIV, and organ transplantation (one study each) have also been included. No observational analytic studies on MBSR were identified

Eleven intervention studies on mindfulness meditation not further specified have been conducted in healthy populations. Other populations included mental health disorders (binge eating disorders [two studies], anxiety disorders, psychosis, substance abuse [one study each])

Musculoskeletal conditions such as fibromyalgia (three studies) and chronic pain (two studies), cardiovascular diseases, cancer (three studies each), psoriasis, infertility, and heterogeneous patient populations (one study each) have been included also. The majority of observational analytical studies on mindfulness meditation techniques not further specified (six studies) have been conducted in healthy populations, with only one observational study conducted in a clinical population (individuals with chronic pain)

The majority of intervention studies (73 percent) on Zen Buddhist meditation have been conducted on healthy participants (11 studies). Clinical conditions that have been studied in intervention studies include hypertension (two studies), coronary artery disease, and insomnia (one study each). All the observational analytical studies conducted on Zen Buddhist meditation (13 studies) have included healthy volunteers

Three intervention studies on MBCT have included patients with a depressive disorder. Other populations that have been examined are individuals with fibromyalgia, stroke, tinnitus, and healthy workers (one study each). No observational studies on MBCT were identified.

Intervention studies on Vipassana meditation have involved healthy populations from the community and patients with migraine or tension headaches (one study each). The observational analytical studies conducted on Vipassana meditation (four studies) have employed healthy populations from the community (two studies), college and university students, and elderly individuals (one study each)

Tai Chi. Intervention studies on Tai Chi have mainly assessed healthy populations (38 studies), particularly the elderly (25 studies). Clinical conditions examined in intervention studies of Tai Chi include musculoskeletal conditions such as rheumatoid arthritis (four studies), osteoarthritis (three studies), chronic pain (two studies), balance disorders, fibromyalgia, multiple sclerosis, and osteoporosis (one study each). Circulatory and cardiovascular conditions have been examined in four studies. Other populations examined in studies on Tai Chi are menopause, postmenopause, depression, miscellaneous psychiatric conditions, developmental disabilities, stroke, type II DM, HIV, breast cancer, end-stage renal disease, and vestibulopathy (one study each). The majority (91 percent, 20/22) of the observational analytical studies conducted in Tai Chi have examined groups of healthy, elderly individuals or other healthy individuals from the community. Two observational studies have been conducted in groups of postmenopausal women

Qi Gong. Intervention studies on Qi Gong have examined populations of healthy participants (seven studies), patients with circulatory and cardiovascular disorders (hypertension [four studies], coronary artery disease [one study]), musculoskeletal conditions (fibromyalgia [two studies], muscular dystrophy and regional pain syndrome [one study each]), type II DM, substance abuse, miscellaneous medical conditions, migraine, and chronic obstructive pulmonary disease (COPD) (one study each). Almost all the observational analytical Qi Gong studies (14/15) were conducted with healthy populations; one was conducted with hypertensives

Meditation practices (ND).Among the 19 intervention studies that failed to describe the meditation practice under study, 12 examined healthy college and university students (nine studies), workers (2 studies), and healthy volunteers from the community (one study). Intervention studies on clinical conditions included patients with hypertension, dental problems, and insomnia (one study each). Two observational studies included respectively, healthy college and university students and individuals with miscellaneous psychiatric conditions

Miscellaneous meditation practices. Five of the six intervention studies that combined different meditation practices were conducted in healthy populations (three studies), miscellaneous psychiatric conditions, and heterogeneous populations of patients (one study each). One intervention study was conducted in patients with breast cancer. All five observational studies on miscellaneous meditation practices examined healthy populations

Table 22

Intervention studies conducted on meditation practices (more...)

Table 22

Intervention studies conducted on meditation practices by populations examined*

Category	Population	Mantra meditation (N)	Mindfulness meditation (N)	Meditation practices (ND) (N)	Miscellaneous meditation practices (N)	Qi Gong (N)	Tai Chi (N)	Yoga (N)	Total (N)	Studies per category (N)
Circulatory and cardiovascular	Hypertension	14*	2	1	...	4	1	13	33	59
	Other cardiovascular diseases	6	6	...	...	1	3	8	24
Dental	Dental problems (NS)	...	...	1	...	...	...	...	1	1
Dermatology	Psoriasis	...	3	...	...	...	...	...	3	3
Endocrine	Obesity	...	1	...	...	...	...	...	1	11
	Type II diabetes mellitus	...	...	...	...	2	1	7	10
Gastrointestinal	Gastrointestinal disorders	...	...	...	...	...	...	1	1	3
	Irritable bowel syndrome	1	...	...	...	...	...	1	2
Gynecology	Infertility	...	1	...	...	...	...	...	1	6
	Menopause	1	...	...	...	...	1	...	2
	Postmenopause	...	...	...	...	...	1	...	1
	Pregnancy	...	...	...	...	...	...	1	1
	Premenstrual syndrome	1	...	...	...	...	...	...	1
Healthy	College and university students	56	23	9	2	2	4	27	124	296
	Elderly	3	...	...	...	1	25	5	34
	Healthy volunteers	36	6	1	1	4	8	34	90
	Army and military	1	...	...	...	...	...	7	8
	Prison inmates	5	...	...	...	...	...	2	7
	Workers	12	5	2	...	...	1	5	25
	Athletes	4	1	...	...	...	...	1	6
	Smokers	2	1	...	...	...	...	...	3
Immunologic	HIV	...	1	...	...	...	1	1	3	3
Sleep disorders	Insomnia	...	1	1	...	...	...	...	2	5
	Chronic insomnia	2	...	...	...	...	...	1	3
Mental health disorders	Anger management	...	...	1	...	...	...	...	1	65
	Anxiety disorders	7	4	...	...	...	...	3	14
	Binge eating disorder	...	3	...	...	...	...	...	3
	Burnout	...	1	...	...	...	...	...	1
	Depression	...	3	...	...	...	1	7	11
	Miscellaneous psychiatric conditions	2	1	...	1	...	1	1	6
	Mood disorders	...	2	1	...	...	...	...	3
	Neurosis	...	...	...	...	...	...	1	1
	Obsessive-compulsive disorder	...	...	...	...	...	...	1	1
	Parents of children with behavior problems	...	1	...	...	...	...	...	1
	Personality disorders	...	1	...	...	...	...	...	1
	Postraumatic stress disorders	1	...	...	...	...	...	...	1
	Psychosis	...	1	...	...	...	...	...	1
	Schizophrenia	1	...	...	...	...	...	...	1
	Schizophrenia and antisocial personality disorders	1	...	...	...	...	...	...	1
	Substance abuse	9	3	2	...	1	...	3	18
Miscellaneous medical conditions	Heterogeneous patient population	1	6	...	1	1	...	1	10	11
	Chronic fatigue	...	1	...	...	...	...	...	1
Musculoskeletal	Balance disorders	...	...	...	...	...	1	...	1	41
	Carpal tunnel syndrome	...	...	...	...		...	1	1
	Multiple sclerosis	...	...	...	...	...	1	1	2
	Muscular dystrophy	...	...	...	...	1	...	...	1
	Chronic pain	...	6	...	...	2	2	10
	Chronic rheumatic diseases	...	...	...	...	...	...	1	1
	Fibromyalgia	...	6	...	...	2	1	1	10
	Regional pain syndrome	...	...	...	...	1	...	...	1
	Rheumatoid arthritis	...	...	...	...	...	4	2	6
	Hyperkyphosis	...	...	...	...	...	-	1	1
	Osteoarthritis	...	...	...	...	...	3	1	4
	Osteoporosis	...	...	...	...	...	1	...	1
	Postpolio syndrome	...	...	...	...	...	...	1	1
	Total hip and knee replacement	1	...	...	...	...	...	...	1
Neurological	Developmental disabilities	...	...	...	...	...	1	...	1	10
	Epilepsy	1	...	...	...	...	...	1	2
	Migraine and tension headaches	...	1	...	...	1	...	1	3
	Stroke	...	1	...	...	...	1	...	2
	Traumatic brain injuries	...	2	...	...	...	...	...	2
Oncology	Cancer	2	7	...	1	...	1	1	12	12
Organ transplant	Organ transplantation	...	1	...	...	...	...	...	1	1
Renal	End-stage renal disease	...	...	...	...	...	1	...	1	1
Respiratory and pulmonary	Asthma	2	...	...	...	...	...	9	11	16
	COPD	...	...	...	...	1	...	...	1
	Chronic airways obstruction	...	...	...	...	...	...	1	1
	Chronic bronchitis	...	...	...	...	...	...	1	1
	Pleural effusion	...	...	...	...	...	...	1	1
	Pulmonary tuberculosis	...	...	...	...	...	...	1	1
Vestibular	Tinnitus	...	1	...	...	...	...	1	2	3
	Vestibulopathy	...	...	...	...	...	1	...	1
Total		172	103	19	6	22	66	159	548	547

*

Only conditions for which studies were available

COPD = chronic obstructive pulmonary disease; HIV = human immunodeficiency virus; ND = not described; NS = not specified

Table 23

Observational analytical studies conducted on meditation (more...)

Table 23

Observational analytical studies conducted on meditation practices by populations examined*

Category	Population	Mantra meditation (N)	Mindfulness meditation (N)	Meditation practices (ND) (N)	Miscellaneous meditation practices (N)	Qi Gong (N)	Tai Chi (N)	Yoga (N)	Total (N)	Studies per category (N)
Circulatory and cardiovascular	Hypertension	1	...	...	...	1	...	...	2	2
Dental	Periodontitis	1	...	...	...	...	...	...	1	1
Gynecology	Postmenopause	1	...	...	...	...	2	...	3	4
	Pregnancy	1	...	...	...	...	...	...	1
Healthy	College and university students	48	6	1	1	2	...	7	65	257
	Elderly	5	1	...	...	1	18	1	26
	Healthy volunteers	104	16	...	4	11	2	23	160
	Prison inmates	3	...	...	...	...	...	...	3
	Workers	1	...	...	...	...	...	2	3
Mental health disorders	Miscellaneous psychiatric conditions	...	...	1	...	...	...	...	1	1
Musculoskeletal	Chronic pain	...	1	...	...	...	...	...	1	1
Total		165	24	2	5	15	22	33	266	266

*

Only conditions for which studies were available

ND = not described; NS = not specified