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Chung M, Balk EM, Ip S, et al. Reporting of Systematic Reviews of Micronutrients and Health: A Critical Appraisal: Nutrition Research Series, Vol. 3. Rockville (MD): Agency for Healthcare Research and Quality (US); 2009 Mar. (Technical Reviews, No. 17.3.)

Cover of Reporting of Systematic Reviews of Micronutrients and Health: A Critical Appraisal

Reporting of Systematic Reviews of Micronutrients and Health: A Critical Appraisal: Nutrition Research Series, Vol. 3.

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2Methods

Literature Search

We searched MEDLINE® from its inception through July 2007 using keywords for micronutrients, multivitamins, and antioxidants. We also searched for SRs, evidence-based reviews and meta-analyses (Supplementary Table). Citations of SRs were reviewed for additional relevant articles. The essential micronutrients included in the analysis were fat-soluble vitamins A, D, E, and K; water-soluble vitamins B (thiamin, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folate, B12) and C; macrominerals (calcium chloride, magnesium, phosphorous, potassium, sodium and sulfur); and trace minerals (chromium, copper, fluoride, iodide, iron, manganese, molybdenum, selenium, and zinc). Multivitamins or minerals and antioxidant supplements were also included. Potentially relevant reviews included those whose abstracts described searches or eligibility criteria for study identification, or included terms such as “systematic,” “evidence,” “evidence-based,” “meta-analysis,” or “pooled analysis.”

Eligibility Criteria

Full-text articles of screened-in abstracts were retrieved and examined to confirm their eligibility according to predetermined criteria. For the purpose of this study, we defined a SR as a study that contained three components: a statement of the research questions (aims or objectives); a description of the literature search; and a listing of the study eligibility criteria. A review that lacked any of these components was excluded. We did not attempt to contact authors for clarification. The following types of reviews were excluded: reviews of foods or diets that did not quantify micronutrient intake, reviews including non-oral routes of nutrient delivery, reviews that did not relate nutrients to health outcomes, reviews of non-human data, and pooled analyses of primary databases (i.e., secondary database analyses of multiple cohorts) that did not include a SR.

Data Abstraction and Collection

The unit of analysis was the SR article. We did not analyze the primary studies within the SRs. The following data were collected from the full-text articles of eligible SRs: topics covered (exposures and outcomes), whether meta-analyses were performed, specific journal, publication date, and number of citations per SR. We categorized the outcomes examined as either clinical outcomes or intermediate outcomes. A clinical outcome was defined as a measurement of how a person feels, functions or survives, or the severity of an existing disease, or the incidence of a new diagnosis. Intermediate disease outcomes included laboratory measurements or physical signs used as surrogates for a clinical endpoint (e.g., plasma cholesterol concentrations or blood pressure for cardiovascular disease, or dark adaptation for night blindness).

A standardized form was used for data collection. From published guidelines for reporting of the meta-analyses such as MOOSE (9) and QUOROM (10), we collected and evaluated 28 reporting items regarding the search and study selection criteria; methods for assessing methodological quality of the included primary studies, methods for quantitative syntheses, and protocols for reporting of results. The primary goal of guidelines for SR reporting is to encourage authors to provide clear and transparent reporting of the factors relating to the literature review and evidence syntheses they carried out. Most widely recognized reporting guidelines reflect consensus opinion of groups of experts in a particular field, including research methodologists and journal editors (13). Because there is no widely accepted guidance for reporting or analyses of variables unique to the field of nutrition in SRs, we included seven items in addition to those identified in MOOSE and QUOROM specific to nutrition or diet variables based on the concern that failure to adhere to the items could lead to biased syntheses and/or interpretation of results in nutrition-related SRs. The definitions and the reasons for selecting these 35 reporting items are described in Table 1.

Table 1. Reporting items for nutrition-related systematic reviews (with or without meta-analyses).

Table 1

Reporting items for nutrition-related systematic reviews (with or without meta-analyses).

Additional data elements collected included the number of primary studies, instruments or methods used to assess the quality of the primary studies, and the types of primary studies (interventional or observational studies). An interventional study was defined as a study with an active intervention, such as randomized or non-randomized controlled trials, crossover trials, quasi-interventional studies (or community trials), and before-and-after studies. Observational studies included cohort, case-control, cross-sectional and ecological studies, case series and case reports, where the intervention was not dictated by the investigator.

For each SR, we also collected citation counts of the SRs and impact factors of the journals that published these reviews from the Science Citation Index and the Institute for Scientific Information Journal Citation Reports® edition 2006. The impact factor of a journal is calculated based on a three-year period, and can be considered to be the average number of times articles published in the journal are cited up to two years after publication. The citation count is the number of times an article was cited by other articles published in journals indexed in Journal Citation Reports®. Citation counts were collected in February 2008. The mean yearly citation number for each SR was calculated [citation count of SR / (2008-publication year of SR)].

Statistical Analyses

Descriptive analyses and summary statistics were performed on the reporting characteristics of SRs, including whether the reporting followed published standards such as MOOSE (9) and QUOROM (10), reporting of nutrition variables, number and types of primary studies analyzed, whether quality assessment of primary studies were performed, and what instruments were used to assess quality or susceptibility to biases. Fishers’ exact test was used to examine differences in the proportion of SRs reporting each item, and comparing the SRs that included observational studies to those that included interventional studies.

We used the Mann-Whitney test to examine the differences in the proportion of reporting criteria met by SRs of different study types (interventional studies, observational studies, or both designs), before versus 3 years after publication of QUOROM and MOOSE (in 1999), and SRs with versus without meta-analyses. Correlation analysis was conducted to examine the association between journal impact factors and citation numbers and the proportion of reporting criteria met among SRs. The maximal number of reporting criteria is 29 (26 SR-reporting factors and 3 nutrition variables) for SRs of interventional studies alone, 30 (26+4) for SRs of observational studies alone, and 33 (26+7) for SRs of both designs. Two reporting items for SRs containing meta-analyses (reporting of models for meta-analyses and data needed to calculate the effect size) were excluded from these calculations.

Median and interquartile range (IQR) are reported when the distributions were skewed. All P-values are two-tailed and considered significant when P<0.05.

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