A. Rationale for Guidelines Development
An estimated 97 million adults in the United States are overweight or obese , a condition that substantially raises their risk of morbidity from hypertension [2–6], type 2 diabetes [7–10], stroke [11–13], gallbladder disease [14, 15], osteoarthritis [16–18], sleep apnea and respiratory problems [19–21], and endometrial, breast, prostate, and colon cancers [22–24]. As a major contributor to preventive death in the United States today , overweight and obesity pose a major public health challenge. Not only is the prevalence of this serious medical condition soaring among adults (between 1960 and 1994, overweight increased from 30.5 to 32 percent among adults ages 20 to 74 and obesity increased from 12.8 percent to 22.5 percent), but it is also affecting ever greater numbers of American youth and exacting a particularly harsh toll from low income women and minorities. The Third National Health and Nutrition Examination Survey (NHANES III) estimated that 13.7 percent of children and 11.5 percent of adolescents are overweight, while a number of smaller, ethnic-specific studies suggest that overweight and obesity may afflict up to 30 to 40 percent of children and youth from minority populations [26 , 27].
The prevalence of overweight and obesity in adults in the United States increased markedly during the last decade. According to NHANES III data, 54.9 percent of U.S. adults aged 20 years and older are either overweight or obese; 32.6 percent are overweight, defined as having a body mass index (BMI)* of 25.0 to 29.9 kg/m2; and 22.3 percent are obese with a BMI of ≥ 30 kg/m2 . The panel acknowledges that overweight and obesity are not mutually exclusive; obese persons are also overweight. Since overweight and obesity lead to increased morbidity and mortality, these figures demonstrate the enormity of the public health problem, as well as the clinical problem, of overweight and obesity in this country.
In this report, overweight is defined as a BMI of 25.0 to 29.9 kg/m2 and obesity as a BMI of ≥ 30 kg/m2. The rationale behind these definitions is based on epidemiological data that show increases in mortality with BMIs above 25 kg/m2 [28–32]. The increase in mortality, however, tends to be modest until a BMI of 30 kg/m2 is reached [28, 31, 32]. For persons with a BMI of ≥ 30 kg/m2, mortality rates from all causes, and especially from cardiovascular disease, are generally increased by 50 to 100 percent above that of persons with BMIs in the range of 20 to 25 kg/m2 [28, 31, 32].
Overweight and obesity result from a complex interaction between genes and the environment characterized by long-term energy imbalance due to a sedentary lifestyle, excessive caloric consumption, or both . They develop in a sociocultural environment characterized by mechanization, sedentary lifestyle, and ready access to abundant food. Attempts to prevent overweight and obesity are difficult to both study and achieve. Indeed, few research efforts have investigated either individual or community-based prevention strategies .
A substantial body of research, however, does exist on the health risks of overweight and obesity, and on methods for treatment. This report, which bases its recommendations primarily on published evidence, emphasizes the important role of primary care practitioners in evaluating all overweight and obese adults and promoting weight control through the use of multiple interventions and strategies tailored to particular patient needs. Although the recommendations and guidelines included in this report focus on the clinical assessment and treatment of overweight and obese patients, a second important goal is to encourage primary care practitioners to take an active role in preventing inappropriate weight gain among all their patients.
B. Objectives of the Guidelines
- To identify, evaluate, and summarize published information about the assessment and treatment of overweight and obesity;
- To provide evidence-based guidelines for physicians, other health care practitioners, and health care organizations for the evaluation and treatment of overweight and obesity in adults; and
- To identify areas for future research.
C. Guideline Development Methodology
The National Heart, Lung, and Blood Institute's (NHLBI) Obesity Education Initiative, in cooperation with the National Institute of Diabetes and Digestive and Kidney Diseases, convened the Expert Panel on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. These guidelines address the treatment of overweight and obesity only in adults, but it is the judgment of the panel that guidelines for treating obesity in children are equally important and should be drafted as soon as possible (see Appendix III for information on overweight and obesity in children). The panel's charge was to develop evidence-based clinical guidelines for primary care practitioners; however, the guidelines should also be useful for certain specialists. The decision to develop “evidence-based” guidelines was based on the increased attention being paid to clinical practice guidelines from methodologists, professional associations, third-party payers and policy makers, and the NHLBI's mission to analyze research results with the goal of providing information that may enable health care practitioners to enhance their ability to detect, treat, and prevent disease . In keeping with this approach, the panel
- defined topics to be included in the guidelines;
- developed an evidence model depicting the strategy of inquiry for each area of scientific interest;
- established criteria for searching and abstracting the literature;
- constructed and reviewed evidence tables of individual studies and summary tables of studies falling within a specific category of evidence; and
- identified the level or strength of the evidence that served as the basis for the recommendations.
A complete description of the methodology used to develop the guidelines is included in the report as Appendix I.A.1.
The guidelines are based primarily on a systematic review of the published scientific literature in English found in MEDLINE from January 1980 through September 1997. This was done in the interest of time and economy. This information was supplemented by material provided by the panel and an ancestral search of appropriate references in eligible articles. The literature was searched and systematically reviewed by
- establishing a priori eligibility criteria for inclusion of studies;
- reviewing titles and abstracts to select promising articles;
- reviewing these full articles; and
- compiling evidence tables summarizing those articles that met the inclusion criteria.
As a priority, the panel identified randomized controlled trials (RCTs) as the strongest level of evidence for the evaluation of treatment efficacy. Only RCTs lasting 4 months or more were considered. The only exceptions were a few 3-month trials in the diet and pharmacotherapy sections. With the assistance of the San Antonio Cochrane Center* , 394 publications of RCTs were reviewed for data abstraction. RCT evidence serves as the basis for the many recommendations contained in these guidelines related to treatment efficacy. Instances when the panel had to make recommendations where RCTs were insufficient or absent are clearly indicated in the text. These instances most often pertain to issues of obesity assessment, classification, and measurement where RCT evidence would not be appropriate to answer the question. These issues are best addressed by epidemiological/observational studies of large population groups. In those few cases where the literature could not credibly support a recommendation, panel members relied on clinical experience and knowledge.
The panel recognizes that by relying primarily on only published literature, a publication bias (a positive result is more likely to be published than is a negative result) may exist, so that treatment efficacy may be overstated [36–39]. However, no other reliable sources of information were available.
The targeted population for the guidelines is all overweight and obese adults (18 years of age and older) with a BMI ≥ 25, with particular emphasis on those with cardiovascular risk factors. While the guidelines are appropriate for patients with a BMI ≥ 40, their care is often complicated and may require surgery. These guidelines are not intended for pregnant women. Excluded from the analysis were adults with pharmacologically induced obesity and those with specific genetic syndromes associated with overweight and obesity.
The selection of weight loss interventions to be considered was determined by the literature review. Namely, the panel considered any topic for which articles meeting inclusion criteria were found, including diet, physical activity, behavior therapy, pharmacological therapy, surgery, and combinations of these modalities. No clinical interventions were excluded at the outset. However, the panel did not consider other interventions such as acupuncture or hypnosis, for which no randomized trial articles were available. Clinical interventions to prevent further weight gain in individuals already overweight were also considered relevant.
The panel also evaluated population factors and clinical situations that might potentially influence the physiological, medical, behavioral, or sociocultural context for obesity identification and treatment. Evidence on special populations and situations was provided from RCTs and non-RCTs when available, but in many cases such evidence was meager. Population factors and clinical situations selected for special consideration for obesity classification and treatment were age, gender, race/ethnicity, socioeconomic status, pregnancy, eating disorders, sleep apnea, extreme obesity (BMI ≥ 40), concurrent treatment of other major conditions (such as heart disease or diabetes), and treatment of obesity in conjunction with smoking cessation. When evidence on these special populations or clinical conditions was insufficient to meet standards for inclusion in the main text of the guidelines, relevant issues are identified for the user, and in some cases are cross-referenced to an appendix (see Appendix III) or discussed in sidebar text as a commentary.
The panel recognized the possibility of an advocacy “bias” due to the large number of panel members drawn from organizations with an advocacy role in the treatment of obesity. As a result, it was agreed to obtain formal external reviews of the document from 59 professional societies, consumer groups and goverment agencies representing a wide spectrum of expertise and concern about obesity.
The format for those sections of the report based on the RCT evidence begins with an evidence statement followed by the rationale for that statement. At the end of a series of related evidence statements, a recommendation is given.
Each evidence statement (other than those with no available evidence) and each recommendation is categorized by a level of evidence (A through D) as described below. Statements for which there is no available evidence are so indicated.
Category A: Evidence is from endpoints of well-designed RCTs (or trials that depart only minimally from randomization) that provide a consistent pattern of findings in the population for which the recommendation is made. Category A therefore requires substantial numbers of studies involving substantial numbers of participants.
Category B: Evidence is from endpoints of intervention studies that include only a limited number of RCTs, post-hoc or subgroup analysis of RCTs, or meta-analysis of RCTs. In general, Category B pertains when few randomized trials exist, they are small in size, and the trial results are somewhat inconsistent, or the trials were undertaken in a population that differs from the target population of the recommendation.
Category C: Evidence is from outcomes of uncontrolled or nonrandomized trials or from observation studies.
Category D: Expert judgment is based on the panel's synthesis of evidence from experimental research described in the literature and/or derived from the consensus of panel members based on clinical experience or knowledge that does not meet the above-listed criteria. This category is used only in cases where the provisions of some guidance was deemed valuable but an adequately compelling clinical literature addressing the subject of the recommendation was deemed insufficient to justify placement in one of the other categories (A through C).
In applying these guidelines, the reader should note some caveats:
- The emphasis of these guidelines was to identify effective interventions, not to rank-order them in terms of their efficacy or effectiveness. The panel chose not to emphasize comparisons among interventions because there were few studies that compared long-term outcomes. Also, since individual preferences and circumstances often dictate choice of therapy, the panel wished to present a menu of options rather than a ranked list of choices.
- When no evidence was available on the efficacy of a treatment, the panel usually rendered no opinion. An absence of studies should not be confused with an absence of effect. While clinicians may prefer to use proven therapies rather than untested ones, the lack of testing does not denote that the untested therapy does not work.
- The limitations of RCTs must be kept in mind. The RCT is the primary method for demonstrating efficacy. Often, participants enrolled in RCTs differ from the individuals in a primary care practice, and effectiveness in the community may differ from efficacy as measured in an RCT.
- The potential exists for misinterpretation of clinical trial results. Analysis of endpoints not specified at the outset, or post hoc or subgroup analysis, should be viewed as hypothesis-generating rather than hypothesis-testing.
D. Statement of Assumptions
The panel has made every attempt to base its recommendations on published evidence, with particular attention to RCTs. Data from RCTs provide the strongest evidence regarding the impact of an intervention. The RCT literature predominantly describes short-term outcomes (< 1 year), although there are a small number of published RCTs of intermediate and long-term treatment and maintenance of weight loss. The panel chose to examine RCTs lasting 4 months or longer as their first priority. However, there are some 3-month studies included in the diet and pharmacotherapy sections.
Evidence of beneficial effects of weight reduction on risk factors and on diseases with which obesity is associated, and evidence of the association of obesity and mortality, are also available in the non-RCT epidemiological literature. Therefore, although the treatment recommendations in these guidelines are derived primarily from RCT evidence, they also come in part from the considered judgment of the expert panel members who weighed the non-RCT epidemiological evidence.
In setting forth its recommendations, the panel assumes that, for most individuals, the benefits of weight loss on overall health outweigh the harmful effects, and that weight loss can be maintained in many individuals with resulting long-term health benefits. The recommendations apply to all segments of the adult population, although apparent differences in applicability have been considered and some guidance is provided with respect to certain sectors of the population such as the elderly and in clinical situations. This additional guidance is in the form of notations within the text and is sometimes supplemented by an appendix (Appendix III).
E. Intended Users of These Guidelines
These guidelines were developed primarily for use by physicians and associated health professionals in clinical practice. They should also be useful to managed care organizations or other groups that define benefit plans for patients or handle health care resources. Users of these guidelines are encouraged to note text and appendix references to situations in which weight reduction treatment may be contraindicated or may involve special treatment techniques or safety considerations (e.g., in older adults or in certain sociocultural contexts).
These guidelines also provide a state-of-the-art review of the scientific basis of the relation between obesity and major disease endpoints and of the scientific rationale for the management of the overweight and obese patients. The systematic assessment of the literature contained in this document should be a valuable resource to health care policy makers and clinical investigators.
The BMI is calculated as follows: BMI = weight (kg)/ height squared (m2). Conversion: [weight (pounds)/height (inches) 2] × 703 (1 lb = 0.45 kg) (1.in = 2.54 cm = 0.0254 m). A BMI of 25 is equivalent to 184 lb in a 6'0" person and to 155 lb in one 5'6". A BMI of 30 is equivalent to 221 lb in a 6'0" person and to 186 lb in one 5'6". (The conversion of BMI according to weight for height is provided in Appendix V.)
The San Antonio Cochrane Center is one of 12 centers around the world that comprise the Cochrane Collaboration. The Cochrane Collaboration is an international organization established in 1993 whose mission is to prepare, maintain, and disseminate systematic reviews and meta-analyses of health care interventions.
National Heart, Lung, and Blood Institute, Bethesda (MD)
NHLBI Obesity Education Initiative Expert Panel on the Identification, Evaluation, and Treatment of Obesity in Adults (US). Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report. Bethesda (MD): National Heart, Lung, and Blood Institute; 1998 Sep. Chapter 1, Introduction.