Section II: Outcome Measures in Obesity: The Evidence Adiposity and Fat Distribution Outcome Measures: Assessment and Clinical Implications Louis J. Aronne and Karen R. Segal Introduction Obesity is the excessive accumulation of adipose tissue to an extent that health is impaired. Obesity is usually deter- mined using body mass index (BMI). A BMI 30 kg/m 2 correlates strongly with obesity-related comorbid conditions and mortality. In 2000, nearly 39 million American adults met the criterion for obesity; namely, a BMI 30 kg/m 2 . Nearly 61% of the U.S. adult population has a BMI 25 kg/m 2 . Thus, overweight and obesity has reached epidemic proportions in the United States, as well as worldwide. Obesity is associated with significantly increased risk of diabetes mellitus, hypertension, dyslipidemia, certain forms of cancer, sleep apnea, and osteoarthritis. In addition, the increas- ing prevalence of obesity and its associated complications places a tremendous burden on health care use and costs. This epidemic of obesity mandates prompt attention from the health care and preventive health services to minimize the rise in the incidence of new cases of diabetes, heart disease, and other obesity-related complications. The National Heart, Lung, and Blood Institute Obesity Education Initiative Expert Panel re- cently developed guidelines for identification, assessment, and treatment of obesity (1). The Clinical Guidelines recommend measurement of BMI and waist circumference as “vital signs” for evaluating the obese patient. This paper reviews these recommended assessment tools including the rationale and evidence supporting their use in evaluating patients, relation- ships to health-related variables, and concomitant changes brought about by intentional weight loss. BMI: Relationships to Health and Disease BMI is a measure of the degree of overweight, in relation to the individuals’ height, but not a surrogate measure of metabolic body size, such as fat-free mass or body surface area. The National Institutes of Health Clinical Guidelines on the Identification, Evaluation, and Treatment of Over- weight and Obesity in Adults define normal weight as a BMI of 18.5 to 24.9 kg/m 2 , overweight as a BMI of 25 to 29.9 kg/m 2 , and obesity as a BMI of at least 30 kg/m 2 (1). In 1998, roughly 97 million American adults met the criterion for overweight or obese, defined as BMI 25 kg/m 2 (1). Data collected by the National Center for Health Statistics indicate that the prevalence of obesity, defined as a BMI 30 kg/m 2 , has increased from 12.8% between 1976 and 1980 to 22.5% from 1988 to 1994 (2). More recent information from a preliminary analysis of 1999 National Health and Nutrition Examination Survey (NHANES) data indicates that the prev- alence of overweight has increased to 34.0% and the preva- lence of obesity has risen to 27.0% (3). Thus, in the United States, 110 million adults are overweight or obese (3). Defining overweight and obesity according to cut-points in BMI is consistent with World Health Organization guide- lines (4), and thus provides a systematic and consistent way to evaluate the worldwide obesity epidemic. It is estimated that worldwide there are roughly 250 million obese adults and 500 million overweight adults. The use of BMI for the classification of weight status is based on epidemiological associations of BMI with morbid- ity and mortality (5). The classification of normal weight, overweight, and obese provides a mechanism to identify those individuals who are at increased risk of being affected (either presently or in the future) by obesity-related health problems. The rationale for the use of the BMI is the demonstrated strong relationship to risk of hypertension, diabetes, dyslipidemia, and overall mortality. As a clinical tool, BMI is simple to calculate, inexpensive, and a well- defined criterion for diagnosis of the severity of obesity. BMI is not a measure of body composition, although it is correlated with body fatness. Recently, Gallagher et al. (6) explored the relationship between BMI and percent fat and developed prediction equations for estimating total body fat- ness from its relationship to BMI. The relationship between BMI and body fat is age- and sex-specific but seems to be consistent across ethnic groups. The limitations of the BMI as an assessment tool are that it does not distinguish between lean and fat tissue and thus could lead to misclassification of over- weight among muscular individuals, and it does not delineate the pattern of fat distribution, which has important health implications (see below). Weill Medical College of Cornell University, New York, New York. Address correspondence to Louis J. Aronne, M.D., Weill Medical College of Cornell University, 1165 York Avenue, New York, NY 10021. E-mail: ljaronne@med.cornell.edu Copyright © 2002 NAASO 14S OBESITY RESEARCH Vol. 10 Suppl. 1 November 2002