HUMAN-BODY COMPOSITION AND THE EPIDEMIOLOGY OF CHRONIC DISEASE

Obesity and body fat distribution (FD) are established risk factors fo r chronic diseases. The body mass index (BMI) and the waist/hip circum ference ratio (WHR) are used conventionally as indices of obesity and FD in epidemiological studies. Although some general limitations of th ese indices are recognized, others that affect their use in relative r isks for disease are not well recognized. These include effects of sex , ethnicity, and especially age on the relationships between these ind ices and body composition, which can result in substantial misclassifi cation of obesity and FD. There is considerable variability in body co mposition for any BMI, and some individuals with low BMIs have as much fat as those with high BMIs. This results in poor sensitivity for cla ssifying levels of body fatness (e.g., too many ''false negatives,'' o r overweight individuals classified as not overweight), and relative r isks are attenuated across all categories of BMI. A more serious probl em, however, is that at different ages the same levels of BMI correspo nd to different amounts of fat and fat-free mass. Data from the Rosett a Study and the New Mexico Aging Process Study show that older adults have, on average, more fat than younger adults at any BMI, due to the loss of muscle mass with age. As a result, the sensitivity of BMI cutp oints with respect to body fatness decreases with age, and the use of a fixed cutpoint for all ages results in ''differential misclassificat ion bias.'' Taken together, these issues suggest that the increases wi th age in the prevalences of overweight and obesity, and in the risks for chronic diseases, may be mis-estimated using BMI. Similar issues m ay affect the use of WHR for estimating prevalences and associated ris ks of FD. New field methods for estimating body composition are availa ble that can be applied in large, epidemiologic follow-up studies of c hronic diseases. These methods will allow epidemiologists to consider, for example, whether it is increased fat, or the replacement of fat-f ree mass with fat, with age that is associated with risk for chronic d isease.