Obesity Epidemiology

MEASUREMENTS OF ADIPOSITY AND BODY COMPOSITION 67by anthropometry and DXA. Among overweight and obese women (BMI > 27), BMI andpercent body fat measured by DXA were similarly correlated with leptin concentrations (0.73vs. 0.79). These correlations were much weaker among women with a BMI 27. In numerousstudies, BMI has been inversely correlated with concentrations of adiponectin, a newlydiscovered adipocyte-derived hormone that is reduced in obese and diabetic subjects. 90The relationship between BMI and cardiovascular risk factors, such as blood pressure,high-density lipoprotein (HDL) cholesterol, fasting glucose, and triglycerides is well established.Spiegelman et al. 91 compared the associations of BMI, absolute FM, percent bodyfat, and regional fat distribution with fasting blood glucose and blood pressure in 1551 menand women between the ages of 15 and 79 years. Percent body fat and absolute FM wereassessed by densitometry. In this study, BMI appeared to be more strongly correlated withabsolute FM adjusted for height than with percent body fat. Also, overall and absolute FMwere stronger predictors of blood pressure and blood glucose levels than was percent bodyfat (after adjustment for age, height, and current cigarette-smoking status). Correlationsbetween body fat variables assessed by densitometry and blood pressure and blood glucosewere not superior to those between BMI and these factors. These findings suggested thatalthough BMI is a measure of both fat and lean body mass, the adverse effects of FM overrodethe potential benefits of LBM in this young-adult and middle-aged population. Thisoutcome may explain a strong association between BMI and cardiovascular morbidity andmortality observed in many epidemiologic studies (see Chapters 9 and 10).Age and Sex DifferencesDespite its established ability to predict body fat and health outcomes, BMI is an indirectand imperfect measure of adiposity. The components of BMI include both FM and leanbody mass (LBM). Given the same BMI, the relative compositions of FM versus LBMappear to depend on age, sex, and ethnicity. 92 It is well known that for the same BMI,percent body fat is higher in women than in men. 84 The sex differences in body compositionare established during adolescence and sexual maturation, when males developmore lean body mass, especially bone mass and skeletal muscle. 93 For children, BMI isa “moving target,” in that normal changes with growth and maturation lead to greaterincreases in LBM than in FM. 94 Because it is inappropriate to use absolute values of BMIas a measure of fatness in children, national and international reference standards havebeen established to define childhood overweight and obesity using age and sex-specificdistributions (see Chapter 20).The validity of BMI as a marker of body fatness in older adults appears to be reduceddue to changes in body composition associated with aging. According to NHANES data, 95mean BMI gradually increases during young and middle-aged adult life, reaching peakvalues at 50 to 59 years of age, then declining slightly after age 60. 96 It is well establishedthat aging is associated with a substantial loss in LBM and with some increase in FM. 84For example, from 20 to 70 years of age, LBM, especially muscle, decreases by up to40%. 96 Decreased muscle mass in the elderly is known as sarcopenia. Several studies haveshown that the prevalence of sarcopenia increases rapidly at ages >60 years. 97,98Janssen et al. 99 observed a reduction in skeletal muscle mass starting during the thirddecade of life, with a more noticeable decrease in absolute skeletal muscle mass at theage of 45 (Fig. 5.1). This decrease was attributed primarily to a decrease in lower bodyskeletal muscle (e.g., thigh muscle). During 4 years of follow-up of the Health ABCcohort, Newman et al. 100 found that weight loss was strongly correlated with lean bodymass loss in elderly men and women, especially in those whose weight loss occurredduring hospitalization.