This study tested the hypothesis that body mass index (BMI) is representative of body fatness independent of age, sex, and ethnicity. Between 1986 and 1992, the authors studied a total of 202 black and 504 white men and women who resided in or near New York City, were ages 20-94 years, and had BMIs of 18-35 kg/m2. Total body fat, expressed as a percentage of body weight (BF%), was assessed using a four-compartment body composition model that does not rely on assumptions known to be age, sex, or ethnicity dependent. Statistically significant age dependencies were observed in the BF%-BMI relations in all four sex and ethnic groups (p values < 0.05-0.001) with older persons showing a higher BF% compared with younger persons with comparable BMIs. Statistically significant sex effects were also observed in BF%-BMI relations within each ethnic group (p values < 0.001) after controlling first for age. For an equivalent BMI, women have significantly greater amounts of total body fat than do men throughout the entire adult life span. Ethnicity did not significantly influence the BF%-BMI relation after controlling first for age and sex even though both black women and men had longer appendicular bone lengths relative to stature (p values < 0.001 and 0.02, respectively) compared with white women and men. Body mass index alone accounted for 25% of between-individual differences in body fat percentage for the 706 total subjects; adding age and sex as independent variables to the regression model increased the variance (r2) to 67%. These results suggest that BMI is age and sex dependent when used as an indicator of body fatness, but that it is ethnicity independent in black and white adults.

This study tested the hypothesis that skeletal muscle mass is reduced in elderly women and men after adjustment first for stature and body weight. The hypothesis was evaluated by estimating appendicular skeletal muscle mass with dual-energy X-ray absorptiometry in a healthy adult cohort. A second purpose was to test the hypothesis that whole body 40K counting-derived total body potassium (TBK) is a reliable indirect measure of skeletal muscle mass. The independent effects on both appendicular skeletal muscle and TBK of gender (n = 148 women and 136 men) and ethnicity (n = 152 African-Americans and 132 Caucasians) were also explored. Main findings were 1) for both appendicular skeletal muscle mass (total, leg, and arm) and TBK, age was an independent determinant after adjustment first by stepwise multiple regression for stature and weight (multiple regression model r2 = approximately 0.60); absolute decrease with greater age in men was almost double that in women; significantly larger absolute amounts were observed in men and African-Americans after adjustment first for stature, weight, and age; and >80% of within-gender or -ethnic group between-individual component variation was explained by stature, weight, age, gender, and ethnicity differences; and 2) most of between-individual TBK variation could be explained by total appendicular skeletal muscle (r2 = 0.865), whereas age, gender, and ethnicity were small but significant additional covariates (total r2 = 0.903). Our study supports the hypotheses that skeletal muscle is reduced in the elderly and that TBK provides a reasonable indirect assessment of skeletal muscle mass. These findings provide a foundation for investigating skeletal muscle mass in a wide range of health-related conditions.

We studied the correlations between body mass index (BMI) and percent body fat (fat%) measured by dual-photon absorptiometry (DPA) in 445 white and 242 Asian adults aged 18-94 y. In addition, comparisons in six circumferences and eight skinfold-thickness measurements between whites and Asians were made to explain the discrepancies. Although Asians had lower BMI, they were fatter than whites of both sexes. The correlations between fat% and BMI varied by BMI and sex and race. Comparisons in anthropometry show that Asians had more subcutaneous fat than did whites and had different fat distributions from whites. Asians had more upper-body subcutaneous fat than did whites. The magnitude of differences between the two races was greater in females than in males. Prediction equations developed for each sex and race, based on BMI alone, gave SEEs ranging from 4.4% to 5.7%. All were significantly improved to the range of 3.5-4.4% when age and several skinfold-thickness measurements were added.