BMC Public Health 2005, 5:41 http://www.biomedcentral.com/1471-2458/5/41 P = 0.004 respectively) after accounting for the influence of body weight and ethnicity (Table 3). In a similarly adjusted model for men, decline in FFM and FFMI were associated with escalating age (P < 0.001; P < 0.001 respectively). This age-related decline in FFM and FFMI were greater for black compared to white women. In men, only FFMI (P < 0.001) but not FFM (P = 0.227) showed a greater decline in black men than white men. Although findings similar to some of our current observations have been reported, as far as we know, we are first to report eth- nicity-related differential decline in FFM and FFMI with advancing age in the same study, using anthropometric measurements from a nationally representative sample (NHANES III). While our estimates of FFM for white men below age 70 are in agreement with reports from the Fels study by Guo and colleagues[23], our estimates for women were slightly higher, but within reported margins of error. Further, the onset of age-related decline in FFM at about age 50 in white men and women in our study are in concordance with the Fels study. The earlier onset of decline in FFM in black men is particularly interesting, given the similarity in mean body weight, standing height, BMI, suprailiac skinfold and body density to that of white men (Table 1). Similarly, white and black women did not differ significantly with respect to mean standing height, thigh skinfold and body density. Of note, is the observa- tion of differential onset and decline in FFM and FFMI in this study that appears to parallel mortality patterns for the United States [7]. While underlying disease processes could potentially explain this observation, it is also possi- ble that age-related decline in FFM and FFMI compared to absolute values may have more important implications for health outcomes. Moreover, low levels of physical activity in Blacks [24] may accelerate the process of sarco- penia and unmask several age-related sub-clinical ill- nesses. Alternatively, differential reduction in androgen and testosterone, reduction in responsiveness to trophic hormones [3], growth hormone [25], and insulin-like growth factors [3] with escalating age may play important roles. The relative increase in FFMI in white women is per- haps related to increased risk of osteoporosis-related com- pression fracture, reduction in standing height, and relative increase in FFMI. Independent reports from Schutz et al, and Hughes et al lend support to the observed age-related changes in FFMI in white women [26,27]. While decline in FFM in women mirrored changes in body weight, we found no consistent pattern of differential weight decline between white men and black men. The finding in women is particularly remarkable, since heavier body weight requires higher FFM for support (Table 2). Nonetheless, a greater reduction in body weight than can be explained by losses in FFM might suggest a greater reduction in absolute body fat with increasing age. Although it is possible that FFMI may overcompensate for standing height, the lack of significant correlation between FFMI and standing height in this sample makes this unlikely. Within each ethnic group, the decline in FFM and FFMI started earlier in men than women (Figure 1). This finding is consistent with published reports from others who showed linear decline in skeletal muscle mass with advancing age[28,29]. Unfortunately, many of the previ- ous studies lacked the ethnic stratification, sample size and representativeness of the current study. Similar to our findings of gender-based differential decline in FFM, a Japanese study reported decreases in lean body mass in Japanese men but not in women[30]. While our current anthropometric estimates of FFM and FFMI from the NHANES III study may be at variance with estimates from a limited number of studies that showed loss of skeletal muscle mass in the 3rd decade of life[17], several other studies that used more robust methods to estimate skele- tal muscle mass[31,32,23] support our current observa- tions. Altogether, our study supports the presence of higher mean FFM in black women than white women, earlier onset of decline in FFM and FFMI in black men than white men respectively (Figures 1 and 2), and greater age-related decline in black men and women than in white men and women. Additionally, it provides support for the use of anthropometric measurements to estimate FFM in large epidemiological studies. Limitations The observations from this study are subject to a number of limitations. First, at extremes of BMI and body fat, there is bias in the use of skinfold-thickness measurements to estimate body density from which percent body fat is determined. This concern is heightened in the aged because of increased centripetal fat distribution. Fortu- nately, Pollock's equation included adjustment for age and the final regression model in our study was addition- ally adjusted for BMI. While population-specific equa- tions are appropriate alternatives and sometimes provide more accurate estimates, they are limited in generalizabil- ity. Though the equation used in the present study has greater applicability to women than men, its selection was based on the available anthropometric measurements in the NHANES III data. Because the Pollock equation was not derived from the NHANES sample, it is possible that it may not predict body density with the same degree of precision compared to the original parent sample [33]. Further, the cross-sec- tional nature of the NHANES III data makes it possible that changes in FFM with advancing age may have been underestimated, given known estimates from longitudinal studies [34]. These concerns are minimized by the similarities of our findings with that of studies Page 7 of 9 (page number not for citation purposes)