FFM (kg) FFMI (kg/m2) Characteristics Ethnicity Age (yrs) BMI (kg/m2) Ethnicity Age (yrs) Body Weight (kg) -0.338 -0.109 1.527 -0.333 0.017 0.122 0.276 0.011 0.039 0.070 0.003 0.003 P Value 0.227 <0.001 0.001 <0.001 <0.001 <0.001 -0.821 -0.125 1.053 -0.456 -0.015 0.479 0.313 0.021 0.032 0.147 0.005 0.006 P Value 0.012 <0.001 <0.001 0.003 0.004 <0.001 BMC Public Health 2005, 5:41 http://www.biomedcentral.com/1471-2458/5/41 Table 3: Regression analysis of fat free mass, and fat free mass index by age in white, and black men and women. The Third National Health and Nutrition Examination Survey (NHANES III 1988–94). Men Beta Coefficient Standard Error Women Beta Coefficient Standard Error Note: Statistical significance was set at P ≤ 0.05; Fat free mass model adjusted for body mass index; and fat free mass index model adjusted for body weight. FFM indicates fat free mass; FFMI indicates fat free mass index; and BMI indicates body mass index. white women, the decline attained significance level ear- lier in black than white women. Overall, Blacks had greater decline in FFM than Whites. Chumlea and colleagues reported higher mean FFM in black men and women compared to white men and women respectively [16]. An inverse association of FFM and FFMI with mortality has been reported [4]. It is not known whether this association is driven by differences in FFM and FFMI, or by dissimilarities in the onset of decline of FFM and FFMI with increasing age. Because mortality rates were reportedly higher for black men than for white men, and for black women than white women, [7] mean differences in FFM and FFMI are unlikely to significantly contribute to FFM-related mortality rates in Blacks. Although several causative factors are likely to be contrib- utory to mortality rates in different populations, it is also possible that age-related changes in FFM and FFMI are important factors that must be considered in future stud- ies on mortality and aging. Since clinical interventions to attenuate age-dependent decline in FFM are now availa- ble, longitudinal studies to examine the influence of age- related decline in FFM and FFMI on mortality patterns is imperative. Because of the availability of more sophisticated measures of body composition, there has been a great deal of debate on the clinical utility of anthropometric measurements. Our findings of ethnicity-related differences in FFM in this study, is remarkably similar to recent estimates of FFM by Chumlea et al who used bioelectrical impedance (BIA) data from NHANES III [16]. Chumlea and colleagues pro- vided the first national estimates of body composition as reference, to compare other studies. In fact, the age spe- cific estimates of FFM in the Chumlea's study are concord- ant with our anthropometric estimates of FFM for white, and for black men and women. The BIA estimates of FFM, together with our anthropometric-derived FFM from the NHANES III, are analogous to DEXA-derived estimates of FFM by Gallagher et al [17]. The close approximation of our estimates with data from studies that used DEXA to estimate FFM supports the use of a generalized equation from anthropometric measurements to estimate FFM in a large representative sample, such as NHANES III, espe- cially when adjusted for age. Despite the known method- ological limitations associated with the use of generalized equations, our study provides valuable data on national estimates of FFM against which other future estimates from more robust measures can be compared. Ethnicity-based differences Men of both ethnic groups were similar in mean FFM and FFMI, whereas, black women had higher body weight, FFM and FFMI than white women. Marshall et al and oth- ers previously reported higher muscle mass in Blacks com- pared to Whites [18,19]. Comparable to our observation of -2.88 kg mean difference in FFM between white women and black women in this study, Gallagher et al reported - 2.22 kg difference in FFM between these 2 groups of women [17]. Also, Gasperino et al observed greater appendicular muscle mass in black women relative to white women [20]. While black women in our study were significantly younger than white women, the presence of differences in FFM that were maintained across several decades of life, argue against differences in age as a likely explanation for the higher FFM in black women. Though Blacks in the study were slightly heavier, and anthropo- metric measurements have been reported to over-estimate body density at extremes of body fatness, we found no sta- tistically significant difference in mean BMI (a measure of body fatness) between white and black women. This sug- gests that bias from differences in body fatness may not explain the observed differences in FFM between the two groups. Additionally, the final multiple regression model confirmed the association of FFMI with ethnicity in men Page 5 of 9 (page number not for citation purposes)