BMC Public Health 2005, 5:41 http://www.biomedcentral.com/1471-2458/5/41 Table 1: Anthropometric measurements and other characteristics of the study sample. The Third National Health and Nutrition Examination Survey (NHANES III 1988–94). Characteristics Age (years)* Body Weight (kg)* Standing Height (cm) Body Mass Index (kg/m2) Triceps Skinfold (mm) Suprailiac Skinfold (mm) Thigh Skinfold (mm) Body Density (g/cm3) Fat Free Mass (kg) Fat Free Mass Index (kg/m2) 65.94 (14.2) 81.20 (15.7) 174.00 (7.1) 26.75 (4.5) 13.49 (5.9) 20.41 (8.9) 15.09 (7.6) 1.05 (0.01) 64.26 (0.3) 20.81 (0.1) Non-Hispanic Whites Men Women Mean ± SE Mean ± SE (N = 2,106) (N = 1,966) African Americans Men Mean ± SE (N = 948) Women Mean ± SE (N = 783) 66.62 (14.5) 68.28 (16.0) 159.54 (7.0) 26.78 (5.8) 23.29 ± (8.1)* 20.03 (9.6) 30.40 (8.8) 1.03 (0.0) 46.65 (0.2)* 17.91 (0.1)* 58.41 (12.9) 81.40 (17.8) 174.48 (7.4) 26.66 (5.2) 12.34 (6.8) 20.40 (10.6) 12.48 (7.1) 1.05 (0.0) 63.87 (0.4) 20.82 (0.1) 58.51 (13.6) 78.64 (20.0) 161.91 (6.5) 29.95 (7.2) 26.08 (9.2)* 25.04 (10.3)* 29.40 (9.7) 1.03 (0.0) 49.53 (0.4)* 18.82 (0.1)* Note: Values are mean (SE); * Indicates statistical significant difference for within gender comparison of Non-Hispanic Whites with non-Hispanic African Americans at two-sided p < 0.05. ethnicity. Separate analyses were conducted for each dec- ade of life beginning from the 4th to the 8th decade. Mean values were calculated for FFM and FFMI. The χ2 and t-test statistics and 95 percent Confidence Interval were used to determine statistical significance. A General Linear Regres- sion Model for continuous variable response, using the method of weighted least squares adapted to survey data [13] was used to test the association between the depend- ent variable (FFM) and independent variables (age, eth- nicity and gender) each in a bivariate model. Similar models were constructed for FFMI as a second dependent variable. Estimates of individual regression coefficients (the vector of beta values) and standard errors were com- puted. Because of our interest in how ethnicity influenced age-related changes in FFM in men and in women, we tested for "ethnicity*gender" interaction (dependent vari- able = ethnicity, gender, ethnicity*gender); and "ethnic- ity*age" interaction (dependent variable = ethnicity, age ethnicity*age). Because of the observed gender- and eth- nicity interaction, and the differences in FFM and FFMI in the initial analyses, final multiple regression models to test for the associations of ethnicity and age, with FFM and FFMI were conducted separately for each gender. The model for FFM included adjustment for BMI. However, because FFMI already adjusted for height, the regression model for FFMI did not include height. This precaution was taken to avoid co-linearity and the bias of structural relationship between FFMI and height. All initial analyses were performed using the Statistical Analysis System (SAS) package [14]. Given the multistage complex survey design of NHANES III, final analyses were conducted in SUDAAN (Proc Descript; Proc Regression) to produce a more robust variance estimate [15]. Sample weights were included in the final analysis to account for over sampling and non-response adjustment. Results Subject characteristics A total of 4,072 Whites (2106 men and 1966 women) and 1731 Blacks (948 men and 783 women) were included in the final analyses (Table 1). Although Blacks had overall higher body weight, standing height and BMI compared to Whites, these differences did not reach statistical signif- icance. Among women, Blacks weighed more, were taller, and had higher body mass index than white women. In contrast, white men had higher waist-hip-ratio and triceps skinfold than black men. Ethnicity-based comparison Collectively, mean FFM and FFMI were similar between Blacks and Whites. However, when stratified by gender- ethnicity, black women had higher FFM and FFMI than White women (FFM difference: 2.88 ± 0.28 kg; p < 0.01) and (FFMI difference: 0.91 ± 0.10 kg/m2; p < 0.01) (Table 1). The significantly higher FFM and FFMI in black women persisted through all decades of life, except in the 8th decade or higher. Overall, Blacks weighed higher than Whites (difference 5.53 ± 0.44 kg; p < 0.001) and this was explained by higher values in black women (difference 10.35 ± 0.59 kg; p < 0.001) (Table 1). Age-based comparison Using age group 40 – 45 as the reference, we observed exponential decline in FFM and body weight with advancing age in men and in women. This decline varied by gender and ethnicity (Figure 1). In Blacks and Whites Page 3 of 9 (page number not for citation purposes)