Lisa M. Stolarczyk and Vivian H. Heyward, University of New Mexico, Albuquerque, NM 87131
Research suggests that there is a strong link between ethnicity and body composition, with documented differences in the composition of the fat-free body. Thus, ethnic variation needs to be considered when assessing body composition of women from diverse ethnic groups serving in the U.S. military.
To assess body composition, body weight is typically subdivided into fat and fat-free body compartments. The fat consists of all extractable lipids, and the fat-free body includes water, protein, and mineral (Siri, 1961). This two-component model assumes that the: (1) densities of fat and the fat-free body components are respectively, 0.901 g/cc (grams/cubic centimeter) and 1.100 g/cc, and these densities remain constant and are the same for all individuals, (2) respective densities and relative proportions of the fat-free body components, water (0.9937 g/cc; 73.8%), protein (1.34 g/cc; 19.4%), and mineral (3.038 g/cc; 6.8%) are constant within and among individuals, and (3) individuals being assessed differ from the reference body only in the amount of fat (Brozek et al., 1963; Siri, 1961). Using two-component equations of either Siri (1961) (percent body fat = [4.95/Db – 4.5] × 100) or Brozek et al. (1963) (percent body fat = [4.57/Db = 4.242] × 100), total body density from hydrostatic weighing can be converted to relative proportions of body fat (percent body fat).
Generally, two-component model equations provide accurate estimates of percent body fat as long as the basic assumptions of the model are met. However, researchers have reported that fat-free body density varies with ethnicity, depending mainly on the relative proportion of water and mineral comprising the fat-free body (Wang et al., 1989).
Research demonstrates that African American women have relatively greater skeletal muscle mass, bone mineral mass, and bone density than Caucasians (Cohn et al., 1977; Ortiz et al., 1992). Thus, the estimated density of the fat-free body is 1.106 g/cc for African American women.
Although the relative hydration of the fat-free body is similar for African Americans and Caucasians (˜74%), the relative mineral content in the fat-free body of younger (7.8%) and middle-aged (7.5%) African American women is somewhat higher than that of their Caucasian counterparts (7.3% and 6.7%, respectively, for younger and middle-aged Caucasian women) (Cote and Adams, 1993; Ortiz et al., 1992). Also, the average bone mineral density of African American women (1.18 to 1.25 g/cm2) is significantly greater than that of Caucasian women (1.09 to 1.16 g/cm2) (Cote and Adams, 1993; Ortiz et al., 1992).