Metabolically active component of fat-free body mass: influences of age, adiposity, and gender.

Fat-free body mass (FFM) is often considered the metabolically active compartment and is widely used to adjust between-subject differences in resting energy expenditure for body composition. The use of FFM as the metabolically active portion of body weight makes the assumption that the body cell mass (BCM) component which is more difficult to measure, maintains a relatively constant relationship to FFM within and between subjects. The aim of this study was to test the hypothesis that BCM and FFM are associated independently of age, adiposity (as represented by body density), and gender in healthy white women and men. BCM and FFM were estimated by whole-body 40K-counting and dual-energy x-ray absorptiometry (DXA), respectively. Multiple regression analysis was used to model the relationships between BCM as the dependent variable and FFM, age, body density, and gender as potential independent variables. FFM alone explained 51% and 63% of between-individual BCM differences in women (n = 269) and men (n = 204) (both P = .0001), respectively. Age contributed significantly (P = .0001) to BCM prediction after adjusting first for FFM in both women and men. Body density also added significantly (P = .004 and P = .0001) to FFM and age prediction of BCM in women and men, respectively. Lastly, gender contributed significantly to the composite model, with 91% of between-individual differences in BCM explained by FFM, age, body density, and gender. Hence, BCM does not maintain a fixed relationship to FFM, as often assumed, but varies significantly and independently of FFM with age, adiposity, and gender. These findings have implications for the study of metabolic indices such as resting energy expenditure.