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 i n resting energy expenditure for body composition. The use of FFM as t he metabolically active portion of body weight makes the assumption th at the body cell mass (BCM) component which is more difficult to measu re, maintains a relatively constant relationship to FFM within and bet ween subjects. The aim of this study was to test the hypothesis that B CM and FFM are associated independently of age, adiposity (as represen ted by body density), and gender in healthy white women and men. BCM a nd FFM were, estimated by whole-body K-40-counting and dual-energy x-r ay absorptiometry (DXA), respectively. Multiple regression analysis wa s used to model the relationships between BCM as the dependent variabl e and FFM, age, body density, and gender as potential independent vari ables. FFM alone explained 51% and 63% of between-individual BCM diffe rences in women (n = 269) and men (n = 204) (both P = .0001), respecti vely. Age contributed significantly (P = .0001) to BCM prediction afte r adjusting first for FFM in both women and men, Body density also add ed significantly (P = .004 and P = .0001) to FFM and age prediction of BCM in women and men, respectively. Lastly, gender contributed signif icantly to the composite model, with 91% of between-individual differe nces in BCM explained by FFM, age, body density, and gender. Hence, BC M does not maintain a fixed relationship to FFM, as often assumed, but varies significantly and independently of FFM with age, adiposity, an d gender. These findings Rave implications for the study of metabolic indices such as resting energy expenditure. Copyright (C) 1996 by W.B. Saunders Company