Body composition and resting energy expenditure in humans: role of fat, fat-free mass and extracellular fluid

OBJECTIVE: The objective of this study was to determine whether there are i ndependent effects of extracellular fluid volume (ECF) and fat mass (FM) on resting energy expenditure (REE) relative to fat-free mass (FFM) in adult men and women. METHODS: Multiple linear regression analysis was used to relate REE, as det ermined by indirect calorimetry, to FFM and FM (measured using dual energy X-ray absorptiometry) and ECF (measured using bromide space and/or the radi osulfate washout space) in 153 women and 100 men with varying amounts of bo dy fat. RESULTS: REE correlated significantly with FFM and FM in women (r=0.65 and r=0.63, both P < 0.001) and men (r = 0.62 and r = 0.48, both P < 0.001, FFM and FM, respectively). In a multiple linear regression analysis FFM, FM an d age significantly contributed to the ability to predict REE in both gende rs. The models that were derived were not significantly different between w omen and men. In women the contribution to REE from FM was easier to detect when FM was greater. Adjustment of FFM for ECF did not improve the relatio nship between FFM and REE. CONCLUSIONS: FFM, FM and age are significant, independent predictors of REE in both men and women. Adjustment of FFM for ECF does not improve the abil ity of FFM to predict REE, which suggests that ECF is a highly integrated c omponent of FFM in healthy adults. Expressing REE relative to FFM alone wil l introduce errors when lean and obese populations are compared.