Metabolically‐active portion of fat‐free mass: a cellular body composition level modeling analysis

The proportion of fat‐free mass (FFM) as body cell mass (BCM) is highly related to resting energy expenditure. However, the BCM/FFM ratio may have been underestimated in previous studies as the Moore equation (BCM = 0.00833 x TBK), which is typically used to predict BCM, underestimates BCM by ~11%. The aims of the present study were to develop a theoretical BCM/FFM model at the cellular level, and to explore the influences of sex, age, and adiposity on BCM/FFM. Subjects were 112 adults who had the following measurements: total‐body water by 2 H 2 O or 3 H 2 O dilution; extracellular water by NaBr dilution; total‐body nitrogen by in vivo neutron activation analysis; and bone mineral by DXA. FFM was calculated using a multicomponent model and BCM was calculated as the difference between FFM and the sum of extracellular fluid and solids. The developed theoretical model revealed that the proportion of BCM/FFM is mainly determined by water distribution ( E/I , the ratio of extracellular to intracellular water). A significant correlation (r = 0.90, P < 0.001) was present between measured and model‐predicted BCM/FFM for all subjects pooled. Measured BCM/FFM (mean +/− SD) was 0.584 +/− 0.041 and 0.529 +/− 0.041 for adult men and women ( P < 0.001), respectively. A multiple linear regression model showed that there are independent significant associations of sex, age, and %fat with BCM/FFM. Variation in the BCM/FFM is mainly dependent on water distribution with a larger E/I associated with a smaller BCM/FFM. Supported by NIDDK PO1‐42618.