Submitted on October 5, 2005
Accepted on March 17, 2006
Metabolically-Active Portion of Fat-Free Mass: A Cellular Body Composition Level Modeling Analysis
ZiMian Wang1*, Stanley Heshka1, Jack Wang1, Dympna Gallagher1, Paul Deurenberg2, Zhao Chen3, and Steven B Heymsfield1
1 Obesity Research Center, St. Lukes-Roosevelt Hospital, Columbia University College of Physicians and Surgeons, New York, New York, United States
2 Nutrition Consultant, Singapore, Singapore
3 Mel and Enid Zuckerman Arizona College of Public Health, University of Arizona, Tuson, Arizona, United States
* To whom correspondence should be addressed. E-mail: zw28{at}columbia.edu.
The proportion of fat-free mass (FFM) as body cell mass (BCM) is highly related to whole-body resting energy expenditure. However, the magnitude of BCM/FFM may have been underestimated in previous studies. This is because Moore equation [BCM (kg) = 0.00833 x total-body potassium (in mmol)], which was 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 the BCM/FFM. Subjects were 112 adults who had the following measurements: total-body water by 2H2O or 3H2O dilution; extracellular water by NaBr dilution; total-body nitrogen by in vivo neutron activation analysis; and bone mineral by dual-energy x-ray absorptiometry. 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 (i.e., 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 mass with BCM/FFM.
