Four-component model of body composition in children: density and hydration of fat-free mass and comparison with simpler models

Background: Body composition in children is generally measured by 2-compone nt (2C) models, which are subject to error arising from variation in fat-fr ee mass (FFM) composition. The 4-component (4C) model, which divides body w eight into fat, water, mineral, and protein, can overcome these limitations . Objective: The aims of our study were to 1) describe 4C model data for chil dren aged 8-12 y; 2) evaluate interindividual variability in the hydration, bone mineral content, and density of FFM; 3) evaluate the success with whi ch 2C models and bedside techniques measure body composition in this age gr oup with use of the 4C model as a reference. Design: Dual-energy X-ray absorptiometry, underwater weighing, deuterium di lution, bioelectrical impedance analysis, and anthropometry were used to de termine body composition in 30 children. The contribution of methodologic e rror to the observed variability in the hydration and density of FFM was ev aluated by using propagation of error. Results: Mean (+/-SD) FFM density and hydration were 1.0864 +/- 0.0074 kg/L and 75.3 +/- 2.2%, respectively, and were significantly different from adu lt values (P < 0.02). Relative to the 4C model, deuterium dilution and dual -energy X-ray absorptiometry showed no mean bias for fatness, whereas under water weighing underestimated fatness (P < 0.025). Fatness determined by us ing skinfold-thickness and bioelectrical impedance analysis measurements al ong with published equations showed poor agreement with 4C model data. Conclusions: Biological variability and methodologic error contribute equal ly to the variability of FFM composition. Our findings have major implicati ons for bedside prediction methods used for children, traditionally develop ed in relation to underwater weighing.