Fat-free mass by bioelectrical impedance vs dual-energy X-ray absorptiometry (DXA).

Body composition (BC) assessment is a useful tool for a careful evaluation of nutrition status. Bioelectrical impedance analysis (BIA) is a safe, low-cost and reliable method for BC assessment. For epidemiological and clinical research in children, paediatric-age specific formula for fat-free mass (FFM) prediction from BIA is needed. Thus, in 35 children (age 7.7-13.0 years) with different levels of body fatness (relative weight for age 70.6-133.8%), FFM was calculated from dual-energy X-ray absorptiometry (DXA). A regression equation from BIA and DXA data was elaborated. The impedance index (ZI = height2/bioelectrical impedance) was the strongest predictor of FFM, explaining 89% of its variance. However, the variance increased to 96% when body weight was added with ZI in the regression model. No variable, including sex and age, contributed to the prediction of FFM in the presence of ZI and body weight. The regression formula [FFM = 2.330 + 0.588 ZI (cm2/omega) + 0.211 Weight (kg) (r = 0.96, SEE = 1.0 kg)], allows a reliable prediction of FFM in children from body impedance values.