Body composition in 5-18-y-old obese children and adolescents before and after weight reduction as assessed by deuterium dilution and bioelectrical impedance analysis.

The aim of the present study was to develop an equation for the prediction of total body water (TBW) from bioelectrical impedance analysis (BIA) in obese children and adolescents before and after weight reduction. In 146 obese subjects with a mean age of 12.7 +/- 3.0 y (5.5-17.8 y), TBW was measured by using deuterium dilution as well as the resistance index (RI; ht2/resistance) using BIA before and after weight loss. Initially, the RI correlated well with measured TBW (r2 = 0.92, P < 0.001). A multiple-regression analysis using forward stepwise selection of the variables RI, sex, age, weight, height, and waist-hip ratio revealed that the equation TBW = 0.35 x RI + 0.27 x age + 0.14 x weight - 0.12 predicts most accurately individual values of TBW before weight loss (adjusted r2 = 0.96, SEE = 1.9 L) with a mean error of predicted TBW of 1.40 +/- 1.38 L. This equation was validated in 1000 random samples (bootstrap-sampling method), giving a mean r2 of 0.95. During the weight-reduction program, which included an energy-restricted diet and an extensive exercise program, the patients lost 7.7 +/- 3.2 kg, leading to a small decrease in TBW of 0.4 +/- 1.5 L. When the developed prediction equation was applied to the data after weight loss, an r2 value of 0.94 between measured and calculated TBW and a mean error of 2.18 +/- 1.89 L was obtained. Validation of the equation in 1000 random samples after weight loss again gave a mean r2 value of 0.95. Individual changes in predicted TBW correlated only weakly with those of measured TBW (r = 0.21, P < 0.05). Thus, individual TBW values before and after weight loss can be predicted by BIA with acceptable accuracy by using the developed equation. However, prediction of small individual changes in TBW during weight loss is not possible by BIA.