Prediction of fat‐free mass in black and white pre‐adolescent and adolescent girls from anthropometry and impedance

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Abstract
As an ancillary project of the National Heart Lung and Blood Institute's Growth and Health Study (NGHS), data of 31 black girls and 38 white girls, 10–16 years of age, were used to develop equations to predict fat-free mass (FFM) from selected anthropometric dimensions and bioelectric impedance measures of resistance and reactance. Subjects were drawn from ongoing projects at the Children's Hospital Medical Center (CHMC) to obtain girls of both races from a wide range of body mass indices. FFM was estimated from measures of dual energy X-ray absorptiometry (DXA). The resulting equations had a root mean square error (RMSE) of 1.9 kg and a coefficient of variation (CV) of 4.9% for black girls using independent variables: stature divided by resistance squared (S2/R), thigh circumference, subscapular and suprailiac skinfold thicknesses. The corresponding values for white girls were 1.1 kg and 3.2% using independent variables: S2/R, arm circumference, weight, and reactance divided by resistance (R/Rc). In addition to cross-validating the ethnic-specific equations using PRESS statistics, the equation for white girls was also cross-validated using an independent sample of 35 girls from the Fels Longitudinal Study selected to match the CHMC sample. The cross-validation yielded a pure error of 2.4 kg FFM and a CV of 7%. The predicted values for FFM from the equations were used to calculate percentage of body fat in the CHMC sample and the pure errors for estimating percentage of body fat were 3% for blacks and 2% for whites.