Use of bioelectrical impedance analysis and anthropometry to measure fat-free mass in children and adolescents with Crohn disease

Objectives: To investigate the precision of published prediction equations for fat-free mass (FFM) from bioimpedance measurements in children with Crohn disease using dual-energy X-ray absorptiometry (DXA) as an in vivo gold standard. Methods: Fat-free mass of 49 white boys and girls ages 7.3 to 16.9 y suffering from Crohn disease was measured by DXA. Body weight, height and bioimpedance measurements were also collected. FFM measured by DXA (FFMDXA) was compared with FFM predicted by the only 5 published prediction equations available for children and adolescents. An equation was developed for predicting FFM and was validated using a bootstrap method. Results: When correlating predicted FFM with FFMDXA, Schaefer's equation showed the highest R-2 (0.950), the smallest standard error of estimate (SEE) (2.05 kg) and the smallest percentage error (0.28%). Our prediction equation for estimating FFM was FFM = 0.652 Ht(2)/Z + 0.0385 Wt+0.586 Age - 0.327, R-2 = 0.951, SEE = 2.08, P < 0.0005, where Ht(2)/Z is the impedance index in cm(2)/Omega, Wt is body weight in kilograms, age is in years. R-2 value from bootstrap method was 0.950 +/- 0.01 (95% confidence interval 0.927-0.968), indicating an acceptable validation of the derived formula. Conclusions: The formula of Schaefer is the best for predicting FFM. The present study provides a new prediction equation for estimating FFM in children with Crohn disease that may be used in clinical settings in which more sophisticated body composition measuring equipments are not available.