Fat-free mass in adolescent athletes: Accuracy of bioimpedance equations and identification of new predictive equations

Objectives: The aims of this study were to evaluate the effect of biological maturity on body composition in Brazilian adolescent athletes, to verify the accuracy of previous bioimpedance predictive equations for estimating fat-free mass (FFM), and to develop new predictive equations, considering sexual and skeletal maturity. Methods: There were 318 Brazilian adolescent athletes (52% male) involved in this study. FFM was determined using single-frequency (50 kHz) bioelectrical impedance analysis (BIA) and dual-energy x-ray absorptiometry (DXA), which was used as the reference method. The adolescents were classified into skeletally mature using bone age (both sexes), and sexually mature using menarche occurrence (female). The effect of maturity on bioelectrical values was tested using bioelectrical impedance vector analysis. Three predictive BIA equations to estimate FFM were selected from the reviewed literature. Lin's concordance correlation coefficient and Bland–Altman test were used to test the concordance and accuracy of BIA equations. Stepwise multiple regression was used to develop new predictive equations, considering BIA vectors, age, skeletal, and sexual maturity. Results: DXA and BIA results showed wide limits of disagreement for FFM for all the three equations. Two new equation models were developed, including age and skeletal maturity for both sexes and menarche status for females. Both models showed high R 2 (males = 0.92 and females = 0.84). Conclusions: The assessment of body composition in adolescent athletes should consider sexual (female) or skeletal (male) maturity. The newly proposed equations showed promising results in Brazilian adolescent athletes. A test in different groups and populations is necessary to evaluate the general suitability of the equations in adolescents