Background: Bioelectrical Impedance Analysis (BIA) is a fast, practical, non-invasive, and frequently used method for fat-free mass (FFM) estimation. The aims of this study were to validate predictive equations of BIA to FFM estimation in Army cadets and to develop and validate a specific BIA equation for this population. Methods: A total of 396 males, Brazilian Army cadets, aged 17–24 years were included. The study used eight published predictive BIA equations, a specific equation in FFM estimation, and dual-energy X-ray absorptiometry (DXA) as a reference method. Student’s t-test (for paired sample), linear regression analysis, and Bland–Altman method were used to test the validity of the BIA equations. Results: Predictive BIA equations showed significant differences in FFM compared to DXA (p < 0.05) and large limits of agreement by Bland–Altman. Predictive BIA equations explained 68% to 88% of FFM variance. Specific BIA equations showed no significant differences in FFM, compared to DXA values. Conclusion: Published BIA predictive equations showed poor accuracy in this sample. The specific BIA equations, developed in this study, demonstrated validity for this sample, although should be used with caution in samples with a large range of FFM.

The phase angle is used to evaluate nutritional status and is an indicator of cellular health. Cardiorespiratory fitness and body composition are strong indicators of health during adolescence. We aimed to evaluate if body composition and cardiorespiratory fitness have an association with phase angle among adolescents. 203 girls and 220 boys (12.8±1.3 years) were evaluated. Peak height velocity, percentage fat mass, fat-free mass, cardiorespiratory fitness, and phase angle measurements were collected. Linear regression adjusted by peak height velocity was used to verify if predictor variables were associated with phase angle among adolescents. Phase angle showed correlation with fat-free mass (girls: r=0.42 and boys: r=0.37); with percent fat mass (girls: r=0.23); and with cardiorespiratory fitness (boys: r=0.19). Linear regression showed that percentage fat mass (in girls) and cardiorespiratory fitness (in boys) had an effect of 11 and 17% in phase angle, respectively, while fat-free mass had an effect of 22 and 26% in phase angle for girls and boys, respectively. Changes in phase angle seem to be more associated with the percentage fat mass in girls, cardiorespiratory fitness in boys, and fat-free mass in both when controlled by peak height velocity.