Validity of the relative fat mass pediatric index (RFMp) for the analysis of body composition in physically active youths at different stages of biological maturation

Abstract Background The paediatric relative fat mass (RFMp) index was valid for analysis of percent body fat (BF%). However, the validation did not consider biological maturation (BM) stages. The present study aimed to verify the validity of the RFMp index in the estimation of BF% in children and adolescents of both sexes at different stages of BM. Methods A cross‐sectional study was conducted with a sample of 146 young (males: 64.5%. females: 35.5%. age: 13.0 ± 2.2 years) practising sports modalities. We tested the validity of four RFMp equations (1: for boys aged 8–14 years; 2: for girls aged 8–14 years; 3: for both sexes aged 8–14 years; and 4: for both sexes aged 15–19 years) to analyse BF% using dual‐energy X‐ray absorptiometry as a reference method. BM was analysed by peak height velocity (PHV). Thus, we created subgroups by BM stage (pre‐PHV, circum‐PHV and post‐PHV). Results Analyses of agreement between methods showed that only the RFMp‐3 equation was reliable to analyse BF% in subjects of both sexes aged 8–14 years at the circum‐PHV BM stage (proportion bias 95% confidence interval = −0.3 to 0.5, p = 0.7. concordance correlation coefficient = 0.3; validity = 0.9). Conclusions The RFMp equation developed for the paediatric population of both sexes aged 8–14 years was valid for predicting BF% in children and adolescents of both sexes at the Circum‐PHV stage of the BM.