Segmental BIA-derived body composition in children and adolescents aged 5–18 years: age- and sex-specific reference values and limitations in obesity

Background/Objectives Bioimpedance analysis (BIA) is a non-invasive method for estimating fat-free mass (FFM) and fat mass (FM) via resistance (Rz) and reactance (Xc). Despite its potential, its usefulness and accuracy in children remain controversial. Subjects/Methods We obtained body composition measurements from 2954 visits involving 1547 participants (aged 5–21 years) using segmental BIA measurements and skinfold-based equations (Slaughter et al.). Four independent analyses were conducted: (1) agreement of Rz and Xc measurements between body segments was assessed; (2) sex- and age-specific reference curves for height-normalized resistance (Rz/H) and reactance (Xc/H) were developed; (3) associations between changes in BMI-SDS, Rz/H, and Xc/H (SDS) were examined; and (4) FM estimates derived from the proprietary algorithm, an alternative formula-based BIA algorithm, and skinfold-based equations were compared with age adjustment. Results Segmental Rz/H and Xc/H measurements showed “excellent” (>0.9) intraclass correlations for all segments. Concordance correlation coefficients indicated variable agreement between methods. Age- and sex-specific Rz/H and Xc/H percentile curves showed similar progressions across body segments. Rz/H (SDS) and Xc/H (SDS) showed a clear association with BMI-SDS, with lower values in participants with obesity. Estimated FFM and FM showed discrepancies between estimation methods depending on weight status rather than age. Conclusion Rz/H and Xc/H showed consistent age-related patterns but were strongly influenced by BMI-SDS. While agreement between FM estimates was good in normal-weight children, substantial discrepancies were observed in those with obesity. These findings suggest that current BIA-based estimation approaches may be unreliable in children and adolescents with obesity.