Segmental BIA-derived body composition in children and adolescents aged 5-18 years: age- and sex-specific reference...

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.

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.

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.

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.