BMI–adiposity discrepancy across weight categories in primary school children: a prospective school-based cohort study

The BMI-for-age z score (BAZ) is widely used to assess adiposity in children; however, whether standardized BMI consistently reflects body fatness across the BMI spectrum remains uncertain. In this prospective school-based cohort study in Chiayi, Taiwan (October 2014-May 2025), we enrolled 3,293 primary school children aged 6-12 years with up to 12 repeated body composition measurements per participant (17,339 total observations) using bioelectrical impedance analysis (BIA). BAZ was strongly correlated with BIA-derived percent body fat (PBF) at baseline (Pearson r = 0.895; P < 0.001). However, discrepancy magnitude differed significantly across BMI categories (Kruskal-Wallis P < 0.001), with the smallest discrepancy in normal-weight children and greater discrepancies at both extremes. Longitudinal mixed-effects models showed that age (β = 0.367% points per year; 95%CI, 0.325-0.409; P < 0.001) and female sex (β = 2.608% points; 95%CI, 2.364-2.852; P < 0.001) were independently associated with higher PBF after adjustment for BAZ (β = 6.058; 95%CI, 5.994-6.122; P < 0.001), based on the primary model with random intercepts and random slope for age. These findings indicate that BAZ captures population-level adiposity variation, but its agreement with individual body fatness varies across the BMI spectrum.