Adiposity heterogeneity and overlap across BMI-defined weight-status categories in adolescents

Body mass index (BMI) is widely used to classify weight status but does not fully capture heterogeneity in adiposity within and between BMI-defined weight-status categories. This study characterised adiposity across weight-status categories in adolescents, focusing on adiposity heterogeneity, distributional overlap, and sex differences. The sample comprised 5740 Polish adolescents (50.8% boys), aged 10–18 years, from the cross-sectional ADOPOLNOR study. Body composition was assessed using bioelectrical impedance analysis. Participants were classified as underweight (UNW), normal weight without obesity (NWNO), normal-weight obesity (NWO), or overweight/obesity (OVOB). Adiposity was evaluated using fat mass percentage (FM%), fat mass index (FMI), and fat-to-muscle mass ratio (FMR). Group differences were assessed using non-parametric tests, with effect sizes ( r ) reported for pairwise comparisons. Density plots visualised adiposity distributions, and overlap coefficients (OVL) quantified overlap between adjacent weight-status categories. Median values of all three adiposity indicators increased from UNW to OVOB in both sexes ( p < 0.001). However, distributional overlap was non-uniform, being lowest between NWNO and NWO (OVL 0.16–0.33) and substantial between NWO and OVOB (OVL 0.54–0.71). Girls had higher adiposity than boys across all weight-status categories, with the largest sex differences in NWNO (FM%, r = 0.49; FMI, r = 0.44; FMR, r = 0.47). Sex differences in overlap were evident for some category transitions, although overall patterns were similar between sexes. Conclusion : BMI provides a framework for classifying weight status, but substantial heterogeneity within weight-status categories and considerable overlap between adjacent categories support the complementary use of body-composition measures for characterising adolescent adiposity. What is Known: • BMI is widely used to classify weight status in adolescents but serves only as a proxy for adiposity. • Consequently, BMI-defined weight-status categories may not fully capture differences in body fatness among adolescents. What is New: • Adiposity is heterogeneous within BMI-defined weight-status categories. • Distributional overlap between adjacent categories provides additional insight into the extent to which BMI-defined weight-status categories differ in adiposity.