Associations between body mass index and body composition among Pacific adolescents in Aotearoa New Zealand

BMI and BMI z-scores are widely used to classify adolescent overweight and obesity (OWOB), but they do not distinguish fat from fat-free mass and may misclassify adiposity. This study aimed to examine how BMI and BMI z-scores relate to body composition among Pacific adolescents in Aotearoa New Zealand (A-NZ). Cross-sectional data from an A-NZ Pacific birth cohort were analysed, assessing body size, body composition, and demographics in 871 adolescents (median age 14.2 years). Body composition was estimated using conventional and vector bioelectrical impedance analysis. Sex-stratified analytical comparisons included fractional polynomial regression of BMI z-scores with fat mass index (FMI) and fat-free mass index (FFMI), Hattori plots, and vector analysis parameters. From BMI z-scores, OWOB was defined in 337 (80.3%) females and 342 (75.8%) males. BMI z-scores fitted FFMI well, but important nonlinear patterns emerged between BMI z-scores and FMI. At any given BMI, Pacific adolescents displayed wide variation in FMI and FFMI combinations, with greater variability among males than females. Mean vector length shortened progressively with increasing body size. Phase angle increased from normal weight to overweight but plateaued from overweight to obesity. BMI-defined OWOB inadequately captured adiposity in this population.