BMI z-score in obese children is a poor predictor of adiposity changes over time

The age and sex standardized body mass index (BMIz) is a simple and widely utilized screening tool for obesity in children and adolescents. The purpose of this study was to evaluate the relationship between the BMIz trajectory versus the percent body fat (%FAT) trajectory, and if BMIz could predict significant changes in %FAT in a sample of obese children and adolescents. In this longitudinal observational study, body composition was measured by dual energy x-ray absorptiometry (DXA) in obese children within a multidisciplinary pediatric fitness clinic at an academic medical center over a 3-year time period. Regression analyses were conducted to evaluate the association between changes in BMIz and changes in %FAT. Baseline assessment was obtained from 515 participants. The reduction observed in BMIz (2.20 to 2.08, p < 0.0001) correlated with the reduction in %FAT (38.5 to 35.8%, p < 0.05) in the first two years. The overall correlation between the slope in BMIz reduction versus %FAT reduction was moderate (r = 0.36, p < 0.0001) over the 3-year follow-up period. The sensitivity of BMIz changes for predicting a decrease in %FAT was acceptable (70, 95% CI: 61–78%), however the specificity was poor (42, 95% CI: 31–54%). These findings advance the understanding of the utility and limitations of BMIz in children and adolescents. While BMIz may be sensitive to changes in adiposity, it is a weak predictor of these changes in total body fat (%FAT) due to the poor specificity. Therefore, clinicians must exercise caution when monitoring changes in a growing child’s body composition to avoid misclassifying or missing substantial change when utilizing BMIz alone.