Background and objectives: Changes in BMI z score (BMIz) are widely used in weight control programs and interventions to monitor changes in body fatness, but this metric may not be optimal. We examined the ability of 3 BMI metrics to assess adiposity change among children with a wide range of BMIs.
Methods: The sample comprised 343 3-year-old children with serial measurements of BMI and body fatness every 4 months over 4 years. We compared correlations between changes in body fatness, calculated with dual-energy-x-ray absorptiometry, and changes in 3 BMI metrics: BMIz and percentage of the 50th (%50th) and 95th (%95th) percentiles in the CDC growth charts.
Results: About 21% of the participants were Black and 79% were white. Changes in body fatness over 4 years were more strongly associated with changes in %50th and %95th than with changes in BMIz. Correlations with %body fat among all children were r = 0.64 for BMIz versus r = 0.77 to 0.78 for %50th and %95th (P < .001 for differences between the correlations). Stratified analyses showed the difference between the correlations were similar among boys and girls, among white children and Black children, and among children without obesity and those with obesity.
Conclusions: Changes in adiposity among young children are better captured by expressing changes in BMI as a percentage of the 50th or 95th percentiles instead of BMIz change. Using the best BMI metric will allow pediatricians to better assess a child's change in body fatness over time.
Copyright © 2022 by the American Academy of Pediatrics.