US pediatric population-level associations of DXA-measured percentage of body fat with four BMI metrics with cutoffs

ObjectiveFour body mass index (BMI) metrics—BMI, BMI z-score, BMI percentile, and BMI%—are commonly used as proxy measures for children's adiposity. We sought to determine a BMI metric that is most strongly associated with measured percentage of body fat (%BF) in the US pediatric population stratified by sex, age and race/ethnicity, and to determine cutoffs that maximize the association for each BMI metric.Subjects, Design and Methods%BF was measured by DXA among N=6120 US boys and girls aged 8.0 to 17.9 years old from NHANES 1999-2004. We fit piece-wise linear regression models with cutoffs to %BF data using each BMI metric as the predictor stratified by sex, race/ethnicity and age. The slopes were modeled differently before and after the cutoffs which were determined based on grid searches.ResultsBMI z-score was in general most strongly associated with %BF for both boys and girls. The associations of the four BMI metrics were lowest for boys aged 12-13.9 years and girls aged 16-17.9 years, and strongest for Mexican-American boys and for non-Hispanic black girls. Overall, the associations were stronger for girls than for boys. In boys, BMI had the lowest association with %BF (R2=0.39) for all ages combined. The fold changes in slopes before and after cutoffs were greatest in general for BMI percentiles regardless of age, sex and race/ethnicity. BMI z-score cutoffs were 0.4 for both boys and girls for all ages combined. Except for BMI, the slopes after the cutoffs were in general greater than those before.ConclusionsAll BMI metrics were strongly associated with %BF when stratified by age and race/ethnicity except that BMI was the least associated with %BF in boys for all ages combined. Overall, BMI z-score was superior for evaluation of %BF, and its cutoff of 0.4 can also serve as a threshold for careful monitoring of weight status