BMI z-scores are a poor indicator of adiposity among 2- to 19-year-olds with very high BMIs, NHANES 1999-2000 to 2013-14

OBJECTIVE: Although the CDC growth charts are widely used, BMIz is known to be uninformative above the 97(th) percentile. We compared the relations of BMIz and other BMI metrics (%BMI(p95), percent of 95(th) percentile, and ΔBMI(p95), BMI minus 95(th) percentile) to circumferences, skinfolds and fat mass. We were particularly interested in the differences among children with severe obesity (%BMI(p95) ≥ 120). METHODS: We used data from 30,003 2- to 19-year-olds who were examined from 1999-2000 through 2013-14 in NHANES. RESULTS: The theoretical maximum BMIz based on the growth charts varied by more than 3-fold across ages. The BMI metrics were strongly intercorrelated, but BMIz was less strongly related to the adiposity measures than were ΔBMI(p95) and %BMI(p95). Among children with severe obesity, circumferences and triceps skinfold showed almost no association with BMIz (r ≤ 0.10), whereas associations with %BMI(p95) and ΔBMI(p95) ranged from r=0.32 to 0.79. Corresponding associations with fat mass ÷ height(2) ranged from r=0.40 (BMIz) to r=0.82 (%BMI(p95)) among 8- to 19-year-olds. CONCLUSIONS: Among children with severe obesity, BMIz is only weakly associated with other measures of body fatness. Very high BMIs should be expressed relative to the CDC 95(th) percentile, particularly in studies that evaluate obesity interventions.