BMI z-Scores are a poor indicator of adiposity among 2- to 19-year-olds with very high BMIs, NHANES 1999-2000 to 2013-2014

David S Freedman, Nancy F Butte, Elsie M Taveras, Elizabeth A Lundeen, Heidi M Blanck, Alyson B Goodman, Cynthia L Ogden, David S Freedman, Nancy F Butte, Elsie M Taveras, Elizabeth A Lundeen, Heidi M Blanck, Alyson B Goodman, Cynthia L Ogden

Abstract

Objective: Although the Centers for Disease Control and Prevention (CDC) growth charts are widely used, BMI-for-age z-Scores (BMIz) are known to be uninformative above the 97th percentile. This study compared the relations of BMIz and other BMI metrics (%BMIp95 , percent of 95th percentile, and ΔBMIp95 , BMI minus 95th percentile) to circumferences, skinfolds, and fat mass. We were particularly interested in the differences among children with severe obesity (%BMIp95 ≥ 120).

Methods: Data was used from 30,003 2- to 19-year-olds who were examined from 1999-2000 through 2013-2014 in the National Health and Nutrition Examination Survey (NHANES).

Results: The theoretical maximum BMIz based on the growth charts varied by more than threefold across ages. The BMI metrics were strongly intercorrelated, but BMIz was less strongly related to the adiposity measures than were ΔBMIp95 and %BMIp95 . Among children with severe obesity, circumferences and triceps skinfold showed almost no association with BMIz (r ≤ 0.10), whereas associations with %BMIp95 and ΔBMIp95 ranged from r = 0.32 to 0.79. Corresponding associations with fat mass ÷ height2 ranged from r = 0.40 (BMIz) to r =0.82 (%BMIp95 ) 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 95th percentile, particularly in studies that evaluate obesity interventions.

Conflict of interest statement

Disclosure: The authors declare no conflict of interest

© 2017 The Obesity Society.

Figures

Figure 1
Figure 1
Sex- and age-specific values of L, M, and S in the CDC growth charts. The lower, right panel shows the maximum values of BMIz that are theoretically possible based on the CDC growth charts.
Figure 2
Figure 2
Calculated BMIz values from the CDC growth charts associated with various levels of %BMIp95 and with the CDC 95th percentile of BMI.
Figure 3
Figure 3
Relation of BMIz, modified BMIz, %BMIp95 and ΔBMIp95 to other body size measures, by sex and BMI status. Values of arm circumference, waist circumference, triceps skinfold and fat mass were adjusted for sex and age.
Figure 4
Figure 4
Relation of various BMI metrics to the sex- and age-adjusted levels of waist circumference among children with severe obesity

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