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Title: 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

Author
item FREEDMAN, DAVID - Centers For Disease Control And Prevention (CDC) - United States
item BUTTE, NANCY - Children'S Nutrition Research Center (CNRC)
item TAVERAS, ELSIE - Massachusetts General Hospital
item LUNDEEN, ELIZABETH - Centers For Disease Control And Prevention (CDC) - United States
item BLANCK, HEIDI - Centers For Disease Control And Prevention (CDC) - United States
item GOODMAN, ALYSON - Centers For Disease Control And Prevention (CDC) - United States
item OGDEN, CYNTHIA - Centers For Disease Control And Prevention (CDC) - United States

Submitted to: Obesity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2016
Publication Date: 2/28/2017
Citation: Freedman, D.S., Butte, N.F., Taveras, E.M., Lundeen, E.A., Blanck, H.M., Goodman, A.B., Ogden, C.L. 2017. 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. Obesity. 25(4):739-746.

Interpretive Summary: The Centers for Disease Control and Prevention (CDC) growth charts are widely used, but were never intended for children with severe obesity, defined as having a body mass index (BMI) above the 97th percentile. In this study, the standard index – BMI z-Scores (BMIz) – was compared against other indices -%BMIp95, percent of 95th percentile, and BMI minus 95th percentile). Analysis showed that BMIz was less strongly related to the adiposity measures than were BMI minus 95th percentile and %BMIp95. Among children with severe obesity, it is recommended to use %BMIp95 to evaluate their BMI status.

Technical Abstract: 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 BMI minus 95th percentile) to circumferences, skinfolds, and fat mass. We were particularly interested in the differences among children with severe obesity (%BMIp95 >= 120). 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). 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 BMI minus 95th percentile 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 BMI minus 95th percentile 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. 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.