Skip to main content
ARS Home » Plains Area » Houston, Texas » Children's Nutrition Research Center » Research » Publications at this Location » Publication #342572
Title: The limitations of transforming very high body mass indexes into z-scores among 8.7 million 2- to 4-year-old children

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 GOODMAN, ALYSON - Centers For Disease Control And Prevention (CDC) - United States
item OGDEN, CYNTHIA - Centers For Disease Control And Prevention (CDC) - United States
item BLANCK, HEIDI - Centers For Disease Control And Prevention (CDC) - United States

Submitted to: Journal of Pediatrics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2017
Publication Date: 4/19/2017
Citation: Freedman, D.S., Butte, N.F., Taveras, E.M., Goodman, A.B., Ogden, C.L., Blanck, H.M. 2017. The limitations of transforming very high body mass indexes into z-scores among 8.7 million 2- to 4-year-old children. Journal of Pediatrics. pii:S0022-3476(17):30451-1. doi:10.1016/j.jpeds.2017.03.039.

Interpretive Summary: Growth charts from the Centers for Disease Control and Prevention (CDC) are used to evaluate the body mass index (BMI) of children with respect to obesity status. Because of the approach taken to develop the growth charts, there is an inherent problem with evaluating children with severe obesity, defined as having a BMI above the 97th percentile. The use of BMI z-scores and percentiles calculated from the growth charts can lead to an erroneous evaluation of children with severe obesity. To demonstrate this problem, data from 8.7 million 2- to 4-year-old children were examined from 2008 through 2011 in the CDC's Pediatric Nutrition Surveillance System. The statistical analysis of these data showed inconsistencies in the use of BMI z-scores and percentiles. An alternative approach is recommended for evaluating very high BMIs relative to the CDC 95th percentile.

Technical Abstract: To examine the associations among several body mass index (BMI) metrics (z-scores, percent of the 95th percentile (%BMIp95) and BMI minus 95th percentile as calculated in the growth charts from the Centers for Disease Control and Prevention (CDC). It is known that the widely used BMI z-scores (BMIz) and percentiles calculated from the growth charts can differ substantially from those that directly observed in the data for BMIs above the 97th percentile (z = 1.88). Cross-sectional analyses of 8.7 million 2- to 4-year-old children who were examined from 2008 through 2011 in the CDC's Pediatric Nutrition Surveillance System. Because of the transformation used to calculate z-scores, the theoretical maximum BMIz varied by >3-fold across ages. This results in the conversion of very high BMIs into a narrow range of z-scores that varied by sex and age. Among children with severe obesity, levels of BMIz were only moderately correlated (r ~ 0.5) with %BMIp95 and BMI minus 95th percentile. Among these children with severe obesity, BMIz levels could differ by more than 1 SD among children who had very similar levels of BMI, %BMIp95 and BMI minus 95th percentile due to differences in age or sex. The effective upper limit of BMIz values calculated from the CDC growth charts, which varies by sex and age, strongly influences the calculation of z-scores for children with severe obesity. Expressing these very high BMIs relative to the CDC 95th percentile, either as a difference or percentage, would be preferable to using BMI-for-age, particularly when assessing the effectiveness of interventions.