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J Pediatr. 2017 Sep;188:50-56.e1. doi: 10.1016/j.jpeds.2017.03.039. Epub 2017 Apr 19.

The Limitations of Transforming Very High Body Mass Indexes into z-Scores among 8.7 Million 2- to 4-Year-Old Children.

Author information

1
Division of Nutrition, Physical Activity and Obesity, Centers for Disease Control and Prevention, Atlanta, GA. Electronic address: dxf1@cdc.gov.
2
Children's Nutrition Research Center, Baylor College of Medicine, Houston, TX.
3
Department of Pediatrics, MassGeneral Hospital for Children, Boston, MA.
4
Division of Nutrition, Physical Activity and Obesity, Centers for Disease Control and Prevention, Atlanta, GA.
5
National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville, MD.

Abstract

OBJECTIVE:

To examine the associations among several body mass index (BMI) metrics (z-scores, percent of the 95th percentile (%BMIp95) and BMI minus 95th percentile (ΔBMIp95) 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).

STUDY DESIGN:

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.

RESULTS:

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 ΔBMIp95. 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 ΔBMIp95 due to differences in age or sex.

CONCLUSIONS:

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.

KEYWORDS:

children; obesity

PMID:
28433203
PMCID:
PMC5572545
[Available on 2018-09-01]
DOI:
10.1016/j.jpeds.2017.03.039
[Indexed for MEDLINE]
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