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Title: Longitudinal changes in BMI z-scores among 45,414 2–4-year olds with severe obesity

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 - BOSTON CHILDREN'S HOSPITAL
item GOODMAN, ALYSON - CENTERS FOR DISEASE CONTROL AND PREVENTION (CDC) - UNITED STATES
item BLANCK, HEIDI - CENTERS FOR DISEASE CONTROL AND PREVENTION (CDC) - UNITED STATES

Submitted to: Annals of Human Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2017
Publication Date: 9/27/2017
Citation: Freedman, D.S., Butte, N.F., Taveras, E.M., Goodman, A.B., Blanck, H.M. 2017. Longitudinal changes in BMI z-scores among 45,414 2–4-year olds with severe obesity. Annals of Human Biology. https://doi.org/10.1080/03014460.2017.1388845.
DOI: https://doi.org/10.1080/03014460.2017.1388845

Interpretive Summary: The Centers for Disease Control and Prevention (CDC) growth charts are used to evaluate the weight status of children using body mass index (BMI) z-scores standardized for age and sex. However, these standardized BMIs do not characterize accurately the BMIs among children with severe obesity. The use of these BMI z-scores is particularly problematic in longitudinal and intervention studies. The objective of this study was to examine longitudinal changes in BMI z-scores compared with alternative indices that address the problem with the CDC growth charts in 45,414 2–4-year-olds with severe obesity. Our results showed that BMI z-scores were particularly problematic in longitudinal analyses of children with severe obesity. Changes in BMI z-scores reflected changes in various parameters of the CDC growth charts rather than changes in body size. To overcome these limitations of the CDC growth charts, very high BMIs can be expressed as a percentage of the CDC 95th BMI percentile (%BMIp95); severe obesity has been defined as a %BMIp95>=120. Among children with severe obesity, CDC BMI z-scores should not be used to evaluate their weight status. Changes in BMI z-scores among these children may not accurately reflect changes in body size and could lead to incorrect conclusions by investigators and clinicians.

Technical Abstract: BMI z-scores (BMIz) based on the Centers for Disease Control and Prevention (CDC) growth charts among children do not accurately characterise BMI levels among children with very high BMIs. These limitations may be particularly relevant in longitudinal and intervention studies, as the large changes in the L (normality) and S (dispersion) parameters with age can influence BMIz. To compare longitudinal changes in BMIz with BMI expressed as a percentage of the 95th percentile (%BMIp95) and a modified z-score calculated as log(BMI/M)/S. A total of 45, 414 2–4-year-olds with severe obesity (%BMIp95'>='120). Changes in very high BMIz levels differed from the other metrics. Among severely obese 2-year-old girls, for example, the mean BMIz decreased by 0.6 SD between examinations, but there were only small changes in BMIp95 and modified BMIz. Some 2-year-old girls had BMIz decreases of >1 SD, even though they had large increases in BMI, %BMIp95 and modified BMIz. Among children with severe obesity, BMIz changes may be due to differences in the transformations used to estimate levels of BMIz rather than to changes in body size. The BMIs of these children could be expressed relative to the 95th percentile or as modified z-scores.