Utility of Body Mass Index in Identifying Excess Adiposity in Youth Across the Obesity Spectrum

Abstract

Objectives: To determine the proportion of youth within a given body mass index (BMI) obesity category with excess adiposity using dual energy x-ray absorptiometry (DXA). Furthermore, to examine whether mean differences in cardiometabolic risk factors based upon various excess adiposity cutpoints were present.

Study design: DXA data from the National Health and Nutrition Examination Survey 1999-2006 (n = 10 465; 8-20 years of age) were used for this analysis. Obesity categories were defined using Centers for Disease Control and prevention definitions for age and sex. Excess adiposity was defined using cohort-specific cutpoints at 75th, 85th, and 90th percentiles of DXA body fat (%) by age and sex using quantile regression models. Additionally, we examined differences in cardiometabolic risk factors among youth (BMI percentile >85th) above and below various excess adiposity cutpoints.

Results: Nearly all youth with class 3 obesity (100% male, 100% female; 97% male, 99% female; and 95% male, 96% female; using the 75th, 85th, and 90th DXA percentiles, respectively) and a high proportion of those with class 2 obesity (98% male, 99% female; 92% male, 91% female; and 76% male, 76% female) had excess adiposity. Significant discordance was observed between BMI categorization and DXA-derived excess adiposity among youth with class 1 obesity or overweight. Elevated cardiometabolic risk factors were present in youth with excess adiposity, regardless of the cutpoint used.

Conclusions: BMI correctly identifies excess adiposity in most youth with class 2 and 3 obesity but a relatively high degree of discordance was observed in youth with obesity and overweight. Cardiometabolic risk factors are increased in the presence of excess adiposity, regardless of the cutpoint used.

Trial registration: ClinicalTrials.gov NCT02496611.

Keywords: NHANES; adiposity; children; dual energy x-ray absorptiometry; obesity.

Conflict of interest statement

Conflict of interest: Dr. Kelly serves on pediatric obesity advisory boards (Takeda; Novo Nordisk) and serves as the signatory author for a pediatric obesity clinical trial sponsored by Novo Nordisk but does not receive personal or professional income for these activities. Dr. Kelly receives research support in the form of drug/placebo from Aztra Zeneca. The other authors declare no conflicts of interest. No honorarium, grant, or other form of payment was given to anyone to produce the manuscript.

Copyright © 2016 Elsevier Inc. All rights reserved.

Figures

Figure 1

Curves for DXA body fat (%) for a given age (years) for females (A) and males (B) and by BMI-percentile (the vertical dashed lines demarcate the 85th and 95th BMI percentiles) for females (C) and males (D). Based on splines with df=6. Each plot contains lines that demarcate the 10th, 25th, 50th (median), 75th, 85th, and 90th percentiles for DXA body fat (%) for each age 8 to 20.

Figure 2

Proportion of individuals within a given BMI-based obesity category with excess adiposity at three cut-points (75th, 85th, and 90th percentile) determined by DXA body fat (%).