Study: BMI ‘very good predictor’ of adiposity in youth
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Key takeaways:
- The positive predictive value between high BMI and high fat mass index ranged from 81% to 90%.
- BMI did not perform as well for predicting body fat percentage, researchers found.
High BMI appears to be a good predictor for high fat mass index in children and adolescents across racial and ethnic groups, according to findings published in The Journal of Pediatrics.
BMI is still commonly used to screen children and adolescents for adiposity, but it has come under scrutiny because it does not distinguish fat mass from lean mass, according to David S. Freedman, PhD, a retired researcher from the CDC, and colleagues. They also wrote that previous research suggested that BMI may be less accurate in screening for adiposity among Black and Asian people compared with white people, and AAP guidelines from 2023 caution that BMI may not be the best screening tool for some racial and ethnic groups.
“We previously showed that a high BMI is a very good indicator of a high fat mass index (FMI)... among 8- to 19-year-olds,” the authors wrote. “Although differences in adiposity across racial and ethnic groups have been examined in these data, we focus on the screening ability of BMI in each group.”
Freedman and colleagues studied data from the 2011-2012 to 2017-2018 National Health and Nutrition Examination Survey cycles on 6,454 children and adolescents aged 8 to 19 years (48% girls). They used FMI— fat mass divided by height squared — and body fat percentage to define adiposity. Freedman and colleagues wrote that they believe FMI is a better indicator of adiposity because a high body fat percentage could be due to high fat mass or low lean mass. Then they evaluated BMI’s association with FMI and body fat percentage to see whether BMI could accurately define adiposity.
Freedman and colleagues defined a high BMI as greater than or equal to the CDC’s 95th percentile for age. High FMI, body fat percentage and lean body mass index (LMI) were defined by the 80th percentile within the study sample.
Overall, the BMI z-score was 0.53 for boys and 0.61 for girls. One-fifth of participants had a high BMI. Black children had the lowest FMI and body fat percentage and the highest LMI compared with other racial and ethnic groups, whereas Asian children had the highest FMI and body fat percentage and the lowest LMI.
Among participants with high FMI, high BMI accurately predicted high FMI for 80.8% of Black children, 84.1% of Asian children, 87.4% of white children and 89.8% of Hispanic children; the percent agreements were between 93% and 96%. In contrast, high BMI had a positive predictive value for high body fat percentage ranging from 66.2% for Black children to 76.7% for Hispanic children. Additionally, the positive predictive value of high BMI for high LMI was 74.3% for Asian children, 75.4% for Hispanic children, 76.5% for white children and 89.1% for Black children.
“Our findings indicate that a high BMI is a very good predictor of high FMI among Asian, Black, Hispanic and white 8- to 19-year olds,” Freedman and colleagues wrote. “BMI was a moderate predictor of body fat percentage because fat mass (positively) and lean mass (inversely) influence this proportion. Because fat and lean mass levels differ by race and ethnicity, the use of body fat percentage exaggerates the group differences in the screening ability of BMI.”
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