BMI still an effective screening tool in youth despite its limitations, researchers find

Key takeaways:

• Those with a high BMI were 29 times more likely to have a high fat mass index, or FMI, than those with lower BMI.
• Age and BMI accounted for 90% to 94% of the variability of FMI and lean mass index.

BMI remains an effective tool for measuring body fat in youth, despite relying only on weight and height, according to research published in Pediatrics.

Although long used as a screening tool, BMI has drawn criticism for not being able to distinguish between fat and lean mass and failing to characterize body fat distribution, according to the authors.

“BMI is based only on weight and height, which can be a poor indicator of adiposity among those with normal or relatively low adiposity,” they wrote. “Further, some investigators have concluded that BMI has low sensitivity for detecting high adiposity, and there are differences in the relation of BMI to adiposity by racial and ethnic identity.”

The researchers reviewed data from 6,923 youth aged 8 to 19 years who participated in the National Health and Nutrition Survey from 2011 through 2018. Study participants were measured by dual-energy x-ray absorptiometry (DXA), and researchers gauged the ability of BMI to predict DXA-calculated adiposity and lean mass. Adiposity was expressed as fat mass index (FMI) and percentage of body fat — abbreviated as %fat — whereas lean mass was expressed as lean mass index (LMI). They then used regression models and 2x2 tables to assess the relation of BMI to FMI, %fat and LMI.

Researchers found that participants with a high BMI were 29 times more likely to have a high FMI than those with lower BMI. Age and BMI accounted for 90% to 94% of the variability of FMI and LMI in each sex. Of participants with a high BMI, 88% had a high FMI, and 76% had a high %fat.

A high BMI in childhood and adolescence is associated with adverse levels of cardiovascular disease risk factors, the initial stages of atherosclerosis and a high BMI in adulthood, the researchers noted, and the findings provide further evidence of the utility of BMI in research and clinical care.

“Although the prediction was not perfect, a BMI [greater than or equal to the CDC’s] 95th percentile was a very good indicator of a high FMI and a good indicator of high %fat,” they wrote.

In an accompanying editorial, Jaime M. Moore, MD, MPH, and Stephen R. Daniels, MD, PhD, MPH, both from the Children’s Hospital of Colorado, noted that in the 50 years of its existence, BMI has been critiqued as a “rudimentary, antiquated metric with a major flaw being its detachment from a direct measure of adiposity.”

“Newer approaches to body composition analysis, including digital anthropometry and associated machine learning (ie, generating a three-dimensional model of the body using a digital scanner and pairing with a database that matches the body shape/measurements to body composition), aim to be fast, accessible, cost-effective and personalized,” Moore and Daniels wrote. “But to date, none have proven better than BMI in the clinical setting.”

They added that the study reinforced the clinical utility of BMI, particularly in identifying increased adiposity in the obesity range.

“Overall, on the basis of these results, pediatricians can feel confident that identification of elevated BMI continues to be an effective way to screen for increased adiposity,” they wrote. “Elevated BMI in childhood persists into adulthood in four out of five children and is strongly linked to the development of numerous, life-limiting cardiometabolic complications.”

References:

Freedman DS, et al. Pediatrics. 2024;doi:10.1542/peds.2024-065960.

Moore JM, et al. Pediatrics. 2024;doi:10.1542/peds.2024-066370.