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How Good is BMI as a Screening Tool for Adiposity?

June 3, 2024

Editor’s Note: Elizabeth Zeichner (she/her) is a former high school teacher. She recently graduated from Emory University School of Medicine and is now a pediatrics resident at the Children’s Hospital of Philadelphia. -Rachel Y. Moon, MD, Associate Editor, Digital Media, Pediatrics

Body mass index (BMI), calculated by weight (kg) divided by height (meters) squared, was first derived in the 1830s by a Belgian mathematician. It was reintroduced in the 1970s as a metric for relative weight and to estimate body fat.

Though BMI has been consistently used over the past 50 years, there is increasing concern about its flaws and limitations. For example, the metric is based on a White/European male population and does not distinguish between fat and lean mass or characterize body fat distribution. A 2023 American Medical Association policy noted concerns surrounding the use of BMI, and the importance of understanding its limitations in providing care for patients.

This week, Pediatrics is early releasing an article, “Screening Accuracy of BMI for Adiposity among 8- to 19-year-olds,” by David S. Freedman, PhD, and colleagues from the CDC, Children’s Hospital of Philadelphia, George Washington University, and Penn State College of Medicine, which looked specifically at how well BMI detects high adiposity (10.1542/peds.2024-065960).

The authors used data from the National Health and Nutrition Survey from 2011 to 2018. They identified 6923 8- to 19-year-olds who were scanned with dual-energy X-ray absorptiometry (DXA), which determines percent body fat (%fat). They identified adiposity through the fat mass index (FMI, fat mass kg ÷ m2) and identified lean mass through the lean mass index (LMI, lean mass ÷ m2).

The authors looked at how well BMI predicted FMI, %fat, and LMI. They found:

  • Participants with a BMI ≥95th percentile (corresponding to the “obesity” category) were 29 times more likely to have a high FMI compared to participants with lower BMIs.
  • 88% of those with BMI ≥95th percentile also had a high FMI.
  • There was not as strong of a relationship between BMI to FMI in lower BMI groups.

In an invited commentary, “Body Mass Index (BMI): Still Going Strong at Age 50”, by Jaime M. Moore, MD, MPH, and Stephen R. Daniels, MD, PhD, MPH, at Children’s Hospital Colorado, the authors reiterate the association between elevated BMI and increased adiposity in youth with BMI ≥95th percentile (10.1542/peds.2024-066370). They acknowledge limitations, such as exclusion of youth younger than 8 years and lack of data on regional distribution of adiposity. They also note that it is important for all of us to use BMI as only one aspect of a holistic patient-centered approach to reduce bias and stigma against patients.

While there remains concern surrounding the use of BMI in a clinical setting, these research findings show that elevated BMI ≥95th percentile can be an important screening tool to identify youth with increased adiposity. However, as we use this information to screen patients, we must also obtain additional history, and work with patients and families to support healthy and sustainable habits.

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