Why Body Fat Percentage Beats BMI in Measuring Health

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Many people use body mass index (BMI) to determine whether their weight is healthy — but new research suggests BMI may not be the best way to predict your long-term health risks, especially for younger adults.

“Body composition has been shown to be associated with risk for cancer, cardiometabolic diseases like Type 2 diabetes and cardiovascular disease (CVD), as well as mortality,” wrote lead investigator Arch Mainous, Ph.D., professor and vice chair for research in the Department of Community Health and Family Medicine at the University of Florida. “Because of this strong association with increased risk, assessments of body composition have long been integrated into clinical practice to risk stratify patients using categories like overweight and obese.”

However, recent research has pointed to the limitations of using BMI to measure body composition, as it may misclassify more muscular individuals as overweight or obese. Further, some people may have a normal BMI but elevated body fat percentage and may therefore be unaware of their increased risk of developing chronic diseases including CVD and diabetes.

The study followed a large, nationally representative group of U.S. adults enrolled in the National Health and Nutrition Examination Survey (NHANES) over 15 years. Healthy BMI was defined as between 18.5 and 24.9 kilograms over the square of height in meters, or kg/m², and overweight/obese was greater than or equal to 25 kg/m². Unhealthy body fat percentage and waist circumference thresholds were greater than or equal to 27% and greater than or equal to 44% and greater than 40 inches and greater than 35 inches for men and women, respectively. This particular group consisted of 4,252 adults aged 20 to 49 years (2,821 men and 1,431 women).

Results showed that body fat percentage and waist circumference were much better at predicting risk of death, especially from heart disease, than BMI.

People with high body fat percentages had almost double the risk of dying from any cause and over triple the risk of dying from heart disease, even after accounting for other health factors. BMI, on the other hand, wasn’t a reliable predictor once those other factors were considered.

“Although BMI is the most commonly used measure of body composition in primary care, it is an indirect measure of body fat percentage and thus has some limitations,” Mainous wrote.

The study provided important insights into how body fat percentage may be a better predictor of health risk than BMI, but investigators noted it’s also important to understand its limitations. First, body fat thresholds aren’t yet standardized, so unlike BMI, there’s no official agreement on what counts as a “healthy” or “unhealthy” body fat percentage. Next, the study focused only on younger adults, so investigators don’t yet know whether the same findings would apply to older people.

The study focused on mortality (death) as the main outcome and did not include other health outcomes. The team encourages future research to explore how body fat relates to conditions like heart attacks or cancer diagnoses. Additionally, some behaviors, such as smoking status, weren’t fully analyzed. However, while factors like smoking can affect health and body composition, the purpose of this study was to compare body fat to BMI the way doctors currently use them in practice.

“Body fat percentage is an easily measured body composition variable that may have a stronger association with downstream mortality in young adults than BMI,” investigators concluded. “This could ultimately lead to a change in how we measure body composition for risk stratification.”