A new study led by Johns Hopkins University researchers has found that nearly all adults in the U.S. who are classified as obese based on their body mass index (BMI) also have too much body fat.
This supports the continued use of BMI in diagnosing obesity, despite growing concerns that it might not always be accurate.
BMI is a simple calculation based on a person’s height and weight. It has long been used by doctors and health organizations to identify people who may be at risk of obesity-related health problems.
But BMI has come under criticism in recent years because it doesn’t tell the difference between fat, muscle, and bone.
For example, a muscular athlete might have a high BMI but very little body fat.
Because of these concerns, experts have recommended that a diagnosis of obesity should also include other measurements, like waist size or direct body fat testing.
To find out how well BMI matches actual body fat levels, the researchers looked at data from 2,225 adults between the ages of 20 and 59 who took part in a national health survey (NHANES) in 2017–2018.
These participants had their height, weight, and waist measured. They also underwent a DEXA scan, a type of body scan that gives a very accurate reading of body fat percentage.
The team defined obesity in most adults as a BMI of 30 or higher. For non-Hispanic Asian participants, they used a slightly lower cutoff of 27.5, based on known health risks in that group. To confirm if someone truly had excess fat, the researchers also looked for at least one of the following: a high waist size, a high body fat percentage, or a very high BMI (40 or above).
The results were clear. Based on BMI alone, 39.7% of participants were classified as having obesity. When researchers added the requirement of excess fat confirmation, the percentage dropped only slightly to 39.1%.
This means that 98.4% of people with obesity based on BMI also had too much body fat, confirming the accuracy of the BMI-based diagnosis for almost everyone.
These findings were consistent across different age groups, genders, and racial or ethnic backgrounds. Even when different ways of confirming excess fat were used, the results were similar. This suggests that for most people, BMI is a dependable tool for identifying obesity.
The study authors point out that while there are special cases—such as very athletic individuals—where BMI might not be the best tool, these cases are rare. For the general public, confirming obesity with extra tests like DEXA scans is usually unnecessary and could make obesity screening more complicated and expensive than it needs to be.
In short, this research shows that despite its limitations, BMI remains a useful and accurate way to identify people with too much body fat. For most adults, doctors can continue to rely on BMI without needing costly or complex extra tests.
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The research findings can be found in JAMA.
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