Much like the old saying goes, looks can be deceiving. Research out today shows that a substantial number of people with so-called “normal” body mass index still have health problems related to obesity.
Scientists at the University of Southern California studied nationally representative data on Americans and found roughly a quarter of people with normal BMI and half of people with an overweight BMI still met the standard for obesity under a recently developed criteria. As a result, the researchers say, many of these people likely aren’t getting the optimal care they need.
“Since most doctors are focused on BMI to screen for obesity-related conditions, this is a population that might not be getting enough attention for screening and treatment of obesity-related conditions,” lead study author Brian Lee, a hepatologist at Keck Medicine of USC, told Gizmodo.
Clinical obesity
BMI is calculated using a person’s weight and height, with obesity classically defined as having a BMI of 30 and over. Lately, though, some doctors and patients have advocated for a more nuanced meaning. Since obesity is actually about having too much excess fat, they argue, simply relying on BMI can miss some skinnier people who would otherwise fit the bill. Likewise, people who are especially tall or muscular can be characterized as obese despite not having excess fat.
In January 2025, a large team of obesity experts around the world, backed by the Lancet, created a new standard for measuring obesity. The new criteria doesn’t abandon BMI but asks doctors to use other bodily measurements, either alongside BMI or in place of it. These are waist circumference, waist-to-hip ratio, waist-to-height ratio, and the DEXA scan, a test that directly measures a person’s bone density and body fat.
The criteria also distinguishes between two different stages of obesity, based on whether it’s actively harming a person. Clinical obesity is defined as having obesity that’s likely tied to health issues, such as tissue damage or conditions strongly linked to obesity like high cholesterol, type 2 diabetes, or chronic knee pain. Pre-clinical obesity means you don’t have these problems, at least not yet.
Lee and his team wanted to see how many Americans would be re-classified under these new definitions. So they examined data from the National Health and Nutrition Examination Survey (NHANES), a nationally representative poll of Americans’ dieting and lifestyle habits run by the Centers for Disease Control and Prevention. They focused on a subset of middle-aged people who underwent more extensive screening tests in addition to BMI, like hip and waist circumference.
Overall, 78% of Americans in general were considered obese under the new criteria, the team found. And around 26% of people with normal BMI (between 20 and 25) met the definition for clinical obesity, while the same was true for just over 50% of people who were overweight (a BMI between 25 and 30). The team’s findings, published Monday in the Annals of Internal Medicine, line up with other recent research showing that many people with normal BMI are likely obese.
“Clinical obesity is common even among people with normal BMI,” Lee said.
What does this mean?
Traditionally, diet and exercise have been used to help people lose weight and/or excess fat, though not with much sustained success. Newer medications like semaglutide (Ozempic and Wegovy) and other GLP-1s have made it significantly easier to lose weight in recent years. But GLP-1s and other reliable interventions like bariatric surgery have been limited to those who meet the classic criteria for obesity, and it’s not clear yet just how effective any of these treatments would be at helping people with normal BMI.
“An important question is whether treatment of excess adiposity with normal body mass index can lead to a decrease in obesity-related conditions and poor outcomes,” Lee said. “Prospective studies and randomized controlled trials to understand how best to treat these patients can fill this knowledge gap.”
All that said, it’s still valuable to know whether you might have too much body fat, even if your BMI seems fine, and to check in with your doctor if so. Though it’s not quite as seamless as having your weight and height memorized, these other measurements aren’t hard to carry out either, Lee notes. The researchers have also published a guide to evaluating these measurements for yourself.
“Using a tape measure to measure your waist, hips, and height is an easy way to see if you have excess adiposity that could be harmful to your health,” Lee said.
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