Experts Propose a New Definition of Obesity—and It Goes Beyond BMI

More than 100 million American adults have obesity, a condition that’s linked to chronic health complications like heart disease and type 2 diabetes. But an international team of researchers is now arguing that the medical community is defining obesity all wrong—and that it’s time for a change.

That’s the major takeaway from a new scientific report published last week in The Lancet Diabetes & Endocrinology. In the report, the global commission argues that the standard way of assessing obesity, by body mass index or BMI, isn’t the most accurate way of determining whether someone has it and its associated health risks. The report also makes the case for other calculations that could help doctors make a more accurate diagnosis.

BMI has been a controversial way of calculating obesity for years. Still, the 58 commission members aren’t advocating for abolishing it altogether, but instead suggest changing its role.

The report and proposed changes have been endorsed by 76 organizations around the world, according to the paper.

Here’s what the report says—plus, what doctors who treat patients with obesity think of the proposed changes.

“Obesity is defined as a complex, chronic disease characterized by an excessive accumulation of body fat that impairs health,” Katherine N. Balantekin, PhD, RD, an assistant professor in the Department of Exercise and Nutrition Sciences at the University at Buffalo, told Health.

The current way of defining obesity is by BMI, which is a calculation of a person’s body weight in kilograms divided by the square of their height in meters. If an adult has a BMI of 30 or above, they are considered to have obesity, based on this measurement.

The problem is that BMI doesn’t break down how much body fat someone has—it just looks at their height compared to their weight, Francesco Rubino, MD, lead report author and chair of metabolic and bariatric surgery at King’s College London, told Health. As a result, people who have high levels of muscle mass, like elite athletes, may end up meeting the criteria for obesity even though they have low amounts of body fat.

Telling someone that they likely have obesity based on a BMI calculation doesn’t fit the bill, Rubino said. “You need to be certain,” he stressed. “And we aren’t with obesity right now based on BMI.”

Richard Siegel, MD, endocrinologist and co-director of the Diabetes and Lipid Center at Tufts Medical Center, agreed that BMI can be flawed. “BMI is really easy to calculate in the clinic, and it will misdiagnose some people,” he told Health. 

The report suggests using BMI as a screening tool instead of a way to diagnose obesity. Under the new recommendation, people who meet the criteria for obesity based on BMI would be screened for excess body fat using tools and measurements like waist-to-hip ratios, waist-to-height ratios, or a form of X-ray called a DEXA (dual-energy x-ray absorptiometry) scan.

People who are prescreened and have a BMI of more than 25 and no health issues would be monitored and counseled on how to avoid gaining more weight, based on the report, which called this diagnosis “pre-clinical obesity.”

To be diagnosed with clinical obesity, people would need to have both excess body fat (confirmed by a BMI at or near 30 and at least another measure such as high waist circumference) and any of the medical conditions caused by obesity, including heart failure, hip or knee pain, and abnormal organ function.

However, people with a BMI of 40 or higher can be given an obesity diagnosis based on that calculation alone, the researchers said.

Body fat also plays a role, with the researchers noting that a woman with a waist measurement of more than 34.6 inches likely has excess body fat, while men with a waist circumference of at least 40 inches also likely have too much body fat.

Currently, no formal changes regarding obesity assessment have been made. The recommendation “is not binding in any way right now,” Balantekin said. “If doctors do adopt this, it likely won’t be in the near future as these changes always take time.”

This new approach can also make diagnoses harder and more time-consuming for healthcare providers, which makes some doctors less likely to use it, Kesavarapu said. “Most people have a 15-minute appointment,” she said. “From a physician standpoint, that makes things more challenging. But we have to be better about addressing all of these things.”

Another potential challenge with the new diagnostic criteria is getting people the treatment they need, including access to weight loss medications. Siegel said insurance companies usually base coverage on BMI. “We will have to see if insurance will change, too—and that will probably take several years,” he added.

Still, some doctors have already begun shifting how they diagnose obesity, Srivastava said. In addition to BMI, they are also considering criteria like waist circumference, body fat percentage, and obesity-related health conditions like type 2 diabetes, cardiovascular disease, and metabolic syndrome.

Experts advocated for more doctors to transition to this line of thinking. “In the majority of people, BMI can diagnose obesity, but this is trying to see if something better can be done,” said Siegel, referring to the commission’s recommendations.

Adopting the commission’s suggested obesity definition could help spark more open conversations with patients about weight loss, he added. While some relatively healthy people with excess body fat are interested in losing weight—others may not be. “There are a number of people with excess body weight and they’re perfectly happy,” Siegel said.

Ultimately, adopting the suggested changes would help people get a more accurate diagnosis and subsequent treatment, Keerthana Kesavarapu, DO, an associate professor of medicine who focuses on nutrition and obesity sciences at Rutgers Robert Wood Johnson Medical School, told Health. “This will be better for patients,” she said. “This could be very good for everyone in the long run.”