New Obesity Definition Challenges Current Use of B.M.I.

Obesity should be assessed in a way that goes beyond the standard measure of body mass index, or B.M.I., according to a new definition of the condition released by an international commission.

Its report, published Tuesday in The Lancet Diabetes & Endocrinology journal, makes the case for focusing on how much body fat and what medical complications the person has, not just their weight.

If the guidelines are widely adopted, they could change doctors’ perceptions of who needs to be treated for obesity. They could also affect the use of prescription drugs that treat obesity, like Wegovy and Zepbound. The new definition of obesity was endorsed by 76 organizations around the world.

The commission proposed that instead of continuing the current use of B.M.I. — as a way to define obesity — it should be a screening tool to determine who should be tested for excess body fat.

The commission said that people who had a B.M.I. over 25 and too much fat, but who are otherwise healthy, should pretty much be left alone. They should be monitored and counseled not to gain any more weight and possibly to lose some.

Their condition would be called pre-clinical obesity.

Other people with any of the 18 medical conditions caused by obesity — 13 for children and adolescents — require medical treatment to improve their health and prevent serious injuries to organs. The conditions include breathlessness, heart failure, hip or knee pain, metabolic abnormalities and poorly functioning organs.

Their condition would be called clinical obesity.

The group said those with a B.M.I. of 40 or greater have clinical obesity on the basis of their B.M.I. alone — there is no need to assess their body fat.

The commission said it did not know the prevalence of the two types of obesity.

The simplest way for doctors to see whether someone has excess body fat is to wrap a tape measure around a person’s waist, the group said. If a woman’s waist is more than 34.6 inches, she most likely has too much fat. For a man, a waist the threshold would be at least 40 inches.

Other tools for health professionals include waist-to-hip ratios, waist-to-height ratios or DEXA scans, a type of X-ray.

The commission’s 58 experts spent years on the report, regularly meeting online. Instead of thinking of obesity as a disease, they wanted to assess it in a different way, said the commission’s chair, Dr. Francesco Rubino, a bariatric surgeon at King’s College London. (Dr. Rubino consults for makers of obesity drugs and medical devices.)

The commission’s approach fits with that of the American Heart Association, which endorsed the report.

“We struggled with the imprecise methods for defining what is a bad weight,” said Dr. Mariell Jessup, chief scientific and medical officer at the association. “How do you define an ideal weight and how do you define a sick weight?”

“We were asked many times, Do you think obesity is a chronic disease? We were unhappy saying ‘yes’ or ‘no,’” she said. “We think it’s more nuanced.”

Rebecca Puhl, deputy director of the University of Connecticut’s Rudd Center for Food Policy and Health, said she saw the commission’s approach as “trying to reduce some misconceptions about obesity that could potentially reduce stigma.”

“Obesity continues to be viewed as a character flaw rather than a complex health condition,” she said.

But it might be difficult for the new definitions to come into common use.

For years, experts have complained about the reliance on B.M.I. to define overweight and obesity.

The index is easy to measure — all that is needed is a person’s height and weight. It soon became the standard for defining whether people were underweight, overweight or obese.

And B.M.I. is established as a major risk factor in diabetes, heart disease, cancer and other conditions, said Dr. David M. Nathan, the professor of medicine at Harvard and founder of the diabetes center at Massachusetts General Hospital.

He added that a large waist, too, was a risk factor. But unlike B.M.I., waist measurements are often performed incorrectly in medical settings.

It is unrealistic, Dr. Nathan said, “to say the whole world is going to change to this.”

Just as unrealistic, he continued, is saying obesity shouldn’t be treated until complications arise. “As they would have it, you wouldn’t treat hypertension until the person has a stroke,” Dr. Nathan said.

Although not everyone who is obese develops a serious health problem, “the number who don’t develop some sort of weight-related complication is pretty small,” Dr. Nathan added.

Using the new standard may also have implications for the new obesity drugs and others headed to the marketplace. They are so expensive that some health systems that initially covered them for people with obesity — defined by their B.M.I.s — decided they could no longer afford to do so.

But Dr. David Cummings, a commission member and an obesity expert at the University of Washington, suggested they could be restricted to patients diagnosed with clinical obesity.

That group’s needs, he said, “are more compelling.”