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

Its report, published this month 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 using BMI not as a way to define obesity, but as a tool to determine who should be tested for excess body fat. It said that people who had a BMI over 25 and too much fat, but who were 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 preclinical obesity.

Other people with any of the 18 medical conditions caused by obesity — 13 for children and adolescents — should get medical treatment to improve their health and prevent serious injury. The conditions include heart failure, hip or knee pain, metabolic abnormalities and poorly functioning organs. Their condition would be called clinical obesity.

The commission said those with a BMI of 40 or greater would have clinical obesity on the basis of their BMI alone; there would be no need to assess their body fat, adding that 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 threshold is 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. (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.”

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