- Posted October 16, 2025
Millions More Americans Might Be Obese Under New Definition, Experts Say
A new definition of obesity could dramatically increase the number of Americans considered obese.
Under the new definition, the prevalence of obesity rose from around 40% to nearly 70% among more than 300,000 people participating in a long-term health study, researchers reported Oct. 15 in JAMA Network Open.
The new definition takes into account additional measures of excess body fat rather than just relying on body mass index (BMI). BMI is an estimate of body fat based on height and weight.
“We already thought we had an obesity epidemic, but this is astounding,” said co-lead researcher Dr. Lindsay Fourman, an endocrinologist at Mass General Brigham in Boston.
“With potentially 70% of the adult population now considered to have excess fat, we need to better understand what treatment approaches to prioritize,” she said in a news release.
Evidence shows that the new definition is on the money, when it comes to health problems resulting from obesity, researchers said.
The people newly classified as obese had a higher risk of diabetes, heart disease and premature death than people without obesity.
“Identifying excess body fat is very important, as we’re finding that even people with a normal BMI but with abdominal fat accumulation are at increased health risk,” Fourman said. “Body composition matters – it’s not just pounds on a scale.”
The new definition could increase the number of people eligible for insurance coverage of weight-loss surgery or GLP-1 drugs like Ozempic, Wegovy and Zepbound, noted Dr. Armando Castro-Tie, chair of surgery and a bariatric surgeon at South Shore University Hospital in New York.
“In the short term, more patients would qualify for certain treatments that they [insurance companies] are now going to be responsible for, as it is their duty to provide these patients coverage for these different types of treatment,” Castro-Tie, who was not involved in the study, said in a news release.
Up to now, obesity has been defined a person’s BMI, which is calculated based on a person’s weight and height.
However, BMI can’t differentiate weight from fat and weight from muscle, making it a less-than-ideal measurement. For example, bodybuilders might be considered obese on a BMI chart even though their excess weight comes from muscle.
In the meantime, other measures like waist circumference, waist-to-height ratio and waist-to-hip ratio have been developed to more accurately assess people’s body fat and calculate obesity, researchers said.
Under the new definition, a person is classified as obese if they have a high BMI plus an elevated reading from one of these newer measures of body fat. A person can also be considered obese if they have a normal BMI but at least two elevated results from the newer measures.
At least 76 organizations have endorsed this new definition, including the American Heart Association and the Obesity Society, researchers noted.
For this new study, researchers analyzed data from the National Institutes of Health’s All of Us Research Program, which is tracking the health of more than 300,000 Americans.
Under the new definition, about 69% of the participants are obese, compared to 43% when looking at BMI alone, researchers found.
The increase was entirely driven by people who qualified as obese based on the newer measures of body fat, researchers said.
The study also found that people classified as obese by the newer measures had a higher risk of diabetes, heart disease and death than people who weren’t obese.
The definition distinguishes between clinical obesity — the presence of physical impairment or organ problems related to excess weight — and preclinical obesity, similar to the difference between clinical type 2 diabetes and prediabetes.
People with clinical obesity under the new definition were six times more likely to develop diabetes or heart disease, and nearly three times more likely to die, the study found.
Those with preclinical diabetes were three times more likely to develop diabetes and 40% more likely to develop heart disease.
“We have always recognized the limitations of BMI as a single marker for obesity because it doesn't take into account body fat distribution,” senior researcher Dr. Steven Grinspoon said in a news release. He’s chief of the Metabolism Unit in the Endocrinology Division at Mass General Brigham.
“Seeing an increased risk of cardiovascular disease and diabetes in this new group of people with obesity, who were not considered to have obesity before, brings up interesting questions about obesity medications and other therapeutics,” he said.
Castro-Tie said the new definition will help get care to the people who need it, helping them lose excess weight before they develop chronic health problems associated with obesity.
“I think it's well overdue in terms of what really defines obesity and how you can really do better work understanding who these patients are,” Castro-Tie said. “Then we can work more targeted in terms of what approach really works for these different levels of obesity at these different population groups."
More information
The World Obesity Federation has more on the new definition of obesity.
SOURCES: Mass General Brigham, news release, Oct. 15, 2025; Northwell Health, news release, Oct. 15, 2025; JAMA Network Open, Oct. 15, 2025
