A new way of defining obesity could dramatically change how many Americans are considered to have the condition.
According to research from Mass General Brigham, applying updated obesity criteria from The Lancet Diabetes and Endocrinology Commission increases the number of adults classified as obese from 40% to nearly 70%.
The findings were published in JAMA Network Open and are raising urgent questions about public health priorities.
Traditionally, obesity has been measured using the body mass index (BMI), a formula based on weight and height. But BMI has long had its critics, since it doesn’t differentiate between fat and muscle and doesn’t account for where fat is stored in the body.
The new definition adds other body measurements—such as waist circumference, waist-to-hip ratio, and waist-to-height ratio—to better understand body fat distribution. This broader approach gives a more complete picture of someone’s health risks.
Under the new guidelines, someone is considered to have obesity if they either have a high BMI plus at least one elevated body measurement (called “BMI-plus-anthropometric obesity”) or if they have a normal BMI but at least two elevated body measurements (called “anthropometric-only obesity”).
The guidelines also separate people with “clinical obesity,” which means their excess fat is already affecting their organs or physical ability, from those with “preclinical” obesity who may be at risk but not yet impaired.
In their study, researchers analyzed data from over 300,000 adults in the National Institutes of Health All of Us Research Program. They found that 68.6% of adults met the new obesity definition, compared to 42.9% using BMI alone. The biggest increase came from people who would not have been classified as obese before—those with normal BMI but abnormal fat distribution.
This shift in classification was especially pronounced among older adults. Nearly 80% of people over 70 fell into one of the new obesity categories. The prevalence also varied by sex and race.
What’s more, the study found that people with anthropometric-only obesity had a significantly higher risk of developing serious health problems—including type 2 diabetes, heart disease, and even early death—compared to those with no signs of obesity.
These individuals had health risks nearly as high as those in the BMI-plus-anthropometric obesity group, despite not meeting the old definition of obesity.
“This is astounding,” said Dr. Lindsay Fourman, co-lead author of the study and an endocrinologist at Mass General Brigham. “We already thought we had an obesity epidemic. With this new framework, almost 70% of adults now qualify, and we need to think more carefully about treatment priorities.”
At least 76 health organizations, including the American Heart Association, support the new definition. The researchers say that understanding who is at risk is the first step toward better care and more targeted treatments. They also emphasize that “body composition matters”—it’s not just about how much a person weighs, but where that fat is located.
The team is now planning future studies to explore how different therapies may help people who fall into the newly defined obesity group. They’ve already developed treatments that can reduce waist size and are looking into how these options might help those with dangerous fat accumulation but normal BMIs.
Dr. Steven Grinspoon, senior author of the study, says that this research shows the limits of using BMI alone and the importance of using better tools to spot hidden health risks.
“Seeing increased risks of disease in people who weren’t classified as obese before makes it clear—we need to rethink how we identify and treat obesity,” he said.
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The study is published in JAMA Network Open.
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