A new definition of obesity released earlier this year by the Lancet Diabetes and Endocrinology Commission could dramatically increase the number of Americans classified as obese. Researchers at Mass General Brigham found that under the updated criteria—which include measures of body fat distribution alongside the traditional body mass index (BMI)—the prevalence of obesity jumped from about 40 per cent to nearly 70 per cent in a study of more than 300,000 adults. The increase was especially notable among older individuals, and those newly classified as obese also faced higher risks of poor health outcomes. The findings were published in JAMA Network Open.
Dr Lindsay Fourman, co-first author and endocrinologist at Mass General Brigham, described the results as “astounding,” noting that the United States may now have an even greater obesity epidemic than previously understood. “With potentially 70 percent of the adult population now considered to have excess fat, we need to better understand what treatment approaches to prioritise,” she said.
Traditionally, obesity has been assessed using BMI, a simple ratio of weight to height. However, this measure fails to distinguish between fat and muscle mass or to account for where fat is stored in the body. The new framework expands on BMI by incorporating other anthropometric measures, such as waist circumference, waist-to-height ratio, and waist-to-hip ratio, which can better reflect unhealthy fat distribution.
According to the updated definition, a person is considered obese if they have a high BMI plus one elevated measure of body fat (“BMI-plus-anthropometric obesity”), or if they have a normal BMI but at least two elevated fat distribution measures (“anthropometric-only obesity”). The definition also distinguishes “preclinical” obesity from “clinical” obesity, the latter involving physical impairments or organ dysfunction associated with excess fat. Over seventy organisations, including the American Heart Association and The Obesity Society, have endorsed this new framework.
The researchers analysed data from over 300,000 participants in the National Institutes of Health’s All of Us Research Program. Under the new definition, obesity prevalence rose to 68.6 per cent, compared to 42.9 per cent using BMI alone. The change was primarily due to individuals classified as having anthropometric-only obesity, with obesity rates reaching nearly 80 per cent among adults over 70. Crucially, these newly identified individuals also faced higher risks of diabetes, cardiovascular disease, and early death compared to people without obesity.
Senior author Dr Steven Grinspoon emphasised that BMI has long been a limited tool for understanding body composition. The discovery that people with normal BMI but high body fat have elevated health risks raises new questions about treatments and preventive care. The team plans further research into therapies targeting fat distribution, including one that reduces waist circumference. As Dr Fourman explained, “Body composition matters—it’s not just pounds on a scale.”
More information: Lindsay Fourman et al, Implications of a New Obesity Definition Among the All of Us Cohort, JAMA Network Open. DOI: 10.1001/jamanetworkopen.2025.37619
Journal information: JAMA Network Open Provided by Mass General Brigham
