The way scientists define obesity is changing, and it could dramatically shift how Americans think about weight and health.
Researchers at Mass General Brigham found that under a new definition expanding beyond body mass index (BMI), nearly 70% of U.S. adults would qualify as having obesity, compared with about 40% under the traditional measure. The updated framework, introduced earlier this year by the Lancet Diabetes and Endocrinology Commission, incorporates not just weight and height but also how fat is distributed throughout the body.
“We already thought we had an obesity epidemic, but this is astounding,” said Dr. Lindsay Fourman, an endocrinologist at Mass General Brigham and co-first author of the new study published in JAMA Network Open. “With potentially 70% of the adult population now considered to have excess fat, we need to better understand what treatment approaches to prioritize.”
The research team analyzed data from more than 300,000 participants in the National Institutes of Health’s All of Us program. Under the new definition, obesity rates rose most sharply among older adults, affecting nearly 80% of people over age 70.
Unlike BMI, which only compares weight to height, the new system adds measures such as waist circumference and waist-to-hip ratio to capture fat distribution, a better indicator of metabolic risk. Many people with a “normal” BMI but higher abdominal fat would now fall under the obesity category.
That shift isn’t just a matter of labeling. The study found that people newly classified as having obesity were also more likely to develop diabetes, heart disease and other metabolic conditions than those with lower body fat.
“We have always recognized the limitations of BMI as a single marker for obesity because it doesn't take into account body fat distribution,” said Dr. Steven Grinspoon, chief of the Metabolism Unit at Mass General Brigham and senior author of the study. “Body composition matters — it’s not just pounds on a scale.”
The new criteria could prompt changes in how obesity is screened and treated, from medication eligibility to early intervention programs. For now, the authors stress that the findings underscore the importance of looking beyond the scale when assessing health.
“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.
This research was supported by the National Institutes of Health, the American Heart Association and the Robert Wood Johnson Foundation.