A major shift in how obesity is defined could dramatically increase the proportion of U.S. adults classified as obese, raising the figure from about 40% to nearly 70%, according to a new large-scale study published in JAMA Network Open.
The revised definition moves beyond body mass index (BMI) alone and incorporates measures of body fat distribution, particularly abdominal fat, which is strongly linked to metabolic and cardiovascular risks.
The study, conducted by researchers at Mass General Brigham, analyzed health data from more than 300,000 participants enrolled in the National Institutes of Health “All of Us” Research Program. When traditional BMI-based criteria were applied, 42.9% of participants met the threshold for obesity. However, under the updated framework proposed by the Lancet Diabetes & Endocrinology Commission, 68.6% met the criteria.
Unlike BMI, which estimates body fat using only height and weight, the new definition incorporates anthropometric measures such as waist circumference, waist-to-height ratio, and waist-to-hip ratio. These indicators provide insight into where fat is stored in the body, particularly visceral fat around the abdomen, which is associated with higher risks of type 2 diabetes, cardiovascular disease, and premature mortality.
Under the revised framework, individuals can be classified as having obesity in two ways. Those with a high BMI plus at least one elevated anthropometric measure fall into a category termed “BMI-plus-anthropometric obesity.” In addition, individuals with a normal BMI may still be classified as obese if they have two or more elevated anthropometric measures, a category referred to as “anthropometric-only obesity”. The latter group accounts for the entire increase observed under the new definition.
The impact of the revised criteria was particularly pronounced among older adults. Nearly 80% of individuals aged 70 years and above met the new obesity criteria, highlighting the limitations of BMI in aging populations, where muscle mass may decline while abdominal fat increases.
Importantly, the study found that individuals classified under anthropometric-only obesity exhibited significantly higher rates of diabetes, cardiovascular disease, and overall mortality compared with individuals without obesity. Approximately half of all individuals identified under the new criteria were categorized as having “clinical obesity,” defined by obesity-related organ dysfunction or physical impairment.
The findings have major implications for public health policy, clinical screening, and treatment strategies. By identifying high-risk individuals who would previously have been overlooked, the revised definition may allow earlier intervention and more targeted prevention strategies. However, researchers emphasize that further studies are needed to determine the most effective treatment approaches for individuals newly classified under this framework.
Overall, the study underscores a growing consensus in obesity research: body composition and fat distribution matter as much as, if not more than, body weight alone.