According to a new study from Mass General Brigham, the use of a revised, expanded definition of obesity could increase the share of American adults classified as having obesity from about 40% to nearly 70%. The results were published in The Lancet Diabetes & Endocrinology Commission.

Traditionally, obesity has been defined using body mass index (BMI) — a ratio of weight to height. But the new model takes into account not only BMI, but also fat distribution — indicators such as waist circumference, waist-to-height ratio, and waist-to-hip ratio.

The researchers analyzed data from more than 300,000 people in the All of Us cohort. Under the old model, 42.9% of participants met the criteria for obesity, while under the new definition the number rose to 68.6%. The increase was especially pronounced among older adults: nearly 80% of people over age 70 would be classified as having obesity under the updated criteria.

Importantly, many individuals who did not previously meet the BMI threshold were found to have “anatomical obesity” — a normal BMI but elevated anthropometric measurements. These individuals also showed higher risks of diabetes, cardiovascular disease, and mortality compared to people who do not meet the criteria under the new model.

The authors note that the revised approach aims to more accurately identify excess fat that BMI alone may fail to detect. They emphasize that the body is not just a number on the scale, and fat distribution matters.

The next step will be studying how the new classification can improve prevention and treatment strategies for people who were not previously considered to have obesity.