Almost 70% of Americans could be considered obese, under a new proposed method for calculating obesity that goes beyond simply measuring body mass index (BMI). The proposed definition relies on adding waist and body fat measurements to the equation, thereby reclassifying individuals as obese who were previously considered healthy.

The new definition is from the Lancet Diabetes and Endocrinology Commission, which suggests that where fat is stored may be just as important as overall weight. Like other individuals considered obese under the standard BMI definition, those newly classified under the updated definition face similar health risks, researchers found.

The findings were published by researchers at Mass General Brigham in JAMA Network Open.

A New Framework

BMI provides a simple estimate of obesity but doesn’t account for the distribution of body fat. Waist circumference, waist-to-height ratio, and waist-to-hip ratio can provide additional insight by distinguishing fat mass from muscle and accounting for abdominal fat, which is associated with disease risk.

The new framework identifies obesity using two criteria: high BMI plus at least one elevated anthropometric measure, or normal BMI with two elevated anthropometric measures, referred to as “anthropometric-only obesity.” This new standard has already been endorsed by organizations including the American Heart Association and The Obesity Society.

Health Risks Present in Newly Identified Groups

Researchers analyzed participants of the National Institutes of Health’s (NIH) All of Us Research Program, which included more than 300,000 Americans. The new definition classified 68.6% of participants as obese, whereas 42.9% were classified under the BMI-based approach. The increase was entirely attributable to individuals with “anthropometric-only” obesity and was most pronounced among adults aged 70 and older.

“Anthropometric-only” obesity was associated with higher rates of diabetes, cardiovascular disease, and mortality compared to people at a healthy weight. Half of all participants who met the new obesity definition were classified as having clinical obesity, which is defined as obesity-related physical impairment or organ dysfunction. Clinical obesity was only slightly less prevalent in anthropometric-only obesity cases versus BMI-plus-anthropometric obesity.

BMI is limited because it doesn’t account for fat distribution, stated Steven Grinspoon, MD, chief of the metabolism unit in Mass General Brigham’s endocrinology division. “Seeing an increased risk of cardiovascular disease and diabetes in this new group of people with obesity, who were not considered to have obesity before, brings up interesting questions about obesity medications and other therapeutics.”

“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,” said Lindsay Fourman, MD, co-first author and endocrinologist at Mass General Brigham. “Body composition matters, it’s not just pounds on a scale.”