New Study Says 1 In 4 People With Normal BMI Actually Have Clinical Obesity

When you get your BMI checked at a routine physical and the number comes back in the "normal" range, you likely breathe a small sigh of relief. But a new study suggests that number may be giving millions of Americans a false sense of security.

Research from Keck Medicine of USC found that one in four people with a normal BMI actually meet the criteria for clinical obesity. And among those classified as "overweight" by BMI, 50% would be reclassified as clinically obese under this definition.

BMI has been the standard tool to measure obesity for decades, but that doesn't mean it exists without controversy. The measurement itself is simple: divide your weight in kilograms by your height in meters squared, and you get a number that slots you into one of four categories: underweight, normal, overweight, or obese.

The problem is that BMI only measures total body weight relative to height. It has no way of distinguishing between muscle, bone, and fat, and the proportions of these things matter.

"A muscular person can have a very high BMI but not have excess fat, while someone without much muscle can have a normal BMI but have excess fat causing health problems," said Brian P. Lee, MD, MAS, a hepatologist with Keck Medicine of USC and the study's principal investigator, in a press release.

Clinical obesity is a term developed in 2025 by the Lancet Diabetes and Endocrinology Commission1, an international group of obesity experts. Rather than relying on height and weight alone, it specifically examines fat around the waist, known as adipose fat, and whether that fat is contributing to health problems.

Why does abdominal fat matter so much? Adipose fat accumulates deep in the abdomen, where it causes dangerous inflammation in the body's organs. This is different from subcutaneous fat, the kind found just under the skin in other parts of the body, which carries far less metabolic risk.

Researchers analyzed data from approximately 5,600 adults in the National Health and Nutrition Examination Survey,2 a nationally representative survey of health and nutrition. The average age of people surveyed was 49, and all of the adults had been tested for both BMI and hip and waist circumferences.

Approximately 26% of people with a normal BMI qualified as clinically obese. Among those with an overweight BMI, that number jumped to 50%.

Obesity isn't just a number on a scale—it's a driver of serious, preventable disease. Obesity can lead to heart disease, diabetes, high blood pressure, high cholesterol, liver disease, and certain cancers, making it one of the leading causes of preventable death in the country.

The concern with BMI-based screening is that people who fall into the "normal" or "overweight" categories may be carrying abdominal fat that is quietly driving inflammation and organ damage, without ever being identified as at risk. They do not currently qualify for pharmacologic or surgical obesity treatments, and their physicians may not flag them as needing the counseling, lifestyle modifications, or early interventions that could make a real difference.

Lee hopes that more physicians will begin diagnosing obesity using the clinical obesity definition rather than BMI alone, to better inform patients of any health risks they may be facing.

The findings suggest that BMI alone may be missing millions of Americans who could benefit from earlier intervention.