LOS ANGELES — Could the way doctors traditionally measure obesity allow a subset of people with obesity-related health risks slip through the cracks?
Medical experts have typically calculated obesity using body mass index (BMI), a calculation based on a person's height and weight.
In the last year and a half, however, some practitioners have shifted away from screening for obesity through BMI and have instead adopted a newer measure known as clinical obesity, which identifies obesity based on fat around the abdomen coupled with weight-related health issues.
A new study from Keck Medicine of USC , published today in the Annals of Internal Medicine, quantifies how often BMI may miss cases of obesity. The research shows that one quarter of people considered to have a normal (healthy) BMI meet the standard of clinical obesity. And 50% of those deemed overweight by their BMI would be reclassified as obese.
"Many people assume that if their BMI says they are not obese, they don't have to worry about the many health problems linked to obesity," said Brian P. Lee, MD, MAS , a hepatologist and liver transplant specialist with Keck Medicine and principal investigator of the study. "Our findings show that millions of Americans may already have obesity-related health impacts and may be missing needed health interventions."
Those with normal or overweight BMIs do not currently qualify for pharmacologic or surgical treatments for obesity, and physicians may not necessarily flag them as needing lifestyle modifications that could improve their health, added Lee.
Old definition of obesity versus new
BMI classifies people into four categories based on their weight to height ratio: a BMI under 18.5 is considered underweight; a BMI between 18.5-25 is considered normal, or healthy; a BMI between 25-29.9 is considered overweight; and a BMI of 30 or over is categorized as obese.
"BMI is problematic because it does not specifically measure body fat and instead reflects total body weight, which includes muscle and bone," said Lee. "So 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."
Clinical obesity, a term developed in 2025 by the Lancet Diabetes and Endocrinology Commission, an international group of obesity experts, specifically examines fat around the waist, known as adipose fat, and is a more accurate way of determining obesity and obesity-linked health problems, said Lee.
Adipose fat accumulates deep in the abdomen, causing dangerous inflammation to the body's organs, unlike under-the-skin or subcutaneous fat, which is found in other parts of the body.
Clinicians determine clinical obesity through three measurements: a person's waist circumference, their waist-to-hip ratio and their waist-to-height ratio.
If a person is found to have extra fat by at least two of the three measurements and has evidence of organ, tissue or other health issues associated with extra fat, such as heart disease or chronic hip or knee pain, they are considered clinically obese, said Lee.
Obesity can lead to many illnesses, including 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.
Toward better long-term health
For the study, researchers used data from the National Health and Nutrition Examination Survey, a nationally representative survey of health and nutrition. They included data from some 5,600 adults with an average age of 49 who had been tested for both BMI and hip and waist circumferences.
When they compared BMI results with clinical obesity status, they reached the conclusion that approximately 26% of people with a normal BMI and 50% of people with an overweight BMI qualified as clinically obese.
In the future, Lee hopes that more physicians will diagnose obesity using the clinical obesity definition rather than BMI to better inform people of any health risks they may be facing.
"The good news is that obesity can be treated," said Lee. "Whether through lifestyle changes, medication or both, we have effective ways to reduce excess body fat and lower the risk of future health problems. The earlier we identify people at risk, the better chance we have of improving long-term health and quality of life."
