A recent study from the University of Southern California has challenged the long-standing use of body mass index (BMI) as the primary measure of obesity. The research, published in the Annals of Internal Medicine, indicates that BMI may significantly underestimate obesity levels, particularly in individuals who appear thin but carry excess abdominal fat.
BMI Has Blind Spots
For decades, BMI—a calculation based on height and weight—has been the standard for classifying underweight, normal, overweight, and obese categories. However, the new study reveals that this method allows many people with obesity-related health risks to go undetected. Researchers from Keck Medicine of USC found that approximately 26% of individuals with a normal BMI and 50% of those classified as overweight by BMI actually meet the criteria for clinical obesity.
Clinical obesity, a term introduced in 2025 by the Lancet Diabetes and Endocrinology Commission, focuses on abdominal fat accumulation rather than total body weight. This approach identifies obesity based on waist circumference, waist-to-hip ratio, and waist-to-height ratio, along with evidence of weight-related health issues such as heart disease, diabetes, or chronic joint pain.
Brian P. Lee, MD, a hepatologist and lead investigator of the study, emphasized the implications: "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. Our findings show that millions of Americans may already have obesity-related health impacts and may be missing needed health interventions."
Why BMI Falls Short
BMI is problematic because it does not distinguish between fat, muscle, and bone. A muscular individual can have a high BMI without excess fat, while someone with low muscle mass may have a normal BMI but carry dangerous visceral fat. Visceral fat, which accumulates deep in the abdomen, triggers inflammation in organs and is linked to serious conditions such as heart disease, diabetes, high blood pressure, liver disease, and certain cancers.
In contrast, clinical obesity assesses adipose fat specifically. This type of fat is more harmful than subcutaneous fat found under the skin. The study used data from over 5,600 adults from the National Health and Nutrition Examination Survey, measuring BMI along with hip and waist circumferences to compare classifications.
How Clinical Obesity Is Measured
Clinical obesity is diagnosed using three key measurements: waist circumference, waist-to-hip ratio, and waist-to-height ratio. If a person shows excess fat in at least two of these measurements and has associated health issues—such as heart disease, diabetes, or chronic pain—they are considered clinically obese.
This method provides a more accurate assessment of obesity-related health risks. Currently, individuals with normal or overweight BMIs do not qualify for pharmacological or surgical obesity treatments, and physicians may not recommend lifestyle modifications. The study’s authors hope that more healthcare providers will adopt the clinical obesity definition to better identify at-risk patients.
"The good news is that obesity can be treated. 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," Lee added.
This research underscores the need for a paradigm shift in how obesity is diagnosed, moving beyond BMI to a more nuanced understanding of body composition and health risks.
