A quarter of Americans with a normal, "healthy" BMI actually have obesity, according to new research from Keck Medicine of USC—a finding that could reshape how millions understand their true health risks.
For decades, doctors have relied on body mass index, a simple calculation based on height and weight, to diagnose obesity and determine who needs treatment. But BMI has a fundamental flaw: it measures total body weight, not fat. A muscular person with significant muscle mass can register as obese on the BMI scale while carrying minimal excess fat. Conversely, someone without much muscle can appear perfectly healthy by BMI standards while harboring dangerous levels of body fat that puts their organs at risk.
A new study published in the Annals of Internal Medicine quantifies the scope of this problem. Researchers from Keck Medicine analyzed data from approximately 5,600 American adults with an average age of 49, comparing traditional BMI measurements against a newer diagnostic standard called clinical obesity. The results were striking: roughly 26 percent of people classified as having a normal BMI actually met the criteria for clinical obesity. Among those deemed merely overweight by BMI, the number jumped to 50 percent—meaning half of them should actually be classified 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 at 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."
Clinical obesity, a term formalized in 2025 by The Lancet Diabetes and Endocrinology Commission, takes a more precise approach. Rather than relying on weight alone, it focuses on abdominal fat—specifically the dangerous adipose fat that accumulates deep in the abdomen and triggers inflammation in vital organs. Clinicians diagnose clinical obesity using three measurements: waist circumference, waist-to-hip ratio, and waist-to-height ratio. If someone shows excess fat on at least two of these measures and has evidence of health issues linked to that fat—such as heart disease, high blood pressure, or chronic joint pain—they are classified as clinically obese.
This distinction matters profoundly for treatment. People with normal or overweight BMIs currently do not qualify for medications or surgery designed to treat obesity, and physicians often fail to recommend the lifestyle modifications that could meaningfully improve their health. That means millions of Americans could be living with unrecognized obesity-related risks ranging from heart disease and diabetes to liver disease and certain cancers.
The good news, Lee emphasized, is that obesity is treatable—through lifestyle changes, medication, or both. But first, people need to know they have it. As more physicians adopt the clinical obesity definition instead of relying solely on BMI, Americans will get a clearer picture of their actual health status and the interventions they might need. For many, that clarity could mean the difference between developing preventable diseases and building genuinely healthier lives.