Millions of Americans may already have obesity-related health impacts
For decades, a number on a scale divided by height squared has served as medicine's shorthand for human health — a blunt instrument in a world of biological complexity. A new study from Keck Medicine of USC, published in the Annals of Internal Medicine, reveals that this shorthand is leaving millions of Americans undiagnosed: a quarter of those with 'normal' BMI and half of those deemed merely 'overweight' carry the abdominal fat and organ stress that define clinical obesity. The findings invite a reckoning with how medicine measures risk, and who gets left out of care when the measuring stick is wrong.
- The standard tool doctors have used for generations to assess weight-related risk cannot distinguish fat from muscle — and that blind spot is quietly endangering millions.
- A USC study of 5,600 adults found that one in four people with a normal BMI and one in two 'overweight' individuals actually meet the criteria for clinical obesity based on abdominal fat and health markers.
- Because BMI alone determines eligibility for obesity medications and surgical interventions, those misclassified as healthy are effectively locked out of treatments that could prevent heart disease, diabetes, and early death.
- A 2025 international commission redefined obesity around waist measurements and organ-level health evidence, offering a more precise lens — but its adoption in everyday clinical practice remains limited.
- Researchers are calling on physicians to integrate clinical obesity screening now, arguing that earlier identification means earlier intervention and meaningfully better long-term outcomes.
A quarter of Americans whose doctors consider them a healthy weight are, by a more precise measure, carrying dangerous levels of abdominal fat and showing early signs of obesity-related disease. Half of those labeled merely overweight would be reclassified as obese under the same standard. These are the central findings of a USC Keck Medicine study published in the Annals of Internal Medicine — and they point to a quiet diagnostic failure unfolding at scale.
The culprit is body mass index, the height-and-weight formula that has anchored obesity medicine for decades. BMI is simple and universal, but it cannot tell fat from muscle, and it cannot see where fat lives in the body. Visceral fat — the kind that accumulates deep in the abdomen — triggers organ inflammation in ways that fat just beneath the skin does not. A person can look unremarkable on a BMI chart while harboring exactly this kind of dangerous fat.
In 2025, an international commission published by Lancet Diabetes and Endocrinology offered a corrective: clinical obesity, defined not by weight alone but by waist circumference, waist-to-hip ratio, and waist-to-height ratio, combined with evidence of weight-related conditions like heart disease, diabetes, or liver disease. Meeting excess fat thresholds on at least two of three measurements, alongside such health markers, qualifies a person as clinically obese.
USC researchers applied this definition to data from roughly 5,600 adults in a nationally representative health survey and found the gaps in BMI-based screening were far larger than previously understood. Lead researcher Brian P. Lee, a hepatologist at Keck Medicine, noted that people misclassified by BMI typically don't qualify for obesity medications or surgery — and their doctors may never prompt them toward lifestyle changes that could reduce their risk.
The stakes are high. Obesity connects to heart disease, diabetes, high blood pressure, liver disease, and several cancers, making it among the most consequential and preventable drivers of early death in the United States. Lee's message, however, is not one of alarm alone: obesity is treatable, and the earlier someone is correctly identified as at risk, the greater the opportunity to intervene — through diet, medication, or both — and protect their long-term health.
A quarter of Americans with what their doctor calls a normal weight are actually carrying dangerous levels of abdominal fat and showing signs of obesity-related disease. Half of those classified as merely overweight would be reclassified as obese under a more precise measurement. These are the findings of a new study from Keck Medicine of USC, published in the Annals of Internal Medicine, and they suggest that millions of people are walking around with undiagnosed health risks because the medical establishment has been using the wrong yardstick.
For decades, doctors have relied on body mass index—a simple calculation based on height and weight—to determine who has a weight problem. It's straightforward, easy to calculate, and has become the standard. But BMI has a fundamental flaw: it cannot distinguish between muscle and fat. A person with significant muscle mass can register as obese on the BMI scale while carrying no excess fat at all. Conversely, someone with little muscle can appear healthy by BMI standards while harboring dangerous fat deposits deep in the abdomen, exactly where they cause the most harm.
Enter clinical obesity, a definition developed in 2025 by the Lancet Diabetes and Endocrinology Commission, an international group of obesity experts. Rather than relying on weight alone, clinical obesity focuses on where fat accumulates—specifically, the visceral fat that gathers around the waist and deep within the abdominal cavity. This type of fat, called adipose fat, triggers inflammation in the body's organs in ways that subcutaneous fat, the kind just under the skin, does not. Clinicians diagnose clinical obesity by measuring waist circumference, waist-to-hip ratio, and waist-to-height ratio. If a person shows excess fat on at least two of these three measurements and has evidence of weight-related health problems—heart disease, diabetes, high blood pressure, liver disease, chronic joint pain—they are considered clinically obese.
For their study, researchers at USC examined data from the National Health and Nutrition Examination Survey, a nationally representative health survey. They analyzed information from roughly 5,600 adults with an average age of 49, all of whom had been measured for both BMI and waist and hip circumferences. The results were striking: 26 percent of people with a normal BMI qualified as clinically obese. Among those classified as overweight, the figure jumped to 50 percent. Brian P. Lee, a hepatologist and liver transplant specialist at Keck Medicine who led the research, noted that people with normal or overweight BMIs typically do not qualify for medications or surgery to treat obesity, and their doctors may not flag them for lifestyle changes that could improve their health.
The implications are substantial. Obesity is linked to heart disease, diabetes, high blood pressure, high cholesterol, liver disease, and certain cancers, making it one of the leading preventable causes of death in the United States. Yet millions of Americans with these risks are being told their weight is fine. Lee expressed hope that physicians will increasingly adopt the clinical obesity definition to better identify who needs intervention. The encouraging part, he noted, is that obesity is treatable. Whether through lifestyle modification, medication, or both, there are effective ways to reduce excess body fat and lower the risk of future illness. The earlier someone is identified as at risk, the better the chance of improving long-term health and quality of life.
Notable Quotes
BMI does not specifically measure body fat and instead reflects total body weight, which includes muscle and bone— Dr. Brian P. Lee, Keck Medicine of USC
Obesity can be treated through lifestyle changes, medication, or both, and the earlier we identify people at risk, the better chance we have of improving long-term health— Dr. Brian P. Lee
The Hearth Conversation Another angle on the story
Why did doctors stick with BMI for so long if it's this flawed?
It's simple and fast. You just need height and weight—no special equipment, no appointments. But simplicity came at a cost. It couldn't see the real problem, which is where the fat is, not just how much weight someone carries.
So a muscular athlete could be flagged as obese?
Exactly. And a sedentary person with normal weight but lots of belly fat gets a clean bill of health. The system was backwards in both directions.
What makes abdominal fat so much worse than fat elsewhere?
It sits deep in the organs and triggers inflammation. It's metabolically active in a dangerous way. Subcutaneous fat—the kind under your skin on your arms or thighs—doesn't do the same damage.
If clinical obesity is better, why isn't every doctor using it already?
Inertia, partly. BMI is entrenched. But also, clinical obesity requires more measurement and clinical judgment. It's not a single number you can plug into a computer. It takes time.
What happens to someone who gets reclassified from overweight to obese?
Suddenly they qualify for treatment options that were closed to them before—medications, surgery, structured programs. They also get the message that something needs to change, which matters psychologically.
Is this study saying BMI is useless?
No. It's saying BMI alone isn't enough. You need to look at where the fat is and what health problems are already present. BMI is part of the picture, but only part.