A substantial portion of people with normal body mass index (BMI) actually meet the clinical definition of obesity, according to new research from the University of Southern California. The findings suggest that millions of Americans may not be getting the health screening and treatment they need because doctors rely too heavily on BMI as the sole measure of obesity risk.

Why BMI Alone Isn't Telling the Full Story?

BMI has long been the standard tool doctors use to screen for obesity-related health problems. It's simple: measure your weight and height, plug in the numbers, and you get a category. A BMI of 30 or higher has traditionally meant obesity. But this approach has a major blind spot.

The problem is that BMI doesn't actually measure body fat. It only measures the relationship between weight and height. This means a muscular person or someone who is especially tall might be labeled obese despite having very little excess fat. Conversely, someone who appears thin on the outside could be carrying dangerous amounts of fat in places you can't see, like around their organs.

In January 2025, a large international team of obesity experts, backed by the Lancet medical journal, created a new standard for measuring obesity that goes beyond BMI. The updated criteria asks doctors to use additional body measurements alongside or instead of BMI.

What Are the New Obesity Measurements?

The new standard includes several measurements that give a more complete picture of where your body is storing fat:

• Waist circumference: A simple tape measure around your midsection reveals how much fat you're carrying in your belly, which is particularly linked to health risks.
• Waist-to-hip ratio: This compares your waist measurement to your hip measurement, showing whether fat is concentrated in your midsection or distributed more evenly.
• Waist-to-height ratio: This measurement accounts for your overall body size and can reveal excess fat even in taller individuals.
• DEXA scan: This medical test directly measures your bone density and body fat percentage, providing the most precise assessment of body composition.

The new criteria also distinguishes between two types of obesity. Clinical obesity means you have excess fat that's likely causing health problems, such as tissue damage or conditions strongly linked to obesity like high cholesterol, type 2 diabetes, or chronic knee pain. Pre-clinical obesity means you have excess fat but haven't yet developed these related health issues.

What Did the Study Actually Find?

Researchers at USC examined data from the National Health and Nutrition Examination Survey (NHANES), a nationally representative study of Americans' health and lifestyle habits run by the Centers for Disease Control and Prevention. They focused on middle-aged adults who had undergone detailed measurements beyond just BMI, including waist and hip circumference.

The results were striking. Overall, 78% of Americans in the study were considered obese under the new criteria. But here's the key finding: roughly 26% of people with a normal BMI (between 20 and 25) met the definition for clinical obesity. Additionally, just over 50% of people classified as overweight (a BMI between 25 and 30) also met the clinical obesity standard.

"Since most doctors are focused on BMI to screen for obesity-related conditions, this is a population that might not be getting enough attention for screening and treatment of obesity-related conditions," said Brian Lee, a hepatologist at Keck Medicine of USC.

Brian Lee, Hepatologist at Keck Medicine of USC

This gap between BMI classification and actual clinical obesity status means many people are walking around with undiagnosed health risks. They may not realize they need intervention, and their doctors may not be offering screening for obesity-related diseases.

How to Check Your Own Body Composition?

You don't need a doctor's office visit to get a basic sense of whether you might have excess body fat despite a normal BMI. Here are practical steps you can take:

• Measure your waist: Use a tape measure around your midsection at the level of your navel. Write down the number and compare it to healthy ranges for your age and sex.
• Calculate your waist-to-height ratio: Divide your waist measurement by your height (both in the same units). A ratio above 0.5 may indicate excess abdominal fat.
• Talk to your doctor about DEXA or other body composition tests: If you're concerned about your body fat despite a normal BMI, ask whether a DEXA scan or other direct measurement would be appropriate for you.
• Track your health markers: Even if your BMI looks good, monitor cholesterol, blood sugar, and blood pressure, which are often better indicators of obesity-related health risks.

Lee emphasized the accessibility of these measurements: "Using a tape measure to measure your waist, hips, and height is an easy way to see if you have excess adiposity that could be harmful to your health".

Lee

What Happens Next for Treatment?

The bigger question now is what to do about this newly identified population. Traditionally, diet and exercise have been the go-to approaches for weight loss, though sustained success has been limited. In recent years, medications like semaglutide (sold as Ozempic and Wegovy) and other GLP-1 drugs have made weight loss significantly easier for many people. However, these medications and other proven interventions like bariatric surgery have typically been reserved for people who meet the classic BMI-based obesity criteria.

It's not yet clear whether these treatments would be effective or appropriate for people with normal BMI who have clinical obesity. Lee noted the critical gap in knowledge: "An important question is whether treatment of excess adiposity with normal body mass index can lead to a decrease in obesity-related conditions and poor outcomes. Prospective studies and randomized controlled trials to understand how best to treat these patients can fill this knowledge gap".

Lee

The research published in the Annals of Internal Medicine suggests that the medical field may need to rethink how it approaches obesity screening and treatment. If nearly a quarter of people with normal BMI are clinically obese, current guidelines could be leaving millions of people without the care they need. The next step will be determining whether treating excess body fat in this population, even when BMI looks normal, can prevent or reverse obesity-related health problems.