You could be obese and not know it — new definitions challenge BMI

Take a calculation based on your weight and divide it by your height.

We’ve long known that little number, your BMI, is not actually that great at doing what it’s designed to: determine whether a patient is underweight, “healthy,” overweight or obese.

Now, a new definition is showing much better success for diagnosing obesity. With it, experts say more people will be accurately diagnosed with obesity and better able to get the treatments suitable for their situation.

A massive new study from participants in China, Spain, the UK and US suggests BMI is missing obesity in up to one in 12 people.

Plus, a third of obesity cases could be “clinical” — a new category under the new framework that was hidden with BMI. Clinical obesity is “an active disease that should be treated as such,” said author Francesco Rubino.

BMI, a calculation invented in the 1830s has long been relied on to determine if a patient is at a healthy weight.

Experts have raised issues for decades, saying it doesn’t tell the full story. For example, BMI doesn’t say how much of your weight is fat, or how and where the fat is distributed through your body.

“BMI-based measures of obesity can both underestimate and overestimate” the amount of fat stored in the body, the commission who proposed a new definition for obesity last year wrote.

Yet BMI is still used for everything from health screening for conditions like heart disease, deciding whether you can have certain medicines, and the dose of those medications, too.

The commission, made up of 56 medical experts from around the world, proposed a new way to diagnose obesity: confirm there’s excess fat, then determine if fat is causing the illness.

Excess fat can be confirmed three ways: a direct measurement like a DEXA scan, BMI plus another measurement like waist circumference, waist-to-hip ratio or waist-to-height ratio, or two waist-based measurements without BMI.

If excess fat is identified, doctors should determine if it’s clinical obesity or pre-clinical obesity — new categories that were previously not differentiated.

Clinical obesity is more serious, requiring immediate attention — determined by obvious signs of reduced organ function and reduced ability to do daily tasks like bathing.

Pre-clinical obesity is still serious, but more because people with it are at risk of developing a number of diseases, rather than having active disease.

What the study showed

A massive study compared how accurately BMI vs. the new framework would assess the weights of 844,108 adults in China, the United Kingdom, the United States and Spain.

Researchers added waist measurements to the standard BMI and found BMI alone misclassified obesity in both directions.

It missed obesity-level body fat in 8.6% of participants (about one in 12). It also incorrectly classified up to 3.1% of people as having obesity.

A separate study done in October 2025 showed that obesity rates could rise up to 30% — from what’s currently considered to be 40% shooting up to 70% — under the new framework.

Plus, whether those with obesity were clinical or preclinical was never measured by BMI.

The new study found that clinical obesity accounted for 23-31% of all obesity cases. Clinical obesity is active disease, and requires treatment for obesity-related impacts on the lungs, heart, joints, bones, kidneys, and other systems.

Importantly, researchers found that those with pre-clinical obesity didn’t have a much higher risk of death and a vast majority, 77 to 91%, of pre-clinical obesity participants did not progress to clinical obesity over 10 years.

These findings make it clear that the differentiation is useful as the two groups require vastly different medical treatment.

“The new definition makes personalised, proportionate care possible whereas a uniform BMI-based definition prevents it,” said Rubino. “People with obesity, like everyone else, deserve care matched to their needs: no less, and no more.”