More people could count as obese under newly proposed definition — here's what you need to know

Obesity is linked to a range of health problems ‪—‬ including heart disease, diabetes and early death ‪—‬ but it can be hard to know who is most at risk for these outcomes. Doctors and researchers often use body mass index (BMI) as a convenient tool to assess patients, but this metric does not account for individual differences in body composition, scientists have long argued.

"BMI misses a lot of information," Dr. Francesco Rubino, a professor of bariatric and metabolic surgery at King's College London, told Live Science.

Rubino recently chaired a team of 56 experts that was assembled to better define obesity and develop new diagnostic criteria for the condition. First proposed in January 2025, their new framework includes more body measurements beyond BMI and distinguishes between "preclinical" and "clinical" obesity.

Now, a new study published Wednesday (Nov. 11) in the journal Nature confirms that these new criteria could detect obesity with greater precision than BMI alone. In other words, with the new approach, more people would be accurately diagnosed with obesity and fewer people would be misdiagnosed than are with BMI alone.

Live Science spoke with Rubino and other experts to break down the new findings and learn what they mean for patients.

What are the limitations of BMI?

At the simplest level, obesity is excess body fat. There are special scans and exams that send weak electrical signals through the body to measure a person's body fat percentage directly, but criteria based on body measurements are more accessible across clinical settings.

BMI is the most common of these measurements, as it simply involves dividing a person's weight by their height squared. However, it does not account for individual differences in lean muscle mass or bone density.

Additionally, the different BMI categories — such as "overweight" and "obese" — don't carry the same risks for all patients. Certain racial and ethnic groups tend to experience health consequences, such as higher mortality rates, at different BMI levels than others do.

What metrics does the new obesity definition use?

For a given patient being assessed for obesity, the proposed guidelines would suggest using at least one additional body measurement alongside BMI and having their doctor review their medical history for other signs of bodily or organ dysfunction.

The proposed body measurements are less influenced by a person's muscle mass or bone structure than BMI is. They include waist circumference, waist-to-hip ratio and waist-to-height ratio. Notably, because different ethnicities are associated with different fat distribution patterns in the body, the latest Nature study incorporated different waist circumference cutoffs based that varied by ethnicity.

In addition to these body measurements, the proposed criteria look at 18 conditions and signs of bodily or organ dysfunction that are related to obesity, such as cardiovascular diseases, joint pain and sleep apnea. (All 18 factors can be found detailed in this chart.)

What is the difference between preclinical and clinical obesity?

Rubino and his colleagues — which together made up what's known as a Lancet Commission — defined "clinical obesity" as a disease state involving at least one of 18 forms of bodily dysfunction caused by excess fat tissue. Preclinical obesity, by contrast, is when a person is at risk of this disease state and may need interventions to avoid obesity-related dysfunction.

That means that two people with the same BMI, waist circumference and waist-to-height ratio may be classified differently, depending on whether they have other conditions. In the new study, between 77% and 91% of individuals with preclinical obesity did not progress to clinical obesity within 10 years, depending on the cohort analyzed.

This distinction can be complicated, though, as the diagnostic metrics for each form of dysfunction contain their own ambiguities.

"There needs to be more consensus on what's actually defining clinical obesity," said Dr. Aayush Visaria, an internal medicine instructor at Rutgers Robert Wood Johnson Medical School in New Jersey. Visaira was not involved in writing the new criteria but authored an analysis of the proposed measures.

The Lancet Commission argues that the categories of preclinical and clinical obesity could help doctors identify the people with the greatest need for treatment. The researchers tracked health outcomes using electronic health records from China, the United Kingdom, the United States and Spain, and they determined that their clinical obesity classification was better at flagging people's risk of death than BMI alone.

They found no clear association between BMI and mortality risk, but clinical obesity was linked to a 69% greater risk of all-cause mortality, compared to no obesity.

Would the new criteria affect the overall prevalence of obesity?

The Lancet Commission's approach could drive a small net uptick in individuals diagnosed with obesity, according to the new study.

Under the current framework, "we are definitely missing some people who actually need obesity treatment but just did not meet the BMI criteria," said Dr. Jennifer Hwang, an instructor of medicine at the University of Chicago who has published her own analysis of the new criteria.

With the BMI-only approach, people with BMIs of 30 or higher are categorized as obese. With the new definition, some people with BMIs under 30 would be newly categorized as clinically obese, while a smaller group of people with high BMIs would suddenly not count as clinically obese, the Nature study predicted.

For instance, the former group might include people who have "overweight" BMIs but also have obesity-related dysfunction, while the latter group might include people with "obese" BMIs but without any dysfunction.

Overall, the new criteria would increase obesity prevalence by about 3.3% to 7.4%, the team predicted. The percentage varied depending on the group analyzed.

What would it take to implement this new definition?

More research is needed to prove that the Lancet Commission's criteria would improve patient care and be practical to use in a clinical setting, Hwang said. She noted potential challenges in implementing both new aspects of the new framework. While the body measurements sound simple enough to use, clinicians would need training to perform them accurately.

Providers also may struggle with the new distinction between preclinical and clinical obesity. A 2025 analysis of the Lancet Commission framework noted that some patients who qualify for obesity medications under the Food and Drug Administration's guidelines may be recharacterized as having preclinical obesity and no longer receive a prescription.

Rubino and the rest of the Lancet Commission plan to continue validating their framework to see how it could best support personalized obesity treatment. For instance, they want to determine which forms of organ dysfunction are of greatest concern and develop a way of scoring each patient's level of risk.

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