A new study supports a more nuanced way to diagnose obesity than body mass index alone, and it would likely bring a modest net increase in the number of people counted as obese, Live Science reported.
Doctors often use BMI as a convenient tool, but the metric does not account for individual differences in body composition. "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 assembled to better define obesity and develop new diagnostic criteria. First proposed in January 2025, their framework includes more body measurements beyond BMI and distinguishes between "preclinical" and "clinical" obesity.
A study published Wednesday, Nov. 11, in the journal Nature found that these criteria could detect obesity with greater precision than BMI alone: more people would be accurately diagnosed, and fewer would be misdiagnosed, Live Science reported.
Waist measures, 18 signs of dysfunction, and two categories
The proposed guidelines would suggest using at least one additional body measurement alongside BMI and having a doctor review a patient's medical history for signs of bodily or organ dysfunction. The added measurements include waist circumference, waist-to-hip ratio, and waist-to-height ratio, which are less influenced by muscle mass or bone structure than BMI. Because fat distribution patterns vary by ethnicity, the Nature study incorporated different waist-circumference cutoffs that varied by ethnicity.
The criteria also look at 18 conditions and signs of dysfunction related to obesity, such as cardiovascular disease, joint pain, and sleep apnea. Rubino and his colleagues, known as a Lancet Commission, defined "clinical obesity" as a disease state involving at least one of those 18 forms of dysfunction caused by excess fat tissue. Preclinical obesity is when a person is at risk of that disease state and may need interventions to avoid obesity-related dysfunction. That means two people with the same BMI and waist measures may be classified differently, depending on whether they have other conditions.
In the new study, between 77 percent and 91 percent of individuals with preclinical obesity did not progress to clinical obesity within 10 years, depending on the cohort. Dr. Aayush Visaria, an internal medicine instructor at Rutgers Robert Wood Johnson Medical School who was not involved in writing the new criteria but authored an analysis of the proposed measures, told Live Science that "there needs to be more consensus on what's actually defining clinical obesity."
A better flag for death risk, and a small rise in prevalence
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 percent greater risk of all-cause mortality, compared with no obesity, Live Science reported.
Under the current framework, 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 with high BMIs would not count as clinically obese. Overall, the new criteria would increase obesity prevalence by about 3.3 percent to 7.4 percent, depending on the group analyzed.
"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. She also said more research is needed to prove the criteria would improve patient care and be practical in clinic, including training so clinicians can take the body measurements accurately, Live Science reported.