An international Nature study has validated a new approach to diagnosing obesity that addresses widespread misclassifications caused by relying solely on body mass index.
Researchers found that conventional BMI-based assessments misclassify millions of adults globally, underestimating obesity in some individuals while overestimating it in others.
The evaluation, published on Nov. 11, 2026, analyzed health records from 844,108 adults across China, Spain, the United Kingdom, and the United States, according to Morningstar. The findings support a framework proposed last year by a Lancet Commission, distinguishing between “clinical obesity”—where excess body fat impairs organ function or daily activities—and “preclinical obesity,” where fat poses health risks but organ function remains intact.
Medical experts have long criticized body mass index as a blunt instrument that fails to account for individual differences in body composition. Globally, about 1.6 billion people are classified as overweight with BMIs between 25 and 29.9, and nearly a billion are considered obese with a BMI of 30 or higher.
According to the new research, BMI alone underestimates obesity in roughly 5% to 9% of adults while incorrectly diagnosing the condition in up to 3%.
“This misclassification can account for tens of millions of people, so the inaccuracy of BMI is a big problem at scale and also for individuals.”
The validated framework combines BMI with additional body measurements—such as waist circumference, waist-to-hip ratio, and waist-to-height ratio—alongside tests for 18 conditions and signs of bodily or organ dysfunction. Because different ethnicities feature distinct fat distribution patterns, the study incorporated ethnicity-specific waist circumference cutoffs.
Under this expanded criteria, overall obesity prevalence would see a modest net increase of about 3.3% to 7.4%. Individuals with “overweight” BMIs who exhibit obesity-related dysfunctions would be newly categorized as clinically obese, whereas a smaller group of people with high BMIs but no dysfunction would no longer count as clinically obese.
Tracking participants over periods of up to 15 years, researchers found no clear association between BMI and mortality risk alone. However, clinical obesity was linked to a 69% greater risk of all-cause mortality compared to having no obesity, while preclinical obesity carried only a modest increase in risk.
Disease progression is not inevitable. The study revealed that 77% to 91% of individuals with preclinical obesity did not progress to clinical obesity over a 10-year span.
Participating institutions in the international research included the University of Colorado Anschutz Medical Campus, Huazhong University of Science and Technology, North China University of Science and Technology, Pennington Biomedical Research Center at Louisiana State University, the University of Girona, and Università Cattolica del Sacro Cuore.
Robert H. Eckel of CU Anschutz said that distinguishing between excess body fat and active disease helps guide appropriate prevention and treatment strategies rather than assuming uniform health risks for everyone sharing the same BMI. Rubino argued that individual-level diagnoses allow clinicians to optimize interventions—whether via lifestyle advice, GLP-1 weight loss drugs, or surgery—making better and safer use of healthcare resources.
Despite the findings, questions remain regarding diagnostic consistency. Dr. Aayush Visaria, an internal medicine instructor at Rutgers Robert Wood Johnson Medical School in New Jersey who was not involved with the study, said that ambiguities remain within the metrics for each form of dysfunction, stating that “there needs to be more consensus on what’s actually defining clinical obesity.”
