A new diagnostic definition of obesity outperforms BMI in identifying differences in health and prognosis, including mortality risk, according to a large international study published in Nature.
The research, led by King's College London, tested the diagnostic framework proposed by a Lancet Diabetes & Endocrinology Commission in January 2025 to address worldwide concerns that reliance on BMI alone may be causing millions of people to be misdiagnosed.
The new definition was designed to improve identification of excess body fat and distinguishes clinical from pre-clinical obesity based on its impact on organ function and daily activities.
The research was undertaken in collaboration with Huazhong University of Science and Technology, Kailuan General Hospital, and North China University of Science and Technology in China; Pennington Biomedical Research Center and the University of Colorado Anschutz in the United States; the University of Girona in Spain; and Università Cattolica del Sacro Cuore in Italy.
BMI does not accurately reflect an individual's body-fat amount or distribution, which strongly influence health risks. It also does not measure effects on organ function and so cannot distinguish illness from health.
The Lancet Commission's diagnostic framework involves two steps: complementing BMI with measures such as waist circumference and waist-to-height ratio to confirm excess body fat; then distinguishing clinical obesity, an active disease in which excess body fat impairs organ function; and pre-clinical obesity, where excess body fat poses increased health risks but organ and whole-body function remain preserved.
The researchers analysed health data from 844,108 adults in China, the United Kingdom, the United States and Spain, comparing BMI-based classification with the new diagnostic framework. To distinguish organ dysfunction attributable to obesity from conditions that arise from other causes, the study used a statistical method to account for the background risk of organ dysfunction among people without obesity.
Adding waist measurements to standard BMI assessments showed that BMI alone misclassified obesity in both directions - missing obesity-level body fat in up to 8.6% of participants and inadequately classifying up to 3.1% of people as having obesity across the study groups. Obesity was linked to a higher risk of death only when identified using waist measurements alongside BMI - not when using BMI alone.
Clinical obesity accounted for 23%–31% of all obesity cases, had an overall population prevalence of 5.6–10.9% and was associated with a substantially higher risk of death, while pre-clinical obesity carried only a modest increase in risk. Most people with pre-clinical obesity, 77%–91% across cohorts, did not progress to clinical obesity over ten years.
Professor Francesco Rubino, Senior Author of the study and chair of Metabolic and Bariatric Surgery at King's College London and honorary consultant surgeon at King's College Hospital, said: "This study shows that up to a third of all cases of obesity are clinical obesity, an active disease that should be treated as such. Most people with preclinical obesity, but not all, remained free of clinical obesity for a decade. Preclinical obesity should therefore be neither treated automatically as active disease nor dismissed as harmless. Some people may need preventive treatment, but its intensity should match their individual risk."
Professor Rubino adds: "Body mass index alone cannot tell us whether someone is already ill or at risk of illness, or what care they need, leaving people vulnerable to both undertreatment and overtreatment. The new definition makes personalised, proportionate care possible whereas a uniform BMI-based definition prevents it. Getting the scope of care right - treating existing disease or reducing risk - and matching its intensity to individual needs matters enormously, both for the billion-plus people affected and for healthcare systems. People with obesity, like everyone else, deserve care matched to their needs: no less, and no more."
Professor An Pan, first author of the study, from Huazhong University of Science and Technology in Wuhan, China, said: "Rapid advances in treatment make a clinically meaningful diagnosis increasingly important to guide proportionate care. The marked differences in prognosis between clinical and pre-clinical obesity, consistently observed across countries and ethnic groups, support global adoption of the new framework to improve obesity care and policy worldwide."
By moving beyond a uniform BMI-based definition, the new framework could help resolve one of medicine's most polarising debates, shifting the question from whether obesity is a disease to when it constitutes disease in an individual.
