BMI alone misdiagnoses obesity in millions, study of 844,000 people finds
Photo: Getty Images (file photo)Body mass index alone is misdiagnosing obesity in millions of people, according to a study of more than 844,000 adults published Wednesday in the journal Nature — and adding a simple waist measurement predicted who was actually at risk of dying far better than BMI on its own.
The research, led by King's College London, tested a new diagnostic framework proposed last year by a Lancet Commission that distinguishes between clinical obesity — excess body fat that is already impairing organ function or daily activities — and preclinical obesity, where excess fat is present but organ function is still preserved.
Across four large health databases in China, the United Kingdom, the United States and Spain, BMI alone missed obesity-level body fat in 5.2% to 8.6% of participants, while wrongly classifying 1.2% to 3.1% of people as having obesity. In the United Kingdom alone, the researchers estimated that works out to about 2.4 million missed cases and 1.3 million people incorrectly diagnosed. Senior author Francesco Rubino, professor of metabolic and bariatric surgery at King's College London, said the misclassification could amount to tens of millions of people worldwide.
The new approach — called the combined anthropometric definition of obesity — pairs BMI with waist circumference and waist-to-height ratio, and checks organ function. Under it, overall obesity prevalence rose in every country studied: in the U.S. group it went from 42.5% to 48.1%, in the U.K. from 24.0% to 27.3%, in China from 17.8% to 21.3% and in Spain from 18.7% to 26.1%.
The sharpest finding concerned the risk of death. Obesity was linked to a higher risk of dying only when it was identified using waist measurements alongside BMI — not when it was defined by BMI alone. Clinical obesity, which made up 23% to 31% of all obesity cases with a population prevalence of 5.6% to 10.9%, carried a substantially higher risk of death, while preclinical obesity carried only a modest increase. Some people with a high BMI were not obese under the new definition and actually had a lower risk of dying than others classified as healthy.
"BMI is not a good measure of obesity at the individual level and is not good in terms of predicting who is going to develop cardiometabolic disease in the long term," said Cátia Martins, an obesity researcher at the University of Alabama at Birmingham, in Nature's coverage of the study.
The researchers drew on health records from 844,108 adults — the Kailuan Study in China, the UK Biobank, the U.S. All of Us Research Program and Spain's SIDIAP database — and tracked their health for up to 15 years, recording more than 50,000 deaths. The authors say the results could help doctors target treatment and resources at the people whose excess weight is actually harming their health, rather than relying on a height-and-weight formula that has long been criticized as a blunt instrument.
Sources: Nature, "Validation of clinical and preclinical obesity across global cohorts" (Nov. 11, 2026, full text verified); Nature, "Move over BMI: new obesity definition is better for predicting your health" (Nov. 11, 2026, full text verified); MedicalXpress, "New clinical definition outperforms BMI in diagnosing obesity" (Nov. 11, 2026, full text verified); The Guardian, "New obesity test could spare millions a wrong diagnosis" (Nov. 11, 2026, full text verified).
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