LONDON (The Thursday Times) — Body mass index on its own gets obesity wrong for tens of millions of people, according to a large international study published in Nature on Wednesday, which backs a new way of diagnosing the condition that also looks at body fat and organ health.
Researchers used health records from almost 850,000 adults in Britain, the United States, China, and Spain, following some for up to 15 years. They tested a definition of obesity proposed in 2025 by a group of international experts and medical organisations.
Under that definition, between 1% and 3% of people now classed as obese by BMI alone would not count as obese, while another 5% to 9%, who carry more body fat than their BMI suggests, would be newly diagnosed.
How the new test works
BMI is a person’s weight in kilograms divided by the square of their height in metres. Under the proposed approach, anyone with a BMI above 40 is treated as obese. Below that, doctors also check how fat is carried, using waist measurement and the ratio of waist to height.
A BMI of 30 or more with a raised waist measurement counts as obesity, as does a BMI between 25 and 29.9 if both the waist measurement and the waist-to-height ratio are high. In most countries a raised waist means more than 102cm for men and 88cm for women, though thresholds vary.
Doctors then separate “clinical obesity”, where excess fat is already damaging the heart, liver, or other organs, from “pre-clinical obesity”, where it is a risk but has not yet caused illness.
Most are not yet ill
Across the four countries, only about 27% of people with obesity had the clinical form, roughly 8.5% of all adults. Around 82% of those with pre-clinical obesity did not go on to develop clinical obesity over the following years, though progression was higher in China and Spain.
Some people with a high BMI were not obese under the new definition at all, and had a lower risk of death than others considered healthy.
“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,” said Francesco Rubino, a professor of metabolic and bariatric surgery at King’s College London and the study’s senior author. He said the framework could “provide a meaningful clinical characterisation of obesity that can inform care and policy”.
The World Health Organization estimates that more than a billion people worldwide live with obesity, a risk factor for type 2 diabetes, heart disease, liver and kidney disease, and some cancers.
