A new diagnostic framework published in Nature flips the obesity status of millions by replacing body mass index with a multi-step assessment. The study, led by Francisco Rubino at King’s College London, analyzed health records from over 800,000 people across the UK, US, China, and Spain. Researchers tracked these individuals for up to 15 years to test a method proposed last year in a Lancet Commission report. This approach combines BMI with waist measurements and organ function tests to distinguish between healthy excess fat and active disease. Rubino reports that standard BMI alone misses obesity in 5% to 9% of adults while wrongly diagnosing the condition in as many as 3%. He states, "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 current metric classifies about 1.6 billion people as overweight and nearly a billion as obese, yet it fails to capture where fat is stored or how it affects specific organs.
Defining clinical versus pre-clinical states
The new definition sets a hard threshold at a BMI over 40 for immediate classification as obese. For those with a lower BMI, doctors check waist circumference; a raised measurement above 102cm for men or 88cm for women, combined with a BMI of 30 or higher, counts as obese. A BMI between 25 and 29.9 also qualifies if both waist measurement and waist-to-height ratio are high, with the latter threshold sitting around 0.57. This initial step identifies who has excess body fat, but the second step determines whether that fat causes harm. Doctors distinguish between "clinical obesity," where excess fat actively impairs organ function, and "pre-clinical obesity," where extra fat poses a risk but has not yet caused damage. Under this system, only a quarter to a third of obese people, or 5% to 10% of the wider population, had clinical obesity with a substantially higher death risk. Reassuringly, 80% to 90% of people with pre-clinical obesity did not progress to active disease within the next 10 years. Some individuals with high BMI were not obese under the new rules and actually faced lower mortality risks than others considered healthy. Three people with a BMI of 32 might receive different diagnoses: one healthy, one pre-clinical, and one clinically obese. While the process adds complexity for clinicians, Rubino argues it allows for targeted interventions like lifestyle advice, GLP-1 drugs, or surgery. He concludes, "If you ask if I think this should be employed in the clinic, I think absolutely, yes."
