A new approach to obesity diagnosis would change the diagnosis for millions of people, according to a study published in Nature on Wednesday. The study tested a framework proposed by a Lancet Commission in 2025, and found that body mass index (BMI) alone misses obesity in 5 to 9 percent of adults while wrongly giving the diagnosis to as many as 3 percent. The findings add weight to calls for doctors to look beyond BMI when they judge whether excess body fat is harming a person’s health.
BMI divides weight in kilograms by height in metres squared, and it cannot tell fat from muscle, show where fat sits on the body, or say whether excess fat is affecting health. Medical experts have long called it a blunt tool. About 1.6 billion people worldwide are classed as overweight, with a BMI between 25 and 29.9, and nearly a billion with a BMI of 30 or more are classed as obese. Two-thirds of adults in Britain and more than 70 percent of adults in the United States are overweight or obese by BMI alone. That gap is why researchers set out to build a better obesity diagnosis.
How the new obesity diagnosis works
The new approach to obesity diagnosis keeps BMI as a starting point and adds waist measurements and checks of organ function. Under the proposed definition, a BMI over 40 counts as obese on its own. A BMI of 30 or higher counts as obese when paired with a raised waist measurement or a high waist-to-height ratio, and the same holds for a BMI between 25 and 29.9 when both of those extra measures are high. In most countries, a raised waist measurement means more than 102 centimetres for men and 88 centimetres for women, while a high waist-to-height ratio sits near 0.57.
A second step in the obesity diagnosis then splits obesity into two groups. Clinical obesity means excess body fat is already impairing how organs work, while pre-clinical obesity means the extra fat is a health risk but has yet to cause problems. The split lets doctors tell apart people who share the same BMI but face very different risks. Three people with a BMI of 32, for instance, would all count as obese under the old rule, yet one might carry no excess body fat and be healthy, a second might carry excess fat with no organ damage, and a third might already have organs under strain.
What the obesity diagnosis study found
Francisco Rubino, professor of metabolic and bariatric surgery at King’s College London, led the study with colleagues who drew on health records from 844,108 adults in China, Britain, the United States and Spain, tracking their health for up to 15 years. The team tested the new obesity diagnosis against those records and found that excess body fat predicted the risk of death better than BMI alone. Under the new definition, overall obesity levels rose only slightly, by 3.5 percentage points in China, 3.3 in Britain, 5.6 in the United States and 7.4 in Spain. Only a quarter to a third of those classed as obese had clinical obesity and a markedly higher risk of death, equal to 5 to 10 percent of the wider population.
The study also found that some people with a high BMI were not obese under the new definition and in fact faced a lower risk of death than some people classed as healthy. Rubino said the percentages translate into tens of millions of people worldwide, adding that in countries with large populations, misdiagnosis on that scale is far from a minor matter. He said people should be assessed one by one so that doctors can choose the right care, whether that means advice on daily habits, GLP-1 weight loss drugs or surgery.
Why obesity diagnosis may change slowly
The new obesity diagnosis makes the task more complex for clinicians, and the researchers accept that their records used diagnostic codes as stand-ins for organ checks rather than direct clinical tests. A researcher commenting on the work said further checks are still needed in groups that were barely represented in the records. Even so, Rubino backed use of the approach in clinics, saying a sharper read of each person could help doctors pick treatments more effectively and use health care resources in ways that improve safety and results.
Four in five people with pre-clinical obesity did not develop clinical obesity in the next 10 years, the study found, which suggests that an obesity diagnosis is a warning to act rather than a verdict on a person’s health. The World Health Organization estimates that more than one billion people worldwide are living with obesity.