An alternative to BMI that combines body measurements with organ function tests more accurately distinguishes healthy people from those with harmful excess body fat and could prevent misdiagnosis in millions worldwide, a large study has found.
The approach, tested in health records from more than 800,000 people in the UK, US, China and Spain, suggests BMI alone misses obesity in 5% to 9% of adults while wrongly diagnosing the condition in as many as 3%.
Francisco Rubino, professor of metabolic and bariatric surgery at King’s College London, said the percentages translate into tens of millions of people worldwide. He said the consequences matter both for population-level healthcare and for individual patients whose needs may be misunderstood when diagnosis depends on BMI alone.
Medical experts have long considered body mass index a blunt instrument for defining obesity, raising concerns that substantial numbers of people with excess body fat are missed while others who are healthy are wrongly diagnosed.
BMI is calculated by dividing weight in kilograms by height in meters squared. About 1.6 billion people worldwide are classified as overweight, meaning their BMI falls between 25 and 29.9, and nearly a billion with a BMI of 30 or more are considered obese. Two-thirds of adults in the UK and more than 70% of adults in the US are overweight or obese based on BMI alone.
Rubino and his colleagues drew on health records to test a diagnostic approach for obesity that he and other world experts proposed last year in a Lancet Commission report. They used the records to track people’s health for up to 15 years. The findings were published in Nature.
How the new definition works
Under the new definition, someone with a BMI over 40 is considered obese, but doctors check people with a lower BMI for excess body fat by looking at their waist measurement. A BMI of 30 or higher with a raised waist measurement or a high waist-to-height ratio counts as obese, as does a BMI from 25 to 29.9 if waist measurement and waist-to-height ratio are high.
In most countries, a raised waist measurement means more than 102 cm (40 in) for men and 88 cm for women, while the threshold for a high waist-to-height ratio is about 0.57.
In a second step of the diagnosis, doctors distinguish between clinical obesity, meaning an active disease where excess body fat is already impairing organ function, and pre-clinical obesity, where the extra fat is a health risk but has yet to cause problems.
The approach allows doctors to distinguish between healthy and sick people who have the same BMI. Three people with a BMI of 32, for example, would all be classified as obese but would have very different diagnoses. One might have no excess body fat and be healthy, another could have excess fat but no organ dysfunction and have pre-clinical obesity, and the third could have excess fat and organ dysfunction and be clinically obese.
What the study found
Under the new definition, overall obesity prevalence rose only slightly. Only a quarter to a third of people classified as obese—equivalent to 5% to 10% of the wider population—had clinical obesity and a substantially higher risk of death.
The scientists described how some people with a high BMI were not obese under the new definition and actually had a lower mortality risk than others considered healthy. Reassuringly, 80% to 90% of people with pre-clinical obesity did not progress to clinical obesity, or active disease, in the next 10 years.
The approach makes the process more complex for clinicians, but Rubino said people should be diagnosed for obesity at the individual level to ensure the best healthcare, whether that involved lifestyle advice, GLP-1 weight loss drugs or surgery.
Rubino supported using the approach in clinical care. He said assessing each person more precisely could help doctors choose interventions more effectively and use healthcare resources in ways that improve both safety and treatment outcomes.