BMI alone misses obesity in millions, new diagnosis method finds

Body mass index is too blunt a tool for diagnosing obesity, according to researchers who tested a replacement on health records from more than 800,000 people in the UK, US, China and Spain. Their method combines BMI with waist measurement, weight-to-height ratio and organ function tests. BMI alone missed obesity in 5-9% of adults and wrongly diagnosed it in as many as 3%, they report in the journal Nature.

“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 Francisco Rubino, professor of metabolic and bariatric surgery at King's College London. The team tracked participants' health for up to 15 years to test the approach first proposed last year in a Lancet Commission report.

Under the new definition, a BMI over 40 counts as obese, but anyone with a lower BMI gets checked for excess body fat using waist measurement and weight-to-height ratio. A BMI of 30-plus with a raised waist or waist-to-height ratio is obese, and so is a BMI of 25 to 29.9 when both are high. In most countries a raised waist means more than 102cm for men and 88cm for women, with high waist-to-height ratio starting around 0.57.

A second step separates clinical obesity — active disease where excess fat is already impairing organ function — from pre-clinical obesity, where the fat is a risk but hasn't caused damage yet. Overall obesity levels barely moved under the new definition. Only a quarter to a third of obese people, or 5% to 10% of the wider population, had clinical obesity and a substantially higher risk of death.

The system lets doctors tell apart people with identical BMIs. Three people at BMI 32 would all be classed as obese today, but one might have no excess fat and be healthy, another could carry excess fat with no organ dysfunction, and a third could be clinically obese with organs already affected. Some high-BMI people weren't obese under the new definition and had a lower mortality risk than others considered healthy, the scientists found. Between 80% and 90% of people with pre-clinical obesity did not progress to clinical obesity within 10 years.

It makes life more complicated for clinicians, Rubino conceded, but he argued diagnosis should happen at the individual level to steer the right care — whether lifestyle advice, GLP-1 weight loss drugs or surgery. “We can optimise our interventions and ultimately make better use of our resources with more safety, more efficacy,” he said. “If you ask if I think this should be employed in the clinic, I think absolutely, yes.”