More than two million people in the UK have “hidden obesity” that is missed by the NHS because of flaws in the body mass index (BMI) measurement, according to new research.
Analysis of 845,000 people by King’s College London found that 8.6% of people were classed as overweight on the BMI scale, but other measurements revealed obesity-level body fat. The study, which included 430,000 people taking part in the UK Biobank project, also found that up to 3.1% of people were overdiagnosed as obese using BMI, but measurements showed no excess fat.
Millions misclassified by BMI
In the UK this translates to 2.4 million people having dangerous levels of body fat that BMI misses, while 1.3 million are classed as obese but have no excess fat and no health risk.
The study concluded that BMI — which divides your weight by your height — should not be used to categorise people without additional measurements such as waist-to-hip ratio or waist-to-height ratio.
Experts call for waist measurements
Professor Francesco Rubino, chair of metabolic and bariatric surgery at King’s College London, said: “BMI doesn't tell you anything about body fat content. It doesn't tell you whether a person's organ is working or not. It doesn't tell you if somebody, as an individual, is ill or healthy. It's just a measure of the overall risk of ill health.”
“Nevertheless, these BMI thresholds are being used to determine who receives treatment, or even who gets coverage for treatment, despite the fact it doesn't accurately measure the individual health.”
The criticism of BMI was also made in a Lancet Commission by 50 top international experts in 2025. The new study found that adding waist measurements to standard BMI assessments showed that BMI alone misclassified obesity.
New framework for clinical obesity
The new diagnostic framework would help distinguish people with clinical obesity — where excess fat impairs organ function — and pre-clinical obesity, where excess weight poses increased health risks, but the organs remain functional.
Prof Rubino said: “This study shows that up to a third of all cases of obesity are clinical obesity, an active disease that should be treated as such. Most people with preclinical obesity, but not all, remained free of clinical obesity for a decade.”
“Preclinical obesity should therefore be neither treated automatically as active disease nor dismissed as harmless. Some people may need preventive treatment, but its intensity should match their individual risk.”
Two thirds of Brits are estimated to be overweight or obese using the current BMI measure. Prof Rubino added: “Because of the size of obesity, even these relatively small percentages in large populations can correspond to millions of people being assigned to one or another category, and being assessed clinically in a way that doesn't correspond to their health status.”
“The numbers can be very substantial, and that's what makes the BMI system all the more problematic because it creates the problem. The inaccuracy of BMI at scale really applies to many, many, many people in every single country.”
