More than two million Brits have ‘hidden obesity’ due to problems with body mass index measure

Résumé détaillé

Two thirds of Brits are officially overweight or obese but new study casts doubt on the body mass index (BMI) measure suggesting that may be all wrong, research says

More than two million people have “hidden obesity” missed by the NHS due to flaws with the body mass index measurement.

Analysis of 845,000 people by King’s College London found 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% people were also overdiganosed as obese using BMI, but measurements showed no excess fat.

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 obese but have no excess fat and health risk.

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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.

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.

The new diagnostic framework would help distinguish people with obesity - where excess fat impairs organ function - and pre-clinical obesity, where excess weight poses increased health risks, but the organs remain functional.

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