Body mass index may have misclassified millions of people in UK, researchers say

Millions of people may have been misclassified on the body mass index (BMI) scale, research has suggested.

The scale may have overestimated and underestimated how excess body fat may harm health, according to the study.

The study showed in the UK there might be more than two million people with dangerous levels of body fat missed by the BMI tool.

There may also be more than one million people classed as obese by the index, but who actually have no excess risk, the research suggests.

BMI is used to assess whether someone has a healthy weight in relation to their height.

Researchers said the "uniform" tool cannot determine if someone is at risk of illness and may be preventing "personalised, proportionate care".

A global team led by King's College London tested the method of using BMI alongside other measurements, such as waist-to-hip ratio or waist-to-height ratio, to diagnose obesity.

The new analysis included 844,108 adults in the UK, China, the US and Spain, including more than 430,000 people taking part in the UK Biobank study.

Researchers found that adding waist measurements to standard BMI assessments showed the index alone misclassified obesity.

Up to 8.6% of people in the study had so-called "hidden obesity", meaning they were classed as overweight on the BMI scale, but measurements revealed obesity-level body fat.

More from Sky News:
Blindness warning on weight loss drugs
Rollout of weight-loss drug needs 'boost'

In the UK, this could translate to 2.4 million people having dangerous levels of body fat that BMI misses, the study suggests.

Meanwhile, up to 3.1% were classed as obese using BMI, but measurements showed no excess fat.

That means an estimated 1.3 million who are classed obese may have no excess fat and no excess risk to their health.

Professor Francesco Rubino, of King's College London, suggested the new method "makes personalised, proportionate care possible, whereas a uniform BMI-based definition prevents it".

He added: "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 of the fact it doesn't accurately measure the individual health."

(c) Sky News 2026: Body mass index may have misclassified millions of people in UK, researchers say