Body Mass Index tests 'misclassifying' people as overweight or obese, new report suggests

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The tool, which measures body fat based on height and weight, was compared to scans which provide an analysis of fat, muscle and bone in the body.

The study, undertaken in Italy and published in the journal Nutrients, included 1,351 adults who had been referred to the Department of Neurosciences, Biomedicine and Movement Sciences at the University of Verona.

Based on BMI, 19 were underweight, 787 were a normal weight, 354 were overweight and 191 were obese.

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People were then reclassified using dual-energy X-ray absorptiometry (DXA) scans, which revealed that more than a third (34 per cent) of patients who were obese based on BMI were misclassified, and should have been in the overweight category.

More than half (53 per cent) of those labelled overweight based on BMI were in the wrong category. According to the study, three-quarters of the misclassified overweight patients were a normal weight when scanned, while the remainder should have been in the obese category.

Researchers said: "Despite the fact that the BMI seems to be reliable in determining body weight status in the normal weight range, over a third of the general population was misclassified, as the current BMI classification appears to inflate the prevalence of underweight, overweight and obesity among the general population."

They suggested healthcare staff should be advised to not rely solely on BMI, and should combine it with measures such as calculating body fat percentage or waist measurements, particularly among people who are considered to have a normal BMI.

According to the NHS, 30 per cent of adults in England were obese in 2024, and 66% were either overweight or obese.