The Body Mass Index (BMI) system may be misdiagnosing people as overweight or obese, according to a study.
Researchers in Italy compared the tool – which measures body fat based on height and weight – to scans providing an analysis of fat, muscle, and bone.
The study, published in the journal Nutrients, involved 1,351 adults referred to the University of Verona's Department of Neurosciences, Biomedicine and Movement Sciences.
Based on BMI, 19 were underweight, 787 were a normal weight, 354 were overweight, and 191 were obese.
Participants were then reclassified using dual-energy X-ray absorptiometry (DXA) scans.
The scans 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 per cent were either overweight or obese.
