People may be misdiagnosed as overweight or obese under the body mass index (BMI) system, a study suggests.
BMI is a tool which measures body fat based on height and weight.
Some 30% of adults in England were obese in 2024, and 66% were overweight or obese, according to the NHS.
Researchers in Italy compared BMI to scans which analyse fat, muscle and bone in the body.
Out of 1,351 adults included in the study, 19 were underweight, 787 were a normal weight, 354 were overweight and 191 were obese when they were measured by BMI.
People were then reclassified by using dual-energy X-ray absorptiometry (DXA) scans.
More than a third (34%) of patients who were obese based on BMI were misclassified, and should have been in the overweight category, the scans showed.
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And 53% of those labelled overweight based on BMI were in the wrong category.
Three-quarters of the misclassified overweight patients were a normal weight when scanned.
And the rest should have been in the obese category, according to the study that looked at adults who were referred to the Department of Neurosciences, Biomedicine and Movement Sciences at the University of Verona.
According to the NHS website, BMI is calculated by dividing an adult’s weight in kilograms by their height in metres squared.
For example, if you weigh 70kg (around 11 stone) and are 1.70m (around 5ft 7in) tall, you work out your BMI by:
1. Squaring your height in metres: 1.70 x 1.70 = 2.89
2. Dividing your weight in kilograms: 70 ÷ 2.89 = 24.22
Your result will be displayed to one decimal place, for example, 24.2.
For most adults, if your BMI is:
Below 18.5 – you’re in the underweight range
18.5 to 24.9 – you’re in the healthy weight range
25 to 29.9 – you’re in the overweight range
30 to 39.9 – you’re in the obese range
40 or above – you’re in the severely obese range
If you have an Asian, Chinese, Middle Eastern, Black African or African-Caribbean family background you’ll need to use a lower BMI score to measure overweight and obesity:
23 to 27.4 – you’re in the overweight range
27.5 or above – you’re in the obese range
What your result means:
A higher BMI increases the chance of developing long-term conditions, such as type 2 diabetes and heart disease.
The BMI calculation is just one measure of health. It cannot tell the difference between muscle and fat.
For example, if you have a lot of muscle, you may be classed as overweight or obese despite having low body fat.
People who are very muscular, like professional sportspeople, can have a high BMI without much fat.
Despite the BMI seeming to be reliable in determining body weight status in the normal weight range, over a third of the general population was misclassified, the researchers said.
They added that the current BMI classification “appears to inflate the prevalence of underweight, overweight and obesity among the general population”.
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The experts suggested healthcare staff should be advised not to rely just on BMI.
And instead they should combine it with measures like calculating body fat percentage or waist measurements, particularly among people who are considered to have a normal BMI.
The study was published in the journal Nutrients.
