GPs underestimate body mass index (BMI), according to researchers who asked 315 doctors to assess 15 standardised photographs.

There were five images each for healthy weight (BMI 19.4-22.4kg/m2), overweight (25.6-28.3kg/m2) and obese (30.5-34.4kg/m2) white British men aged between 18 and 30 years. GPs estimated BMI, said whether the man was a healthy weight, overweight or obese, and whether they would be likely to suggest weight management.

The 248 trainee GPs correctly classified, on average, only 3.9, 2.3 and 1.6 of the healthy weight, overweight and obese images respectively. The 67 qualified GPs fared a little better, correctly classifying, on average, 4.2 of the five healthy weight men, but just 2.8 of overweight males and 2.1 of the obese.

For each 1kg/m2 increase in BMI, GPs underestimated BMI by 0.21kg/m2. In other words, GPs underestimated the BMI of a 30kg/m2 man by about 2.5kg/m2.