Who are you calling fat? We are bigger than ever, but think we're thinner

We are getting fatter but we think we are thinner, and the trend could have serious implications for the obesity epidemic, a study has found.

Growing obesity in the population has increased what is perceived as "normal" weight, which is leading to large numbers of people underestimating how fat they really are. As a result, more people are failing to recognise they are overweight, despite an increase in the prevalence of obesity in the population.

However, thin women who are prone to feeling they are overweight have benefited from the change. The findings show they are less likely to perceive themselves as fat than in the past.

Professor Jane Wardle, of the Health Behaviour Research Centre at University College London, said: "Recognising you are overweight is the first step to doing something about it. Those who do not see themselves as carrying excess pounds will not be motivated to act."

Comparison of two household surveys conducted in 1999 and 2007 showed the proportion of respondents who had a body mass index (BMI) of 25 or over, defined as overweight, increased from 43 to 53 per cent. But the proportion who correctly identified themselves as overweight fell from 81 to 75 per cent.

The proportion who were obese (BMI of 30 or over) nearly doubled from 11 to 19 per cent but most of these were aware they were too fat. Body mass index is a composite measure calculated as height in metres squared, divided by weight in kilograms.

Professor Wardle said: "The whole population weight has been moving up and as a result people in the mid-range with a BMI of 26 or 27 [classed as overweight] think of themselves as being normal. They can't believe that what is normal among their friends and neighbours could be overweight.

"The increase in population weight has changed the norm. I have conversations about obesity with friends who have BMIs over 30 yet they never appear to be aware it is a problem that applies to them. It always seems to be about other people."

Professor Wardle said that the publication of pictures of grossly obese men and women in the media may have contributed to the trend. "There is such a lot of publicity about obesity and the images used are always of someone with a very high BMI of 40 or 45 because they have the most impact. Someone with a BMI of 30 looks fine beside that."

It was essential people had an accurate perception of their own weight, she said, so that those who needed to shed pounds would be motivated to do so.

Only 75 per cent of women and 40 per cent of men with BMIs of 26 perceived themselves as carrying excess weight in 2007, down from 85 per cent and 65 per cent in 1999. "We would like people to be taking action [to control their weight] when their BMI is in the range of 26 or 27," Professor Wardle said. The study was based on self-reported weights and heights. Research shows people commonly underestimate their weight and overestimate their height, so the situation was likely to be even worse than it appears.

Perception and reality

43 per cent were overweight in 1999

81 per cent identified themselves as overweight in 1999

53 per cent were overweight in 2007

75 per cent identified themselves as overweight in 2007