The eating disorder anorexia nervosa could be caused by a brain dysfunction, rather than social pressures, according to a radical new theory.

Researchers at St George's hospital in Tooting, south London have found that sufferers from anorexia have an abnormality in the blood flow to an area of the brain which affects body image.

In results presented for the first time yesterday at an international conference in London, they say this points to a biological cause for the condition.

More than a million people in Britain alone suffer from eating disorders which have been thought to be caused by social and cultural pressures to be slim.

There are 11 new cases of anorexia diagnosed annually per 100,000 population, 90 per cent of them women, according to the Eating Disorders Association.

Bryan Lask, professor of child and adolescent psychiatry at St George's Hospital and an expert on anorexia who led the latest research, said the finding of a biological cause could change the way it was treated.

"The biological cause has been under-emphasised and the socio-cultural pressures have been over-emphasised. There has to be a biological contribution otherwise everyone would be anorexic given the socio-cultural pressures in our society," he said.

"I am not saying people are born anorexic any more than they are born asthmatic or diabetic. But I am saying some people are born with a genetic pre-disposition to anorexia which makes it more likely to develop.

"If you live in a society which promotes thinness as the ideal then you are more likely to develop anorexia if you have the biological predisposition."

The study of 60 patients, some of whom have been followed for a number of years, is to be published in the International Journal of Eating Disorders.

The researchers used brain imaging and neurological testing which revealed the abnormal blood flow.

"There is a low blood flow in one specific part of the brain on one side. It only occurs in people with anorexia and does not occur in people without anorexia," Professor Lask said.

The part affected, called the insula, links other key parts of the brain involved in the disorder.

Anorexia is marked by intense feelings of anxiety, linked with the amygdala, restlessness and irritation (basal ganglia), obsessive thoughts (frontal lobe), visuo-spatial difficulties (parietal lobe) as well as body image (the somato-sensory cortex).