Anorexia nervosa is a form of intentional self-starvation. It affects about 5% of young girls in Britain. It is the psychological illness with the highest death rate.

Bulimia nervosa involves a cycle of starving and eating binges and is also highly dangerous.

Some campaigners blame the incidence of such eating disorders on the prevalence of slim role models in modern society.

But research indicates that the disorders are a problem even in a society where fat is considered beautiful and has produced evidence that some people could be genetically more likely to develop them.

The research is examined in the Horizon programme Living on Air, which will be broadcast on Thursday.

Genetic risks

Doctors at the Maudsley Hospital in London have been looking for genetic causes of eating disorders.

Researchers are looking for further genes involved

They focused on the serotonin system. Its functions include determining appetite levels and sexual behaviour.

Dr David Collier is a member of the research team.

He said: "We were involved in a genetic study to look through the whole human genome to try and find genes for anorexia.

"But because of our own interest in the serotonin system, and its role in eating, we thought we'd look at that and see if we can identify any genes from their role in behaviour.

"The one we picked first was the 5HT2A receptor, because that's known to be involved in regulating feeding, and drugs that block it cause weight gain."

Dr Collier found variations in the gene for serotonin receptors in anorexic patients. They were twice as likely to have the variant gene than women without eating disorders.

Dr Collier said the next stage of the research would involve looking at how genetic risk factors interact with social factors.

Anxiety

People with high levels of serotonin are prone to anxiety.

Dr Janet Treasure, director of the eating disorders unit at the Maudsley, believes this could be behind anorexic patients' ability to suppress appetite.

Raised levels of serotonin in the brain could be to blame

She said: "In anorexia nervosa the drive to eat can be inhibited, but we know that in normal people who are starved they will kill each other and do all sorts of morally repugnant things, and eat all sorts of foodstuffs that you wouldn't normally touch.

"Yet that doesn't happen in anorexia nervosa, so there's some aspect of the appetite system that isn't working."

The unit looked at the biology of stress mechanisms, in particular the fight or flight response.

This is where the body prepares itself for action when confronted by a stressful situation.

Heart rate and blood pressure rise and two of what are usually humans' highest priorities, eating and reproducing, are put on hold.

It is possible that anorexic people are chronically in an acute state of stress reaction - they are constantly in a fight or flight state of mind.

Born with a disorder

There is further evidence to suggest that eating disorders run in the family and are something with which sufferers are born.

Dr Walter Kaye looked at family histories of eating disorders

Dr Walter Kaye, a doctor from Pittsburgh in the US, set up an international study centre to see if eating disorders ran in the family.

He found that 10% of his patients with either anorexia or bulimia had a relative who also had an eating disorder.

And Dr Hans Hook used to believe that anorexia only existed in Western countries where there was a pressure to be thin.

That was until he conducted a study on the Caribbean island of Curacao, where fat is considered attractive.

He studied the medical notes of 144,000 cases looking for signs of eating disorders. He examined 291 of them in detail and was able to confirm eight cases of anorexia nervosa.