The Princess of Wales says exercise has enabled her to overcome bulimia. Perhaps it is just another way of subduing her body. By Jerome Burne
Can someone with an emotional problem decide when they are cured? If faulty brain chemistry is involved, where does that leave psychotherapy? What difference does it make if there is a genetic link? Such questions are prompted by the latest news about the Princess of Wales's eating disorder - via yet another treacherous confidant.

Last week she revealed to patients at Priory Hospital in south-west London that, although she had spent many hours and thousands of pounds on a variety of brands of psychotherapy, in the end she found it pointless because the practitioners she had seen had not known what she had gone through. Instead, she announced, exercise was what kept her bulimia at bay.

So a slap in the face for therapy? Not necessarily, because once you enter the psychotherapeutic hall of mirrors, where nothing is quite what it seems, there are several way of looking at that. It could indeed be the simple truth. Exercise is a good way of dealing with depression and it might be that the Princess has got it sorted. But therapists have perfected an irrefutable defence when faced with ungrateful clients. "In denial" they say, or "counter-transference".

Here, for instance, is an expert writing in the venerable American Family Physician: "Anorexic patients frequently deny the severity of their illness, which makes therapy difficult. They perceive their thinking as being entirely rational and the rest of the world as dysfunctional." However, even when they are not blaming the victim, the experts' own performance is not that impressive - the cure rate for anorexics is reckoned to be between 45 per cent and 60 per cent and long-term about 20 per cent die from the condition.

But maybe the Princess's therapy was successful in that it gave her the confidence to look after herself. However, here the issue gets even more tricky because a well-known sign of anorexia is an obsession with exercise. The sufferer just replaces one method of subduing and controlling their alienated body with another. So the question then becomes, how much is too much?

Faced with all this conceptual confusion, it is not surprising that many eating disorder researchers are starting to take a more biologically based approach and are trying to find the key to anorexia in the chemical workings of the brain. Another of the revelations from the Priory - that the Princess had idolized her older sister Sarah who was anorexic - fits in well with the latest thinking. "There definitely seems to be a genetic element in anorexia and bulimia," says Nigel Brown at the Eating Disorders research unit at Maudsley Hospital in south London. "Identical-twins studies show that it does run in families."

As yet there is no clear idea of what the genes might be coding for but a popular candidate is one of the brain's messenger chemicals. "After we've eaten, serotonin is released in the brain which tells the appetite centre 'I'm full'," Brown says. "Now it could be that in anorexics' brains the signalling system sends the message out too soon or that the receiving end is over-sensitive and responds to tiny amounts." All of which raises the possibility of a drug treatment to restore serotonin functioning to normal, but whether that would be more effective than therapy and/or exercise no one knows yet.

But for all our neurochemical sophistication, a glance at history suggests that some things never change. Next year is the 100th anniversary of the death of "Sisi", Empress of Austria, who after four children still had a 22-inch waist at the age of 60, after a lifetime battling with her weight. Her story sounds remarkably familiar.

From a bourgeois background, she married very young into the rigidly orchestrated protocol of the imperial court where her mother-in-law firmly pulled the strings. She was a great beauty, very shy, who at one point wanted to devote her life to caring for mental patients. Feeling stifled and unloved at court, she soon separated from her husband and spent much of her time visiting the spas and health centres of Europe. Her recipe for keeping slim, besides fasting, was constant exercise - horse-riding, lengthy walks, gymnastics and fencing. Could her genes have found their way into the Spencers?