Experts say that these can have an effect on how people perceive themselves, but the causes of eating disorders are usually more complex and are linked to general feelings of self-worth.

Anorexia nervosa

Around 5% of young girls in the UK are estimated to have anorexia nervosa. Boys and children from ethnic minorities are much less likely to be affected.

The condition results in death in 20% of cases after 20 years of onset of the illness.

Only around 60% of anorexics recover. The illness is also one of the most controversial areas in mental health.

Psychiatrists have singled out several characteristics which they say are typical of anorexics.

These include: a dominant, over-protective and critical mother and a passive or withdrawn father and a tendency to perfectionism, a strong desire for social approval and a need for order and control.

However, many of these characteristics have been the subject of dispute.

The media and its emphasis on super-thin models is also blamed by some for influencing the way people, particularly girls, see themselves and making them believe looks are all-important.

Feminists argue that it is not the images in themselves which are harmful, but the fact that women still derive much of their sense of self-worth from whether they please men.

Anorexia nervosa is a form of intentional self-starvation. What may begin as a normal diet is carried to extremes, with many reducing their intake to an absolute minimum. It is also characterised by obsessive behaviour. The majority of anorexics deny they have a problem.

Lack of food deprives the body of protein and prevents the normal metabolism of fat. The effects of this can include:

• an irregular heart beat caused by a change in the heart muscle. This can lead to heart failure and death.
• ceasing of menstruation
• dehydration, kidney stones and kidney failure
• the growth of fine downy body hair, called lanugo, on the face and arms
• wasting away of muscles, leading to weakness
• constipation or bowel irritation
• osteoporosis caused by lack of calcium.

Symptoms of anorexia

Symptoms of anorexia range from extreme weight loss for no discernable medical reason; ritualistic food habits, such as excessive chewing; denying hunger and exercising excessively to choosing low calorie food and hiding feelings. A person with anorexia may be excessively thin but still see themselves as overweight.

The average age for onset of the illness is thought to be 16, although the age range of anorexia is between 10 and 40. Around 90% of cases are female. Most have no history of being overweight.

Treatment for anorexia

Over 25% of anorexics are so weak that they require hospitalisation. This may involve force feeding as well as advice on healthy eating and counselling.

Many doctors believe that once a person's bodyweight has fallen below a certain level, they are no longer capable of making rational decisions.

There has been wide-ranging debate over whether anorexics should be force-fed or whether they have the right to literally starve themselves to death.

In 1997, guidance was offered to doctors, telling them that they can force feed anorexics over the age of 18 under the Mental Health Act 1983. The anorexic must be shown to be incapable of making rational decisions about their condition.

Other forms of treatment range from group therapy, family counselling and psychotherapy to antidepressants.

Around one third of patients recover fully; another third improve significantly and the last third do not recover.

Bulimia nervosa

Bulimia is thought to be two to three times more common than anorexia, but is not generally as physically dangerous.

However, excessive use of laxatives and self-induced vomiting can cause rupture of the oesophagus, mineral deficiency and dehydration, which can have serious effects on health.

Bulimia was only officially recognised in the 1970s and is characterised by a cycle of bingeing and starving.

Many bulimics seem fine, but experts say that, under the surface, they often feel worthless.

Bulimics may have irregular periods or stop having periods at all because of excessive use of laxatives and vomiting.

Using laxatives can also cause kidney and bowel problems and stomach disorders.

Laxatives do not cause people to lose weight, but remove water and essential minerals, such as potassium, from the body, giving the appearance of weight loss.

Excessive vomiting can cause tooth decay, bad breath, mouth ulcers, sore throats and stomach disorders and may have serious long-term health implications.

Some experts believe bulimia is the result of an imbalance of chemicals to the brain, but others think the illness is more likely to be linked to a lack of self-worth.

It is thought that up to half of anorexics also suffer from bulimia and some 40% of bulimics are reported to have a history of anorexia.

Sufferers tend to be older than anorexics, take a longer time to recover and are more likely ot commit suicide.

Treatment for bulimia includes the use of drugs such as antidepressants, counselling which examines the cause of bulimic behaviour and behavioural modification, including education about healthy eating.