Staff in adolescent units have become adept at finding food hidden in clothing or about the ward. Treatment programmes for anorexics are usually strictly controlled.
Their fear of getting fat is so overwhelming that they fiercely resent pressure to make them eat. Up to 10 per cent will eventually starve to death or commit suicide.
Although skeletally thin and dangerously underweight, their self-image can be so distorted that they believe that at best they are of normal weight, or at worst that they are obese.
Their beliefs about how food can make them gain weight are also faulty. Most are genuinely frightened that one small meal will make them enormous.
Anorexia nervosa affects about 1 in 100,000, mostly women. But among middle-class teenagers and young women, the rate may be 1 in 100, rising to one 1 in 20 among ballet dancers, models and athletes.
The cult of female thinness, going back to the fashions of the 1960s, is held by most experts to underpin anorexia, a culture so pervading that even little girls who need their calories for normal growth are 'on diets'.
A recent study of nine-year-olds found one in three thought they were overweight. Half of them were underweight for their age.
There is no wide agreement among psychiatrists and psychologists about the real nature of anorexia.
Some believe it is a true phobia about putting on weight, but many experts feel it is a manifestation of depression or a personality disorder, or even schizophrenia.
It is defined by the loss of a third or more of the body's weight with, initially, people being very active or exercising constantly.
With extreme weight loss, sufferers become weak. They grow fine body hair, called lanugo hair, while their normal hair thins and their skin dries. Women stop menstruating.
The young risk damage to sexual organs, the development of which is harmed by lack of nourishment. Eventually, in extreme cases, the liver and other organs sustain damage.
Some believe that the refusal to eat relates to children's problems with their parents, since the practice is almost guaranteed to make parents anxious and exert control.
Young anorexics are usually treated in hospital psychiatric units, with a range of psychological and family therapies being used to persuade them to return to a normal diet.
Relapse is common and full recovery can take many years of psychiatric care.