Anorexia and starvation can cause menstruation to stop (amenorrhoea). In order for menstruation to function normally about 22 per cent of a woman's body should consist of fat. If menstruation does not return after anorexia has been cured, the problem may be lack of iron. If menstruation does not return despite normal weight and normal iron values, a gynaecologist should be consulted.

Menstrual cycle interruption (amenorrhoea) can have many causes; in the case of anorexia it is due to the reduction of the normal production of female sex hormone precursors by the hypothalamus (the part of the brain that controls glandular secretions), causing a reduction of sex hormone levels in the blood.

This reduction is caused by weight loss and food restriction. Menstruations usually return to normal a few months after weight recovery.
The principal effects of these reductions are the perturbation of all physiological activities controlled by sex hormones.

The body stops the menstrual cycle in order to protect blood and protein reserves.

Amenorrhoea is combined with reduced sexual desire (in men it is linked with sexual impotence). This is also due to the reduction of hormone levels in the blood. This phenomenon is called sexual anorexia.

A progressive reduction of secondary sexual traits (fat and muscular mass distribution and voice timbre distribution) may also occur as a result of reduced secretion of sex hormones.

The drop in hormone levels, linked to malnutrition, causes bone and teeth demineralisation (teeth are often attacked by the acids that are present in vomit) and can result in osteoporosis (loss of bone mass).

Since the changes described are caused by malnutrition it is useless, if not detrimental, to try to resolve them by giving female hormones (the "pill", to be precise). Hormone therapy would only encourage new monthly losses that the body can not recover and, in the long-term, will only worsen the situation.