The term oestrogen refers not to one hormone but to a group of hormones: oestradiol, oestrone and oestriol, which promote female characteristics and reproductive functions. Oestrogen is made in the ovaries in women, and in the adrenal cortex and peripheral body fat in both men and women. It is also made by the male testes.

During the first half of the menstrual cycle, oestrogen is produced in response to the stimulus provided by follicle stimulating hormone, and renders the female reproductive system ready for fertilization, implantation and nutrition of the early embryo. Female sex drive is not dependent on oestrogen but on male hormones (androgens) synthesized by the female adrenal glands. Poor adrenal function or removal of the adrenals can therefore greatly reduce the sex drive.

Small amounts of testosterone are produced by the ovaries, which then use enzymes to convert them to oestrogen. In the female body an excess of oestrogen (or of progesterone) promotes fluid retention, while an oestrogen deficiency causes hot flushes of menopause.

Pharmaceutical preparations of oestrogen are used in oral contraceptives and as 'hormone replacement therapy' (HRT) to relieve menopausal discomforts and treat osteoporosis. Oestrogens in oral contraceptives have an adverse effect on circulation, and have caused venous thrombosis in many Pill users. HRT is associated with dependency problems by suppressing the body's own natural oestrogen production. Discontinuing the medication can lead to withdrawal symptoms which are identical to the original hot flushes and other problems but more severe. This makes the patient reluctant ever to discontinue the treatment and she may require higher and higher doses to maintain a feeling of well-being. Both oral contraceptives and HRT are associated with a higher risk of some cancers.

Cow's milk is the most significant source of natural oestrogens in the western diet. However phyto-oestrogens (plant oestrogens also known as isoflavones) are found in many foods, including wheat, rice, soya products, oats, barley, carrots, potatoes, apples, cherries, plums and parsley, and in herbs such as sage leaf, hops and liquorice. Vegetable oils, including safflower, wheatgerm, corn, linseed, peanut, olive, soya and coconut oils, may also be high in phyto-oestrogens. One of the most potent phyto-oestrogens, genistein, is found in soya products. Dietary phyto-oestrogens have been found to have an oestrogen-balancing effect on female health. Both problems of excess oestrogen, such as breast cancer, and problems of inadequate oestrogen, such as hot flushes, are reduced in populations with a high soya intake. Soya products also seem to be effective in preventing cancer of the prostate in men. A large body of scientific research now indicates that phyto-oestrogens are generally protective against cancer and reduce cholesterol levels.

In experimental animals, oestrogen has been shown to influence food intake. On days of the animal's cycle when oestrogen levels are high there is a decrease in food intake, likewise when animals are given oestrogen-like compounds in their diet.

An excess of oestrogen (in its oestradiol or oestrone rather than oestriol form) can promote tumours of the female reproductive system. In fact these cancers account for half of all cancers in women. Breast cancer is one of the major causes of mortality in the West. Many cases of breast cancer are hormone-dependent; that is to say, removal of the ovaries can, at least temporarily, check their growth. In populations with a high incidence of breast cancer, it has been found that women excrete a larger quantity of total oestrogens in their urine but a smaller quantity of oestriol than women in low-risk populations. The most active form of oestrogen, known as oestradiol, is normally metabolized to oestrone, and then to oestriol. Low oestriol levels suggest that the liver is not performing this metabolizing function adequately. The higher levels of circulating oestradiol may encourage tumour formation. Recent research shows that dietary broccoli may help the liver to metabolize oestrone more efficiently. In a study carried out on 16 men and women, 500 grams of broccoli a day added to their diet resulted in an increase in the cytochrome liver enzymes involved in oestrone metabolism. (Kall MA et al: Effects of dietary broccoli on human in vivo drug metabolizing enzymes: evaluation of caffeine, oestrone and chlorzoxazone metabolism. Carcinogenesis 17(4):793-9, 1996.) High fibre diets can also help to reduce blood oestrone and oestradiol levels (Rose DP et al: High-fibre diet reduces serum estrogen concentrations in premenopausal women. Am J Clin Nutr 54(3):520-5, 1991).

Excess body fat is associated with a higher incidence of female reproductive cancers, and obese women may have elevated oestrogen levels. Because adipose (fat) tissue is a major site of oestrogen synthesis even after the menopause, obese women may have a later menopause with fewer hot flushes and other symptoms.

Oestrogen and antibiotics. Normally over 60 per cent of circulating oestrogen is excreted into the intestines through the bile, to be deconjugated (broken down) by bacterial enzymes. Some of the oestrogen is reabsorbed and excreted in the urine. However, when antibiotics are used, oestrogen reabsorption drops, lower levels are found in the urine, and the quantity found in faeces may be up to 60 times greater than normal. Because of this excessive loss of oestrogen, antibiotics can lead to breakthrough bleeding in women.