Soya beans contain large amounts of phyto-oestrogens which may have a protective effect. The traditional East Asian diet is particularly high in isoflavonoids, one type of phyto-oestrogen. Levels of isoflavonoids in the blood have been found to be 7 to 110 times higher in Japanese men with a low incidence of prostate cancer, compared to Finnish men. However, just how phyto-oestrogens help is not yet fully understood. Professor Norman Blacklock from Manchester University believes they may exert a ' weak oestrogen effect' : there is evidence that phytooestrogens may block oestrogen receptor sites, thereby lowering body levels of active oestrogen. If this proves to be so, it is consistent with accumulating evidence that many modern diseases, including breast and prostate cancer, are the result of too much oestrogen.

There is also evidence that a protease inhibitor in soya, Bowman-Birk Inhibitor (BBI), may be another key anti-cancer compound. A study at the University of Pennsylvania School of Medicine added BBI to the diet of rats that had previously been fed a substance known to induce colon cancer. None of the rats developed tumours. In another similar study, BBI suppressed the formation of tumours by 71 per cent.

Whatever the mechanism, phyto-oestrogens have consistently been associated with reduced cancer risk. Women whose diets are abundant in soya beans have a lower risk for breast cancer, while men with a high soya intake have a substantially lower risk of prostate cancer. Research is beginning to focus on two isoflavonoids - genistein and daidzein. Japanese women, who generally have a lower risk of breast cancer than women in other industrialised societies, have been found to have higher levels of these in their bodies. They may protect against the harmful effects of unopposed oestrogen. In fact, a recent study from Singapore, which monitored a group of women for early signs of breast cancer, found that the more soya a woman ate, the less chance there was of having pre-cancer changes in breast cells.

A likely ideal intake for cancer prevention is around 5mg a day of genistein and daidzein, which you can get from a 120z serving of soya milk or a serving of tofu. Soya milk can be used in drinks and on cereal like cow' s milk, while tofu is excellent in stirfries. Tofu is the richest source of isoflavones, while very processed soya products are the poorest source.

However, I don' t advise having more than this. Even plant oestrogens could be oestrogenic in excess, and you can develop allergies to soya if you eat too much of it.