Soy foods, phyto-oestrogens and cancer

What are phyto-oestrogens and where are they found?
Phyto-oestrogens are compounds found in plants that may act like the hormone oestrogen when consumed. Foods high in phyto-oestrogens include soy products (soy milk or yoghurt, tofu and tempeh), flaxseed, legumes and whole grains. The phyto-oestrogens in soy foods are also known as isoflavones.

Phyto-oestrogens have a very similar structure to the body’s own oestrogen. The effects of phyto-oestrogens on the body are not fully understood, it is believed that they may act like weak oestrogen in some situations, but also block the actions of oestrogen in others.

Do phyto-oestrogens and soy foods protect against breast and prostate cancer?
Lower rates of breast and prostate cancer in some Asian countries, where soy is very common in the diet, led scientists to investigate the link between eating soy foods and protection against breast and prostate cancer. Due to their different dietary intakes, and also different genetics, it is difficult to determine whether it is the soy in the diet or some other factor. Overall in large studies on people it seems like a high consumption of soy foods may lower the risk of breast and prostate cancers, but only a little. Animal and test tube studies do support an anti-cancer effect.

More studies are needed to examine if phyto-oestrogens have a protective effect against breast and prostate cancer. From the current evidence, it is believed that a moderate consumption of soy foods (e.g. 1-2 serves of soy foods/day) along with an overall healthy eating plan is unlikely to have adverse effects. This is consistent with Cancer Council’s recommendations and dietary guidelines to eat a diet rich in plant foods.

There is no evidence supplements that contain high doses of soy or soy isoflavones are effective in preventing cancer, and are therefore not recommended.

Should women with breast cancer eat soy foods?
It is not clear if eating a diet high in phyto-oestrogens is safe for women who have breast cancer. Tamoxifen is a common treatment for women with oestrogen receptor positive breast cancer. Tamoxifen works by blocking the actions of oestrogen, and therefore stopping or reducing tumour growth.

For women with oestrogen receptor positive breast cancer who are taking Tamoxifen, it is still unclear whether eating soy foods or taking soy supplements will block or enhance the actions of Tamoxifen. Results of scientific studies are contradictory. A moderate consumption of soy foods, as part of an overall healthy eating plan, is unlikely to have any harmful effects.

Should women with breast cancer take soy supplements?
High dose soy or soy isoflavone supplements have not been tested for safety in women who have breast cancer or are taking Tamoxifen. The best advice is to eat soy foods in moderation as part of an overall healthy eating pattern, and not to suddenly increase the amount of soy phyto-oestrogens in the diet. Cancer Council recommends that women with breast cancer avoid soy and phyto-oestrogen supplements.

What can women with breast cancer do to improve their health?
Research is underway looking at the types of eating patterns that are protective for women who have had breast cancer. Evidence is starting to emerge that maintaining a healthy weight by eating a low-fat diet with plenty of fruit and vegetables and being physically active can improve survival and the overall health of breast cancer survivors.

See the  Soy, Phyto-oestrogens and Cancer Position Statement for more information.