Soy for breast cancer survival

Soy is a source of chemicals known as isoflavones that are known to mimic oestrogen. For this reason it has been used widely for women’s health conditions and now new research suggests that it can reduce recurrence of breast cancer.

Isoflavones have a chemical structure that is similar to oestrogen which means that they can trigger oestrogen-like effects in the body. However, the oestrogen action of isoflavones is much weaker than that of oestrogen itself.

Nevertheless there has been concern that foods like soy may be problematic in hormone-dependant breast cancers which rely on oestrogen and progesterone to grow. A test done on the tumour itself will determine whether or not it is hormone dependant. Estimates are that between 50 and 70 per cent of breast cancers are sensitive to either oestrogen or progesterone.

To shed some light on this issue researchers from the Harbin Medical University, China monitored more than 500 women who had undergone breast cancer surgery. The aim of the study was to see what impact soy isoflavones might have on the women receiving post-surgical hormone therapy. Food frequency questionnaires were used to establish the isoflavone intake of these women. The study began in 2002-3 and the women were followed for five to six years.

What emerged was that for women who consumed less than 15mg of soy isoflavones per day there was hormone dependant breast cancer recurrence rate of 18.7 per cent. By contrast, among women who consumed more than 42mg of soy isoflavones per day the recurrence of hormone dependant breast cancer was only 12.9 per cent.

In case you are wondering how many isoflavones you consume from soy, here is a guide to working it out. Raw soybeans contain between two and four milligrams of isoflavones per gram. Foods made from soy differ in their concentration of isoflavones. In a 100g serve raw soy beans will have around 200mg of isoflavones, tempeh will have around 50-60mg and tofu will have around 40mg, but all of the traditional soyfoods, such as tofu, soy milk, tempeh and miso, are rich sources of isoflavones providing between 30 and 40 milligrams per serving. One-half cup of soy flour contains approximately 50mg of isoflavones and soy milk has around 20mg of isoflavones per cup. Soy sauce and soy oil do not contain isoflavones.

In the case of hormone sensitive breast cancer preventing oestrogen from binding to cancer cells is the aim of treatment post surgery. It may be that by binding to oestrogen receptors on cancer cells but exerting a weaker effect the soy isoflavones are exerting the blocking effect that is desired.

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