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Soy isoflavones reduce risk of breast cancer recurrence

Soy foods are rich in isoflavones and have been hypothesized to reduce the risk of breast cancer, but their similar structure to that of estrogen and a possible interaction between isoflavones and anti-cancer drugs have caused concern about soy food consumption by breast cancer patients. The soy isoflavone genistein has been shown to stimulate the proliferation of cultured breast cancer cells and to promote growth of estrogen-dependent mammary tumors implanted in ovariectomized rats. A study published in the Canadian Medicinal Association Journal investigated the effect of soy isoflavones intake on the recurrence of breast cancer and death for patients receiving adjuvant endocrine therapy.
Xinmei Kang and co-workers conducted a study on 524 women who underwent surgery for breast cancer and who were receiving the drugs tamoxifen and anastrozole. They measured dietary intake of soy isoflavones, which was coming mostly form soy milk, tofu and soy meal, and divided the women in for groups depending on soy isoflavones intake. The lowest quartile had a daily soy isoflavones intake of less than 15 mg, whereas women from the highest quartile consumed more than 42 mg. The average daily isoflavones intake for all women was 26 mg, which falls within the range of soy isoflavones intake in Asian countries.. They found that 5 years after the operation the overall death rate of premenopausal women was not related to intake of soy isoflavones. For postmenopausal women they found a protective action of isoflavones: the risk of recurrence in the highest quartile was significantly lower (-12%) than in lowest quartile. Women with estrogen and progesterone receptor positive cancer and women receiving anastrozole therapy also showed significant lower recurrence rates. The scientists suggest that the beneficial effects of soy isoflavones is exerted through their interaction with estrogen and progesterone receptors.

The study concluded that high dietary intake of soy isoflavones was associated with lower risk of recurrence among postmenopausal women with estrogen and progesterone positive breast cancer and those receiving the anti-cancer drug anastrozole.

Source: Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. CMAJ. 2010 Oct 18.

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