Soy based foods are a good source of vegetarian and vegan protein, and have been shown to lower cholesterol, blood pressure, and reduce symptoms of menopause. These health benefits are related to soy isoflavones, primarily genistein, daidzein, and glycitein.
Intake of soy has been controversial for those at risk for breast cancer, or with a history of breast cancer, however. The concern for breast cancer is due to the chemical structure of these isoflavones, which is similar to estrogen and can possibly stimulate estrogen type activity by binding to estrogen receptors.
Those women with estrogen receptor-positive breast cancer may be at risk with intake of soy. On the other hand, some researchers think that soy isoflavones may act as anti-estrogens (by inhibiting local production of estrogen) and reduce cancer growth.
A recent study looking at estrogen-receptor positive breast cancer patients taking Tamoxifen or Anastrozole and soy intake showed no association with premenopausal patients, but a 12.9% lower recurrence rate with postmenopausal patients who had the highest soy isoflavone intake. Other studies have shown high intake of soy isoflavones during adolescence may reduce risk of breast cancer in adulthood.
Since research is still limited and contradictory, however, the American Cancer Society has suggested that while intake of 3 or less servings of soy food is most likely safe for the breast cancer survivor, high intakes should be avoided, especially concentrated sources such as soy powders and isoflavone supplements. Intake of 3 servings of soy food (such as tofu and soy flour) is comparable to Asian countries that have low incidence of breast cancer.
To know what is best for you, be sure to consult with your physician for guidance for intake of soy products in your diet.
American Cancer Society (www.cancer.org/)
Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. Canadian Medical Association Journal 2010. DOI:10.1503/cmaj.091298