The debate between soy food consumption and breast cancer risk has been a hot debate for years. Now, new research from Tufts University on dietary isoflavones, found primarily in soy, confirms a positive take on the soy and breast cancer discussion—there’s an association between high intake of soy and reduced-all cause mortality in breast cancer survivors. The epidemiological study, led by nutrition and cancer epidemiologist Fang Fang Zhang, M.D., Ph.D., was published on March 6th in the journal Cancer. Dietary isoflavone intake post-breast cancer diagnosis was examined using food frequency questionnaires for 6,235 American and Canadian women enrolled in the Breast Cancer Family Registry Dietary.

Participants were divided into four groups based on isoflavone intake, those in the highest group consumed at least 1.5 milligrams of isoflavones per day, equivalent to a few dried soybeans. A 21 percent decrease in all-cause mortality was observed among women in the highest group of dietary isoflavone intake, which was limited to those with hormone-receptor-negative tumors who were not treated with endocrine therapy. This research supports a previous large-scale prospective cohort analysis of over 9,500 American and Chinese breast cancer survivors published in the American Journal of Clinical Nutrition in 2012. High post-diagnosis dietary isoflavone intake was associated with a non-significant reduced risk of breast cancer-specific mortality and a statistically significant reduced risk of breast cancer recurrence.

Despite other supporting research on the health benefits of dietary isoflavone intake from soy consumption, it has remained a controversial topic when it comes to breast cancer, and confusion about its health effects continues. Soybeans are one of the richest sources of isoflavones – phytoestrogens (plant estrogens) that share a similar chemical structure and similar qualities to the hormone estrogen, yet are not the same. Because of these estrogen-like compounds, intake of soy was once thought, and still is by many, to increase risk of breast cancer. Since higher blood levels of estrogen are linked to increased risk of breast cancer, primarily in post-menopausal women, it’s natural to assume that phytoestrogens may increase risk as well, which is why their effects remain an active area of research.

Isoflavones are thought to act as natural SERMS, or selective estrogen receptor modulators. SERMS are compounds with only “selective” estrogenic effects, meaning they can selectively inhibit or stimulate estrogen-like actions in various tissues. Soy isoflavones have unique anti-estrogenic and anticancer properties; and, any estrogenic effects they possess are also much milder than the effects of the body’s own estrogen, but have still raised concern. Currently, there is little to no evidence to suggest that these weak estrogenic effects have a clinically relevant impact on the breast tissue of women with or without history of breast cancer.

The bottom line—according to the American Institute for Cancer Research, the evidence consistently supports that moderate consumption of soy foods does not increase cancer risk and is safe for cancer-free women and breast cancer patients and survivors. In fact, moderate soy consumption may even offer modest protection against breast cancer.