Are Soy Foods Safe for Breast Cancer Patients?

Many women with breast cancer have been told to avoid soy foods. This warning is based on the assumption that plant estrogens (phytoestrogens) found in soybeans could be harmful. The assumption is that soy food sources of estrogens might somehow "feed" cancer cells and act in opposition to anti-estrogen medications like tamoxifen, thereby increasing breast cancer risk.

But what really happens when women eat these soy products? Biologically, the isoflavones in soybeans are phytoestrogen particles that bind to both estrogen receptors alpha and beta, but with a preference for the latter. Therefore, they are selective estrogen receptor modulators (SERMs), similar to tamoxifen and raloxifene, drugs used in breast cancer treatment and prevention. Therefore, rather than "feeding" cancer cells and acting in opposition to SERM medications and aromatase inhibitors, dietary intake of soy foods would be expected to reduce breast cancer risk by having antiestrogenic effects. In fact, breast cancer risk is lower in countries where soy consumption is high.

Recent research also supports the idea that soy consumption is not only safe, but can be beneficial. A recently published study (Zhang FF 2017) showed an inverse association between dietary soy intake and all-cause mortality in a cohort of 6235 women with breast cancer in North America. Women with the highest dietary isoflavone intake had a 21% decrease in all-cause mortality compared to women with the lowest intake. Another study (Nechuta SJ 2012) found that soy food consumption after a diagnosis of breast cancer was associated with improved treatment outcomes and lower recurrence rates. They found that higher post-diagnosis soy intake was associated with a 25% reduction in tumor recurrence.

Keeping this in mind, how should health care providers respond to women with breast cancer who ask whether it is safe to consume soy foods? We now have the answer: It is probably safe (Kucuk O 2017). The general message to patients with cancer should be: "Be physically active, have a normal body weight, consume a healthy diet (rich in vegetables and low in sugar), and reduce stress." Soy foods can be consumed as part of a healthy diet and a healthy lifestyle.

We are also beginning to get the question "How much soy should I eat to obtain the most benefit?" because many women have become aware of the results of recently published studies. A large variety of soy foods are readily available in stores including soy milk, edamame, tofu, and others. For example, an 8-ounce glass of soy milk typically provides 25-30 mg of soy isoflavones. Therefore, it should be easy to consume sufficient amount of soy foods as part of a healthy diet. The results of recent studies in North America showed that even small quantities of soy foods (containing 1-2 mg soy isoflavones) could improve the outcome of breast cancer treatments (Zhang FF 2017, Nechuta SJ 2012).

In summary, the safety and health benefits of soy foods are well established and suggest it is reasonably safe for women with breast cancer to consume soy foods.

References

1 Zhang FF, Haslam DE, Terry MB, Knight JA, Andrulis IL, Daly M, Buys SS, John EM. Dietary Isoflavone Intake and All-Cause Mortality in Breast Cancer Survivors: the Breast Cancer Family Registry. Cancer 123(11): 2070-2079, 2017

2 Nechuta SJ, Caan BJ, Chen WY, Lu W, Chen Z, Kwan ML, Flatt SW, Zheng Y, Zheng W, Pierce JP, Shu XO. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr 96(1):123-132, 2012

3 Kucuk O. Soy foods, isoflavones and breast cancer (Editorial). Cancer. 123(11):1901-1903, 2017

4 Hsieh CY, Santell RC, Haslam SZ, Helferich WG. Estrogenic effects of genistein on the growth of estrogen receptor-positive human breast cancer (MCF-7) cells in vitro and in vivo. Cancer Res. 58(17):3833-8, 1998

5 Andrade JE, Ju YH, Baker C, Doerge DR, Helferich WG. Long-term exposure to dietary sources of genistein induces estrogen-independence in the human breast cancer (MCF-7) xenograft model. Mol Nut Food Res. 59:413-23, 2014