Are Soy Foods Safe for Breast Cancer Patients?

The SIO Research Committee is pleased to offer this second installment in a new blog series known as "Myths of Cancer". In this series we will address some of the most common myths and misperceptions that arise around cancer risk and treatment related to diet and natural health products, as well as other complementary therapies such as yoga, acupuncture and meditation. If you have a question you'd like us to address or comments about this post, please send your suggestions to: info@integrativeonc.org.

We hope you enjoy the series!
Linda Carlson and Eugene Ahn, Research Co-Chairs.

Written by Omer Kucuk, MD

Omer Kucuk, MD is a Professor in the Department of Hematology and Medical Oncology at Emory University School of Medicine/Winship Cancer Institute. He is a veteran member of SIO and has a primary research focus on nutrition and cancer, conducting preclinical and clinical studies with soy isoflavones, lycopene and other nutritional and botanical compounds for over 20 years. Disclaimer: The opinions expressed here are the author's own, and not necessarily those of the Society of Integrative Oncology or Emory University School of Medicine. The SIO supports open dialogue between health care practitioners and patients to make sure patients can make informed decisions. As always, your comments and feedback are welcome.


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.

 

Quick Answer Box

The safety and health benefits of soy foods are well established and it is probably safe for women with breast cancer to consume soy foods. However, greater caution is advised regarding use of soy derived isoflavone supplements such as genistein. 

 

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. However, the use of soy isoflavone supplements has not been evaluated well in human studies and precautionary findings have been published regarding a specific isoflavone genistein (aglycone of the main soybean isoflavone genistin) in breast cancer animal models (Hsieh CY 1998, Andrade JE 2014). Therefore, caution is advised regarding the intake of soy isoflavone supplements in women with breast cancer, and dietary consumption of soy-containing foods is preferred.

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