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Is Soy Safe for Cancer Patients?

Stephanie Meyers, MS, RD/LDN
Senior Nutritionist
Dana Farber Cancer Institute

“What should I do about soy?” is a common question among cancer survivors. On one hand, consuming soy foods like edamame, tofu and unsweetened soy milk have been shown to reduce the risk of certain cancers such as breast cancer, prostate cancer and gastric cancer. On the other hand, questions about the potential estrogenic activity of soy can lead to confusion about the safety of eating soy foods, particularly for those with hormone sensitive cancers, such as estrogen-receptor positive breast cancer.

Soy contains compounds called phytoestrogens. It is important to realize that when people eat soy it does not turn into estrogen in the body. Phytoestrogens, specifically genistein and daidzein, are structurally different and significantly weaker than human estrogen. No one food, soy included, is capable of single-handedly disrupting hormones linked to cancer growth. Nonetheless, non-evidence based sources make claims about soy that can create unnecessary fear amongst cancer patients. Let’s take a closer look at the scientific research to date.

Ongoing research on soy intake and cancer risk began decades ago when scientists observed lower rates of certain cancers, such as prostate and breast cancer, in Asian countries where soy foods were a regular part of an overall healthy diet. Current research continues to support inclusion of soy foods in the diet for general cancer prevention and for people with cancer.

When deciding about inclusion of soy in your diet, it can be helpful to think about three distinct categories of soy products:

  1. Soy Foods like edamame, tofu and unsweetened soy milk
  2. Soy Protein Supplements like protein powder or nutrition bars made with soy protein isolate
  3. Soy Condiments or by-products, such as soy sauce, soybean oil and soy lecithin

Soy Foods

Current research supports including soy foods in the diet of cancer survivors and does not suggest harmful effects, even for those experiencing estrogen-receptor positive breast cancer. In fact, research in patients with breast cancer patients suggests possible benefit to overall survival with consuming moderate amounts of soy foods, or 1-2 servings per day. Examples of serving sizes for soy foods are ½ cup of edamame, 1 cup of soy milk or ¼ cup of tofu. The bottom line is that soy foods like edamame, tofu and unsweetened soy milk can safely be included as an alternative protein or dairy source, even for those going through cancer treatment.

Soy Protein Supplements

The effect of soy protein supplements and soy-derived protein powders on cancer growth is less clearly understood. This type of powder is typically used to make a smoothie or shake but can also be the source of protein in nutrition bars, certain pre-packaged frozen veggie burgers and vegetarian/vegan meat alternatives. Research is less clear on the effects of consuming soy protein in this form, as levels of phytoestrogens in soy products are variable. Theoretically, these products could provide higher levels of phytoestrogens if taken consistently due to their concentrated nature. While consensus on clinical guidelines for soy do not yet exist, some patients elect to minimize intake of soy protein powder supplements (soy protein isolate) given the lack of research to support inclusion.

Soy Condiments

Soy sauce, soybean oil and soy lecithin are examples of soy products that do not contain significant levels of phytoestrogens.

For those who do include soy in their diet, additional questions about genetic modification (GMO) and conventional versus organic options sometimes arise. Undeniably, soy is a crop that undergoes genetic modification in U.S. agriculture. The short and long-term effects of genetic modification as it pertains to soy and cancer risk have not been well studied and are subject to much debate. Regardless, those seeking to avoid GMO in soy foods can elect to purchase organic options.

The take home message regarding soy and cancer is that eating tofu stir-fry, an edamame appetizer or having unsweetened soy milk as a replacement for dairy is safe for cancer survivors. This is true for women with estrogen-receptor positive breast cancer and all others. Those undergoing treatment for ER+ breast cancer may want to avoid soy protein isolate in powdered form as well as soy protein enriched nutrition bars or vegetarian meat replacements until further data is available.