To Soy or Not to Soy

by Pat Battaglia

Survivors of breast cancer, especially those whose tumors were estrogen-receptor positive, are often advised by well-meaning friends and family members to avoid soy foods. This myth persists despite evidence to the contrary.

It is true that soybeans contain a significant amount of phytoestrogens, which are plant-based compounds similar to human estrogen, but there are important differences. While early studies conducted in mice models suggested that phytoestrogens may have contributed to the growth of estrogen-receptor-positive breast cancer tumors, further research has shown that humans and mice metabolize these compounds differently. More recent studies revealed that breast cancer rates are lower in Asian countries, where soy foods are a staple, than in the United States. The overall body of evidence indicates that unprocessed soy foods, eaten in moderation, do not increase the risk of breast cancer recurrence.(1) Sue Czap, RD, a registered dietitian at the Integrative Oncology and Wellness Center of the Wilmot Cancer Institute, says “Statements issued in 2012 by the American Institute for Cancer Research and the American Cancer Society said that moderate intake of soy is safe for patients with hormone sensitive breast cancer.”

It may be helpful to know that soy is not converted to estrogen in the body. No single food is capable of the type of hormone disruption that may lead to cancer growth.(2)

Although foods derived from whole soybeans can be part of a healthy eating pattern, processed soy isoflavones contained in products such as protein powders, nutritional bars, some meatless entrees, and dietary supplements have not been evaluated in large, randomized clinical trials, and their effect on cancer growth is unknown.(3) These may be listed on ingredient labels as ‘soy protein isolate’ or ‘soy protein concentrate’ and are best avoided, at least until more is known about them. “Be a label detective,” advises Czap.

Whole soy foods include such things as edamame, tofu, soy milk, miso, and tempeh. “Moderate intake of soy foods is one to two servings a day, ”Czap shares. Some examples of serving sizes for these foods are one-half cup of edamame, one cup of soy milk or one-quarter cup of tofu.(2) Soy-based condiments and by-products such as soy sauce, soybean oil, and soy lecithin are also regarded as safe, Czap confirms, and do not contain appreciable amounts of phytoestrogens.

For those taking Tamoxifen or an aromatase inhibitor, studies do not show any harmful interactions between soy and these anti-estrogen medications. In fact, a few studies suggest there may be some benefit to consuming soy foods in these cases, but more research is needed to confirm this.(4)

“Instead of being fearful of food, I encourage patients to become empowered by their food choices,” says Czap. “Adding more plant-based foods to the diet can be an empowering choice. Foods based on whole soybeans are a plant-based source of high quality protein; they contain all the essential amino acids.”

Food choices are personal, and fortunately there is a wide array of healthy foods to choose from. Soy foods are just one option among many. Breast cancer survivors who choose to consume small-to-moderate amounts of whole soy have solid evidence to alleviate any concerns they or others may have about their risk of recurrence. And an informed choice is an empowered choice.

1. https://www.roswellpark.org/cancertalk/201905/should-cancer-patients-avoid-soy
2. https://www.cancernutrition.org/2018/03/30/is-soy-safe-for-cancer-patients/
3. https://acsjournals.onlinelibrary.wiley.com/doi/full/10.1002/cncr.30614
4. https://www.aicr.org/cancer-prevention/food-facts/soy/