Soy and Breast Cancer: Should breast cancer survivors eat soy foods?

The Tracey Birnhak Nutritional Counseling Services
Ultima Vez Modificado: 21 de junio de 2017

What are soy foods?

Soy is one of the only plant based food source of complete protein. Meat, fish, poultry, eggs and dairy are also sources of protein. Soy is rich in fiber, potassium and magnesium.

Examples of soy foods include: soybeans (also called edamame), soybean sprouts, tofu, soymilk and fermented soybeans (also called tempeh). These traditional soy foods have been used in many cultures as good sources of protein for thousands of years. More recently, processed soy protein has been added to a variety of foods, such as frozen meals, side dishes, soups, protein powder drinks, and snack bars.

Is there a concern about eating soy foods?

One of the many compounds found in soy is a plant chemical called isoflavone. According the American Cancer Society, beans or legumes, soy, and foods derived from soy, are excellent sources of protein and a healthy alternative to meat. Isoflavone has, weak estrogen-like activity and may help protect against hormone-dependent cancers. There is growing evidence that eating traditional soy foods may lower the risk of cancers of the breast, prostate, or endometrium (lining of the uterus). There is also evidence it may lower the risk of some other cancers. It is not known whether this is true for processed soy foods, such as foods that contain soy protein isolates or textured vegetable protein derived from soy.

Should you eat soy foods?

  • Most experts agree that it is probably safe, and possibly beneficial, for breast cancer survivors to eat traditional soy foods. See the table below for serving sizes of soy foods.
  • A recent study (Zhang, F.F, et al) looked at more than 6,200 American and Canadian women with breast cancer. These women filled out questionnaires about what they ate and other lifestyle habits. Those women who ate the highest amounts of isoflavones had a 21% lower risk of having died from any cause, compared to the women who ate the lowest amount of isoflavones.
  • The Iowa State University Database includes a list of the isoflavone content of food.

Soy food

Portion equal to one serving

Tofu

½ cup

Soybeans (edamame cooked)

½ cup

Miso

1 tablespoon

Soybeans (roasted)

¼ cup

Soymilk

1 cup

Soy yogurt

1 cup

Soy cheese

1 ounce

Soy burger*

1 (3 ounces)

Soy flour *

¼ cup

Tempeh

½ cup

*Processed soy food

Are processed soy foods safe for survivors?

  • Highly processed soy foods that include only the protein, or soy protein isolates, are used in many snack foods and vegetarian products. The isoflavones in these foods are very concentrated and may not be as safe as traditional soy foods.

Should you avoid soy supplements?

  • Supplements are not regulated by the Food and Drug Administration and should be used with caution. Please consult with your physician before using any type of supplement, as they may interfere with your medications.
  • Supplements may contain high doses of concentrated sources of soy. These include soy powders, soy protein powders and isoflavone supplements. Genistein and daidzein are specific types of soy isoflavones. They are often sold as dietary supplements.
  • There is little data to support the use of supplements of isolated soy phytochemicals for reducing cancer risk.
    • It is recommended that you avoid these supplements due to the potential estrogen-like effects and lack of safety data.

What about other soy products?

  • Soy is often used as a food additive (soy lecithin, soy oil) and can be found in processed food, such as salad dressing and baked goods. These forms of soy do NOT contain isoflavones. Also, soy sauce does NOT contain isoflavones.
  • You do not have to avoid foods that have these additives.

Should you avoid soy if you take Tamoxifen?

  • Tamoxifen and similar medications are prescribed for some breast cancer survivors because they can block the effects of the body’s estrogen. According to the study authored by Zhang, eating soy did not have a negative impact on the effectiveness of Tamoxifen.

Still have questions? Everybody’s medical history and treatment plan is different. Talk with your medical team about what is right for you.

Resources for more information

Referencias

Chi, F. Wu, R. Zeng, Y.C., Xing, R. Liu, X. and Xu, Y.L Post-diagnosis soy food intake and breast cancer survival: A meta-analysis of cohort studies As Pacific J of Cancer Prevent:2013;24017-2412. http://dx.doi.org/10.7314/APJCP.2013.14.4.2407

Zhang, F.F, Haslam, D.E., Terry, M.B., Knight, J.A., Andrulis, I.L., Daly, M.B., Buys, S.S., and John, E.M. Dietary isoflavone intake and all cause mortalitiy in breast cancer survivors: The Breast cancer family registry Cancer 123:2017;2070-2079 http://dx.doi.org/10.1002/cncr.30615

American Cancer Society ACS guidelines for nutrition and physical activity for cancer prevention https://www.cancer.org/healthy/eat-healthy-get-active/acs-guidelines-nutrition-physical-activity-cancer-prevention/common-questions.html Accessed June 22, 2017

USDA database for the isoflavone content of selected foods http://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/isoflav/Isoflav_R2.pdf Accessed 6/22/17

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