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Is soy safe for cancer patients?

  • edamame salad
  • Eating soy foods like tofu, edamame and soy milk has been linked to reduced risk of certain cancers including breast cancer, prostate cancer and gastric cancer. Many patients worry, however, that eating soy might be harmful if they have estrogen-receptor positive breast cancer. Let’s clear up confusion about the safety of eating soy foods as it relates to cancer risk.

    Soy contains something called phytoestrogens, which are the plant version of estrogen. Here are 3 important things to know:

    • Phytoestrogens are structurally different and significantly weaker than human estrogen.
    • Phytoestrogens do not turn into estrogen when you eat them.
    • Moderate intake of soy, in food form, does not increase cancer growth.

    The scientific research to date suggests:

    Prostate and breast cancer rates are lower in Asian countries where soy foods are a regular part of an overall healthy diet.

    Soy in natural food form such as tofu, edamame and soy milk is safe for consumption, even for people with a cancer diagnosis.

    Cancer patients do not need to eliminate all sources of soy food from their diet.

    For more detail on specific categories of soy products, please see this excerpt previously published in the Cancer Nutrition Consortium Newsletter.

    It can be helpful to think of soy products in three distinct categories:

    1. Soy foods like edamame, tofu and unsweetened soy milk.
    2. Soy protein supplements like protein powder or nutritional 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. One serving of soy is equivalent to 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 these more concentrated sources of soy. 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, many healthcare providers favor minimizing intake of soy protein powder supplements (soy protein isolate) in the diets of patients with hormone sensitive cancer.

    Soy condiments

    Soy sauce, soybean oil and soy lecithin are examples of soy products that do not contain significant levels of phytoestrogens. This means they do not pose risk in terms of fueling cancer growth. At the same time soy sauce and soybean oil have other nutritional drawbacks. Soy sauce is high in sodium and soybean oil is comprised mostly of polyunsaturated fat, which makes it less desirable than other oils such as olive oil. Soybean oil is an ingredient in many packaged foods such as crackers, cookies, breads and salad dressings. Unfortunately, soybean oil added in the manufacturing of these foods is often partially hydrogenated. This form of soybean oil is known as "trans fat," and should be avoided for general health purposes. Even though soybean oil isn’t likely to exacerbate cancer growth it is not considered a healthy fat overall. Soy lecithin is an emulsifier, meaning it is used to help keep things like salad dressing stable in the bottle. Soy lecithin does not contain phytoestrogens and has no documented association with cancer risk.

    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 as well as men and children. 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.

    Weng KG, Yuan YL.
    Medicine (Baltimore). 2017 Aug;96(33):e7802. 
    Applegate CC, Rowles JL, Ranard KM, Jeon S, Erdman JW.
    Nutrients. 2018 Jan 4;10(1
    Zhang FF et al.
    Cancer. 2017 Jun 1;123(11):2070-2079