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

The Tracey Birnhak Nutritional Counseling Services
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: April 3, 2013

What are soy foods?

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.

Why worry about soy foods?

One of the many compounds found in soy is a plant chemical called isoflavone. Researchers like to study isoflavones because the shape of an isoflavone is similar in structure to estrogen and it is known to compete with estrogen in the body. Research about isoflavones has been confusing. Most studies show that eating traditional soy foods that have isoflavones may help reduce the risk of breast cancer and breast cancer recurrence. Unfortunately, some research has shown that very high amounts of processed or concentrated isoflavones can make breast cancer cells grow.

Should you eat soy foods?

  • If you are eating traditional soy foods, a moderate amount (up to 3 servings per day) is considered safe and acceptable. See the table below for serving sizes of soy foods.
  • At the present time, most experts agree that it is probably safe and possibly beneficial for breast cancer survivors to eat up to 3 servings of soy foods per day.
  • The Iowa State University Database includes a list of the isoflavone content of food

Soy Food

Portion equal to one serving


½ cup

Soybeans (edamame cooked)

½ cup


1 tablespoon

Soybeans (roasted)

¼ cup


1 cup

Soy yogurt

1 cup

Soy cheese

1 ounce

Soy burger*

1 (3 ounces)

Soy flour *

¼ cup


½ cup

*Processed soy food

Should you avoid processed soy foods?

Highly processed soy protein, or soy protein isolate, is 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 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.
  • We recommend 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 an estrogen receptor antagonist such as Tamoxifen?

  • Tamoxifen and similar drugs are prescribed for some breast cancer survivors because they can block the effects of the body’s estrogen. At this time researchers are unsure whether soy can work with Tamoxifen to block estrogen or if soy makes Tamoxifen less effective which could increase the chance of cancer recurrence.
  • Until there is more research, if you are on Tamoxifen, we recommend you follow the same guidelines and limit soy foods to less than 3 servings a day and avoid soy supplements.

Still have questions? Talk with your doctor or dietitian.

If you are thinking about adding soy to your diet, it is important to consider:

  • The stage of your cancer
  • Your usual dietary intake including your history of eating soy foods
  • Your menopausal status
  • Whether you have a positive or negative receptor status tumor
  • Your weight history


Rock C, Doyle C, Denmark-Wahnefried, Meyerhardt J, Courneya K, Schwartz A, Bandara E, Hamilton K, Grant B, McCollough M, Byers T, Gansler T. Nutrition and Physical Activity During and After Cancer Treatment: An American Cancer Society Guide for Informed Choices. CA Cancer J Clin2012; 62: 242-274. Available at:

For isoflavone content of foods:

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