Soy and breast cancer: Is there a connection?

In the United States, breast cancer continues to trend as the most common type of cancer diagnosis in women. While treatments have improved overall outlook and survival statistics, care for those who are at risk or in remission often places an emphasis on healthy lifestyle modifications. Along with regular physical activity and a healthy weight, diet has been shown to positively influence cancer risk. Research has focused on the impact of different foods, with one of these being soy. Thoughts on inclusion of dietary soy with breast cancer have changed over time, so let’s take a closer look at the most current recommendations. 

Soy Food & Benefits

Experts recognize the numerous health benefits of soy. When looking at different foods, there are a variety of choices that include soy as a main or added ingredient. Each offers a wide variety of nutrients including: 

• Isoflavones
• Protein 
• B vitamins 
• Iron 
• Potassium 
• Magnesium  
• Fiber 

In addition, soy can be used as a plant-based alternative for animal protein sources. Depending on food processing, changes in the quantity of these nutrients may occur and foods typically fall within the categories of fermented or unfermented. Examples of each type are: 

• Fermented: miso, cooked tempeh, natto  
• Unfermented: edamame, tofu, soybeans, soy milk, soy cheese or soy burgers 

Fermented choices tend to be more concentrated with isoflavones. For example, miso (1/2 cup) and tempeh (3 oz) contain over 50 mg total. In comparison, certain soy products such as cheddar cheese (1 oz), a burger (1 patty), and low-fat milk (1 cup) have less than eight total isoflavones per serving. 

Breast Cancer and Soy 

Recommendations for soy consumption with breast cancer have evolved with more research. One of the initial concerns with soy is that it naturally contains isoflavones, a source of plant estrogens. Plant estrogen activity can appear similar to that of human estrogen but in reality, it’s different. While an increase in estrogen levels is recognized as a risk for breast cancer, isoflavones from soy-containing foods don’t have this same effect or risk. Instead, isoflavones in soy are recognized for having an anti-estrogenic influence by competing for and binding to estrogen receptors, therefore lowering overall risk. 

American Institute for Cancer Research Recommendations

General recommendations from the American Institute for Cancer Research suggest that soy can be included for the patient with or at risk for breast cancer. While research remains ongoing, soy contains valuable antioxidant and anti-estrogenic properties that are beneficial as part of a healthy diet. Each source also offers different nutrients supportive of the patient at risk or with breast cancer.  

Patient Application

Understanding some of the theories between soy and its influence on breast cancer can better help you, as the registered dietitian nutritionist, discuss this topic with your patients. Many of the available studies done in this area focus on an Asian diet because soy is consumed more consistently with this female population. Timing and age of soy consumption, menopausal status, and type of breast cancer all factored into findings of eating soy with these individuals. If your patient is interested in trying to integrate this food into their diet, moderate intake is suggested to start. This includes incorporating one to two servings of a soy source per day with a goal of 10 to 40 mg of isoflavones.  

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References:

Centers for Disease Control and Prevention. Basic Information About Breast Cancer. https://www.cdc.gov/cancer/breast/basic_info/index.htm. Accessed May 11, 2024.  

Brown L. Medical Nutrition Therapy for Breast Cancer. In: Oncology Nutrition for Clinical Practice. 2nd edition. United States: Academy of Nutrition and Dietetics; 2021:346-373. 

American Institute for Cancer Research. Soy: Intake Does Not Increase Risk for Breast Cancer Survivors. https://www.aicr.org/cancer-prevention/food-facts/soy/. Accessed May 15, 2024.  

Harvard T.H. Chan School of Public Health. Straight Talk About Soy. https://www.hsph.harvard.edu/nutritionsource/soy/#:~:text=Of%20all%20ethnicities%2C%20Asian%20American,eating%20smaller%20amounts%20of%20soy. Accessed May 17, 2024.