Breast Cancer Mini Series: Soy and Breast Cancer Recurrence Prevention

As a cancer thriver I totally understand. I understand the fear that what you’re eating could possibly be pushing cancer to you rather than away. Soy and breast cancer recurrence has been a topic widely discussed recently.

This is where research and evidence based information can help us feel more empowered and informed to feel good about our choices and move forward more boldly post treatments.

The popular thought around soy for a number of years was that it may help promote the growth and spread of estrogen sensitive cancer cells. 

One of the first clues we had pointing to the opposite was the fact that in Asian countries where soy intake is the highest, we also see breast cancer incidences at much lower numbers than Western countries.  What’s more – we see breast cancer incidences in Asian women become similar to that of Western women with the Westernization of their diets.  All pointing to the fact that lifestyle and diet plays a large part not just genetics!

Since then we have come to a more united front around soy with sites like the American and Canadian Cancer Society, and the American Institute for Cancer Research now all expressing the safety and maybe even benefit of moderate soy consumption.

So where did this initial idea of soy feeding estrogen driven cancers come from?

Soy has been a big topic with regards to breast cancer because it contains isoflavones.  Isoflavones in soy include daidzein, genistein and glycerin and have structures that look a lot like estrogen and are called phytoestrogens for this reason.  

The fear came from the idea that if these structures look similar to estrogen and high estrogen levels have been linked to increased risk for cancers like breast cancer then they may impact cancer cells the same way as the estrogens in our body.  Leading to growth and spread of breast and other estrogen driven cancers. 

However, phytoestrogen studies have shown that:

  1. They don’t turn into estrogen when you eat them.
  2. They are structurally different and significantly weaker than human estrogen.
  3. While they have similar structure as estrogen, they bind differently to estrogen receptors.
  4. Once they bind they may actually act to suppress tumors.

Furthermore, studies have found:

  1. Eating moderate amounts have not been shown to increase risk of cancer whether breast or otherwise but incorporating moderate amounts of soy is safe and may be beneficial for prevention.
  2. Research does NOT currently support avoidance of soy for cancer patients or survivors.
  3. Soy can either have no effect or block potent natural estrogens in the blood.  
  4. Soy foods have been linked to lower rates of heart disease and may help lower cholesterol.  This is important as some breast cancer survivors may have an increased risk for heart disease and high cholesterol as side effects of their hormone treatments!
  5. Soy might reduce hot flashes by as much as 20-26%!  Hot flashes can be a side effect of hormone therapies for breast cancer and one of the top complaints I get in my practice so anything natural we can do that has impact here too is a bonus!
  6. Higher isoflavone intake was related with lower breast cancer risk in women at high risk of hereditary breast cancer (ie. BRCA1 and 2 mutation).
  7. Soy does not appear to interfere with tamoxifen and anastrozole therapy.
  8. A study by Zhang et al. of 1954 breast cancer survivors showed that among postmenopausal women treated with tamoxifen there was an almost 60% reduction in breast cancer recurrence when comparing those with the highest daidzein intake to the lowest.
  9. One study by Kang et al. showed a 12.9% lower recurrence rate of estrogen and progesterone positive breast cancer survivors with the highest intake of soy isoflavone (> 42.3mg of soy isoflavones which is approximately 2-3 servings of tofu or edamame a day) 
  10. The above same study showed postmenopausal women on Anastrozole had an 18.7% lower recurrence rate among those with the highest intake of soy vs. the lowest intake.
  11. Interesting to note, at least a 21% decrease in deaths from any cause was seen in hormone negative breast cancer survivors who had a high isoflavone intake vs those that had the lowest intakes.

Now that we see soy could be beneficial – what is a moderate amount of soy?

Moderate amounts equal 1-3 serving a day of whole soy foods like tofu, tempeh, soy milk and edamame.

For your reference 1 serving equals:

  • 1 cup of soy milk
  • ½ cup cooked edamame/soy beans
  • ⅛ cup of soy nuts
  • ⅓ cup of tofu

These whole soy foods are rich in potassium, magnesium, fiber, B vitamins and is a complete protein as it contains all nine essential amino acids that our bodies need to get from our diet.

For whole soy foods ensuring they are certified organic is also very important as it means they are not genetically modified and not exposed to harmful pesticides that could causes unhealthy cell changes. 

It’s also important to note that whole soy foods are not the same as processed soy products.  Caution should be taken with soy supplements and soy protein isolates (found in nutrition bars, protein powders, vegan and vegetarian meal alternatives) as they may not have the same benefits and more research is needed to ensure their safety with regards to breast cancer. 

It’s important to remember however that if you don’t love soy – it’s okay not to eat it but if you do love it it is safe to continue to include it!  

As with all foods, soy alone is not enough to help prevent cancer recurrence.  The approach needs to be multi-pronged with action around maintaining a healthy weight, incorporating a nutrient dense diet, lowering chronic inflammation, balancing sugar regulation and insulin response, ensuring physical activity, and cultivating stress resiliency.  All things to not only help lower your recurrence risks, boost the effectiveness of medications but also help optimize your health so you can feel your best! 


  1. Simon, Stacy. Soy and Cancer Risk: Our Expert’s Advice. 2019 April 29. American Cancer Society. Accessed 2021
  2. Eating Well After Breast Cancer. Canadian Cancer Society. Accessed 2021
  3. Soy: Intake Does Not Increase Risk for Breast Cancer Survivors. 2021 April 8.  American Institute for Cancer Research – Accessed 2021.
  4. Zhang, Fang Fang et al. “Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry.” Cancer vol. 123,11 (2017): 2070-2079. doi:10.1002/cncr.30615
  5. Applegate CC, Rowles JL, Ranard KM, Jeon S, Erdman JW. Soy Consumption and the Risk of Prostate Cancer: An Updated Systematic Review and Meta-Analysis. Nutrients. 2018 Jan 4;10(1):40. doi: 10.3390/nu10010040. PMID: 29300347; PMCID: PMC5793268.
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  8. Kang X, Zhang Q, Wang S, Huang X, Jin S. Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy. CMAJ. 2010;182:1857-62.
  9. Dong JY, Qin LQ. Soy isoflavones consumption and risk of breast cancer incidence or recurrence: a meta-analysis of prospective studies. Breast Cancer Res Treat. 2011;125:315-23.
  10. Sim EJ, Ko KP, Ahn C, Park SM, Surh YJ, An S, Kim SW, Lee MH, Lee JW, Lee JE, Kim KS, Yom CK, Kim HA, Park SK. Isoflavone intake on the risk of overall breast cancer and molecular subtypes in women at high risk for hereditary breast cancer. Breast Cancer Res Treat. 2020 Nov;184(2):615-626. doi: 10.1007/s10549-020-05875-0. Epub 2020 Oct 17. PMID: 33068197.
  11. Okekunle AP, Gao J, Wu X, Feng R, Sun C. Higher dietary soy intake appears inversely related to breast cancer risk independent of estrogen receptor breast cancer phenotypes. Heliyon. 2020 Jul 2;6(7):e04228. doi: 10.1016/j.heliyon.2020.e04228. PMID: 32642579; PMCID: PMC7334424.
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Dr. Becky Lee, ND

Dr. Becky Lee ND is a naturopathic doctor, cancer coach and cancer thriver, and founder of the Femme Thrive Method post treatment cancer recovery program. She was a medical advisor for the Colorectal Cancer Canada for a number of years, and enjoys volunteering her time and expertise at Gilda's Club and Wellspring. Her passion is helping women feel empowered, informed and supported to recover and reclaim their health and lives post cancer treatments and in doing so lower side effects of treatments, optimize overall health and lower recurrence risks. She is also a mom of three beautiful girls, a chaser of joy and beautiful memories.