How Does Soy Affect Breast Cancer Risk?

One of the most controversial topics in nutrition and cancer is soy.

An oncologist says one thing. A dietitian says another. Plus, thousands of nutrition articles on the internet tell both stories. All of the back and forth is enough to pull your hair out and ultimately you decide to avoid soy completely.

In my professional opinion, you should include soy in your diet.


Soy contains protective benefits against breast cancer and other hormone fueled cancers, like prostate cancer.

Let me explain.

Soy naturally contains the phytoestrogen isoflavones. Isoflavones are phytochemicals with powerful anti-cancer properties. When people hear the word “estrogen” many automatically associate phytoestrogens to human estrogen.

Phytoestrogens from soy (and other foods like flaxseeds) are not the same as human estrogen.

Instead we should be more concerned with estrogen found in animal based products since it is identical to human estrogen. In fact, human estrogen and chicken estrogen are identical (1).

Soy milk is a great non-Dairy option

Soy milk is a great non-Dairy option

Although phytoestrogens look similar in chemical structure, it doesn’t work the same as human estrogen. I reviewed this in our Facebook Live and in our dairy post.

But what does research reveal about soy and breast cancer?

Phytoestrogens from soy have been found to lower breast cancer risk (2).

Breast cancer patients who ate the most soy lived significantly longer and had significantly lower risk of recurrence than those who ate less soy (3). In fact, drinking just 1 cup of soy milk (or consuming the equivalent amount of phytoestrogens), may actually reduce the risk of breast cancer recurrence by 25% (4,5).

Even better, this risk reduction was found in both women with estrogen positive (ER+) and triple-negative breast cancer. (6)

Why does soy intake reduce risk? Research believes the reduction in risk is related to the BRAC1 and BRAC2 genes (7). These genes are tumor suppressor genes and are responsible for repairing DNA damage. If someone has genetically mutated (or, changed) BRAC1 or BRAC2 genes, their risk for breast cancer is greater than those who do not have the gene mutation. Soy is believed to reactivate these BRCA genes to repair DNA damage--just as they were intended to. The great news is that research also demonstrates that soy consumption can benefit even those with BRAC1 or BRAC2 gene mutations (8).

Tofu is a very common soy product, but you should moderate your consumption as it is a processed food.

Tofu is a very common soy product, but you should moderate your consumption as it is a processed food.

What about GMO (genetically modified) soy? Good question.

Unfortunately, when pesticides are sprayed on crops in soy fields, it has been found to have toxic effects on human tissue (9). Although by the time the soy reaches our table the pesticides are highly diluted, even at an incredibly small dose, the pesticides were still found to have estrogenic effects and stimulate the growth of ER+ human breast cancer cells (10).

Thankfully, when choosing non-GMO and organic soybeans, high levels of the pesticides have not been found (11). But, you should know there are no direct human studies that suggest any harm in eating GMO crops--however, no studies have been done (12).

“Okay, Alison, what’s the bottom line?”

One shouldn’t be fearful of consuming whole, soy food products such as tofu, tempeh, edamame, etc. In fact, the consumption of soy has demonstrated a decreased risk in hormone positive cancers and the development of recurrence for those who have encountered a diagnosis. And although studies still need to be done in GMO crops, your safest bet would be to choose organic, non-GMO products until we have long-term research proving otherwise.

Please enjoy some organic edamame on your salad or Pad Thai!

Still have pending questions? Ask them in the comments below!

Have an amazing recipe that includes limited processed soy? Share it below!


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  2. Nagata C, Mizoue T, Tanaka K, et al. Soy intake and breast cancer risk: an evaluation based on a systematic review of epidemiological evidence among the Japanese population. Jpn J Clin Oncol. 2014;44(3):282-95.

  3. Chi F, Wu R, Zeng YC, Xing R, Liu Y, Xu ZG. Post-diagnosis soy food intake and breast cancer survival: a meta-analysis of cohort studies. Asian Pac J Cancer Prev. 2013;14(4):2407-12.

  4. Bhagwat, S., Haytowitz, DB, and Holden, JM. 2008. USDA Database for the Isoflavone Content of Selected Foods, Release 2.0. U.S. Department of Agriculture, Agricultural Research Service, Nutrient Data Laboratory Home Page:

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  7. Bosviel R, Dumollard E, Dechelotte P, Bignon YJ, Bernard-Gallon D. Can soy phytoestrogens decrease DNA methylation in BRCA1 and BRCA2 oncosuppressor genes in breast cancer? OMICS. 2012;16(5):235-44.

  8. Magee PJ, Rowland I. Soy products in the management of breast cancer. Curr Opin Clin Nutr Metab Care. 2012;15(6):235-44.

  9. Richard S, Moslemi S, Sipahutar H, Benachour N, Seralini GE. Differential effects of glyphosate and Roundup on human placental cells and aromatase. Environ Health Perspect. 2005;113(6):716-20.

  10. Thongprakaisang S, Thiantanawat A, Rangkadilok N, Suriyo T, Satayavivad J. Glyphosate induces human breast cancer cells growth via estrogen receptors. Food Chem Toxicol. 2013;59:129-36.

  11. Bohn T, Chura M, Traavik T, Sanden M, Fagan J, Primicerio R. Compositional differences in soybeans on the market: glyphosate accumulates in Roundup Ready GM soybeans. Food Chem. 2014;153:207-15.

  12. Butler D, Reichardt T. Long-term effect of GM crops serves up food for thought. Nature. 1999;398(6729):651-6.