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Should women with a high risk of breast cancer avoid soy?

breast cancer and soy

Many people believe that consuming soy-containing foods can increase breast cancer risk. However, I consider it one of the biggest misconceptions. So, where does the misconception come from?


A well-researched type of phytochemical in soybean is isoflavone, a naturally occurring polyphenolic, non-steroidal phytoestrogen with estrogen-agonist and antagonist properties. Isoflavones are sugar-molecule-bound glycosides, including genistin, daidzin, and glycitin. through digestion or fermentation, these glycoside sugar molecules are metabolized into isoflavone aglycones, including genistein, daidzein, and glycitein (Soy Isoflavones | Linus Pauling Institute | Oregon State University, n.d.).


Isoflavones are structurally and functionally similar to human estrogens (17β-estradiol) and can bind to estrogen receptors, mimicking the action of endogenously produced estradiol (Mostrom & Evans, 2011). This has been the main reason for the misconception of the soy and breast cancer link. However, research has shown the opposite effect of soybeans, as many studies demonstrated that soy isoflavones exert protective functions against breast, uterine, and ovarian cancer development. Here are some of the main mechanisms of soybean’s cancer protective effect:

  • Soy isoflavones exert the effect of competing with endogenous human estrogens for 17β-estradiol receptor binding with high affinity but producing ligand/receptor complexes with low biological activity. Compared to estradiol, isoflavones are of much lower potency and bind to the estrogen receptors with a low degree of activation of the receptor-ligand complex.

  • Estrogen receptors (ER) are found in tissues of reproductive organs and others such as bone, liver, heart, and brain. Isoflavones can preferentially bind to and transactivate ER-β rather than ER-α. This mimics the estrogen effects in specific tissues while blocking it in others (Soy Isoflavones | Linus Pauling Institute | Oregon State University, n.d.).

  • After being ingested into the human body, isoflavone daidzein is metabolized into equol by the colonic bacteria. Equol is reported to bind to both estrogen receptors (ERα and Erβ), with a higher binding affinity for ERβ, which has been implicated in inhibiting proliferation and induction of apoptosis in breast cancer cells. A systemic review article published in 2021 reports evidence that equol has more anticancer than oncogenic properties by acting on multiple pathways in breast cancer cells (Hod et al., 2021). Other studies also suggested that equol can induce apoptosis and even enhance tamoxifen’s apoptotic effect, indicating that equol’s apoptotic action against ER-positive breast cancer cells might lower the risk of developing breast cancer (Charalambous et al., 2013).

  • Isoflavonoid metabolites, such as daidzein and genistein, also have biological activities unrelated to their binding ability with estrogen receptors, such as antioxidant effect, inhibition of tyrosine kinase enzymes in the signaling pathway for slowing cell proliferation, anti-anagenesis, activation of natural killer cells, etc., which all have been proven to work against breast cancer (Soy Isoflavones | Linus Pauling Institute | Oregon State University, n.d.) (Takagi et al., 2015).

My Final Thought:

Women with elevated risks of breast cancer do not need to completely avoid soy consumption. However, considering the importance of a balanced diet and that science is still evolving fast, and new findings are coming out at any minute, I recommend a small to moderate consumption of unprocessed organic soy foods for this patient population.




Charalambous, C., Pitta, C. A., & Constantinou, A. I. (2013). Equol enhances tamoxifen’s anti-tumor activity by induction of caspase-mediated apoptosis in MCF-7 breast cancer cells. BMC Cancer, 13(1), 1–10.

Hod, R., Maniam, S., & Nor, N. H. M. (2021). A Systematic Review of the Effects of Equol (Soy Metabolite) on Breast Cancer. Molecules, 26(4).

Mostrom, M., & Evans, T. J. (2011). Phytoestrogens. Reproductive and Developmental Toxicology, 707–722.

Soy Isoflavones | Linus Pauling Institute | Oregon State University. (n.d.). Retrieved March 19, 2023, from

Takagi, A., Kano, M., & Kaga, C. (2015). Possibility of Breast Cancer Prevention: Use of Soy Isoflavones and Fermented Soy Beverage Produced Using Probiotics. International Journal of Molecular Sciences 2015, Vol. 16, Pages 10907-10920, 16(5), 10907–10920.



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