Soy and Breast Cancer: Our gut bacteria determine how protective soy is

When I was completing my masters program last year I took a class on plant based nutrition. My professor followed a vegan lifestyle and at times I felt that his personal beliefs about veganism interfered with teaching non-biased material.

Around that time, the Netflix show “The Game Changers” had just come out too. That was something my professor recommended we all watch to further convince us that animal protein was not necessary. I was open to learning about and trying out a vegan diet. I was diligent about adding in nuts, seeds, lentils, peas, those meatless meats (gluten free ones). I tried soy milk, despite the cautionary things I had heard about it.

Although a vegan diet did not work well for my body, it helped me learn about what did by trial and error. While I was doing the vegan thing I noticed if I didn’t eat something every 2 hours my blood sugar would crash and i’d get a massive head ache, brain fog and I felt like my head was floating off of my body. I tried it for a month but came to find my body just feels better with animal protein & my blood sugar was more stable. I have found a gluten free paleo type diet works well for me. But that’s not to say a vegan diet doesn’t work for others. I do often see imbalanced blood sugar and amino acid/mineral deficiencies in people who eliminate animal protein from their diet. One diet does not fit everyone and I encourage everyone to find what works best for their body!


Should You Eat Soy?tofu

One of the main staples of protein in a vegan lifestyle is soy, and like so many people out there, I had heard that soy was “bad.” I had heard “Soy caused breast cancer”, it was “not good for men to consume soy” and if you had a history of breast cancer you should avoid it.  So I decided to look into the research about it. This is what got me so interested in the gut microbiome and discovering that there is a gut connection to every organ system in the body.

I narrowed my research down to 11 studies that discovered all of the large scale studies had found soy isoflavones to be protective against breast cancer, regardless of Estrogen Receptor Positive status, pre and post menopausal status. They even found that those women who had mutations in a liver enzyme CY1B1, which metabolizes estrogen, were protected against breast cancer if they consumed soy!

Boy all of this was making me feel pretty good about soy! Until I found the bottom line of every study…every study that found soy protective was done on all ASIAN women. Every single study. Then I came across 2 studies that tested Western women (American) vs. Eastern women (Asian) to see if they derived the same benefit from consuming soy. Western women did not derive the same protection from consuming soy as did Eastern women. Why???


Gut Bacteria Metabolize Soy Differently

It all comes down to the difference in their gut bacteria. Western women don’t have the same type of microbiome as Eastern women, so their guts have a reduced ability to metabolize the soy isoflavones into the protective metabolite “S-equol.” So all of these studies that are using  70,000 Asian women cannot be generalized to all women everywhere.

How fascinating is it that it comes down to gut bacteria?! I think epigenetics is at play here, there are generations that have been consuming soy products for thousands of years, which is probably why their gut microbiome can produce more protective s-equol than westerners, who have not been eating soy for that long.

Soy Isoflavone Benefits: Asian Culture vs. Western

stomachDong & Qin, 2010 conducted a meta-analysis of 4 studies that observed breast cancer recurrence and 14 studies of breast cancer incidence between 2001 and 2009. The median length of follow up time was between 1.57 years to 13 years and total participants involved were 369,934 with 5,828 incidents of breast cancer. They found that soy isoflavones consumption was associated with 11% reduction of breast cancer. In 4 studies of breast cancer recurrence involving 9,545 participants and 1,226 recurrent breast cancer cases, only 1 study found a 16% reduction in recurrence among women with the highest isoflavone consumption. However they noted that the protective effect of soy was observed only in Asian populations and not Western populations, this may be because the average intake of soy isoflavones is much lower in Western culture than in Asian culture. They stated that a metabolite of daidzein called S-equol, may affect the soy and breast cancer association because of it’s superiority to all other isoflavones. As previously mentioned, 50-70% of Asian populations have gut bacteria that can produce higher amounts of S-equol when compared to that of Western populations, which is 20-30% (Sekikawa et al., 2019).

Bahrom et al., 2016 conducted a meta-analysis of 57 studies including those in Asian countries which found soy isoflavone consumption to be inversely associated with breast cancer in both premenopausal women  and postmenopausal women . But in Western women found no statistically significant association between soy consumption and breast cancer risk.


Is It The Soy? Or That It’s GMO?


In the United States, 94% of all soy is genetically modified. Sometimes there are also allergies to soy, but is it the soy or is it that its a GMO?

Soybeans are now being genetically altered to be resistant to RoundUp (glyphosate). This is so that when the farmers spray the soy crops with all these pesticides, the soy crops do not die. Not only are you eating a genetically altered organism, but on top of that you’re consuming something sprayed with a cancer causing pesticide.

Similar questions have been postulated about wheat. I have had clients say they eat the wheat in the USA and they have bloating, running nose, etc. But when they eat the wheat in Europe (where it is not GMO) they have no issues. A paper was published in 2013 when RoundUp was being questioned, and it found that the GMO wheat crops were sprayed heavily with RoundUp and that the glyphosate chemical in the pesticide was binding with the wheat protein to create a “neo-antigen.” Something our immune systems have not gotten used to.

For a complete list of Genetically Modified crops go here:



This research indicated that soy isoflavone consumption has an inverse relationship with risk of breast cancer and hormonally related cancers regardless of HER2 status, pre and postmenopausal status in Asian women. Earlier life consumption of soy isoflavones offers greater protection from breast cancer later in life. Soy isoflavone consumption also protects against risk of breast cancer even with high risk enzyme mutation CYP1B1 in Asian women. More studies should be done with Western women due to all of the study populations being Asian women. Research suggests Western women may not have the exact benefit from soy isoflavones due to the reduced ability of their gut microbiota’s ability to metabolize soy isoflavones into S-equol. Further studies are warranted.

If you’re going to eat soy, always choose organic and non-GMO soy.



  1. Baglia, M. L., Zheng, W., Li, H., Yang, G., Gao, J., Gao, Y.-T., & Shu, X.-O. (2016). The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status. International Journal of Cancer139(4), 742–748. Retrieved from The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status.
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  9. Sekikawa, A., Ihara, M., Lopez, O., Kakuta, C., Lopresti, B., Higashiyama, A., … Cui, C. (2019). Effect of S-equol and Soy Isoflavones on Heart and Brain. Current Cardiology Reviews15(2), 114–135. Retrieved from of S-equol and Soy Isoflavones on Heart and Brain.
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Author: Melissa Mullin

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  • Very good article, Melissa on Soy and breast cancer. I’m on all this since I’ve had breast cancer.

    How are you? I certainly miss you. Come back soon!