Is Soy Safe After a Cancer Diagnosis? Separating Fact from Fear

Soy is a controversial topic in cancer care.

This is because soy is a phytoestrogen, meaning that it can mimic or modulate estrogen in the body. 

What Is a Phytoestrogen? 

Estrogen is a growth factor, which is why certain drugs such as aromatase inhibitors are used in care to reduce circulating estrogen, and potentially halt cancer growth.

A phytoestrogen is a very weak estrogen that comes from plants. It can mimic estrogen by blocking (filling) an estrogen receptor site. This is useful because it causes a downregulation of estrogen production. (Tells the body to produce less.)

Other phytoestrogens such as flaxseed, can bind to estrogen in the GI system and eliminate it via waste. By eliminating it this way, the excess estrogen is not available for reuptake and circulation in the body. 

Examples of Phytoestrogens

Flaxseed, organic soy, black cohosh, and red clover.

Why Soy Research Is So Inconsistent

Advice about soy consumption is varied because of the following factors: 

• Estrogen effects certain cancers differently 
• Studies on soy are often conducted on Asian-only populations
• Studies may fail to detail the other factors influencing the participants (microbiome, consumption of fermented foods and sea vegetables)
• Studies sometimes do not detail the type of soy product they are testing (organic, non-organic, fermented, milk, processed soy isolates)

The Benefits of Fermented Soy

Consumption of fermented soy has been associated with benefits to cardiovascular, hormonal, and bone, and digestive health. It is also used in some hormone-dependent cancer care.

Why Equol Matters

When assessing these benefits, it's very important to note that: 

Many of the studies showing benefits from soy consumption were conducted on Asian populations, who have a very different microbiome than that of a Caucasian, due to inherited microbiome from the mother and a vastly different diet over their lifetime.

Equol is an isoflavone from soy, which has been praised for its health benefits. Equol is converted from daidzein, another isoflavone found in soy. Specific microbes in the intestines make this conversion possible. 

However, only 40-60% of individuals of Asian descent are able to convert daidzein to equol. Even less of the Caucasian population in the U.S. is able to make this conversion: 25-35%. A contributing factor may be that those of Asian descent tend to eat more seaweed, which enhances the intestine’s conversion of isoflavones into equol.

What Determines Whether You'll Benefit from Soy

Your ability to benefit from soy intake depends on:

• your genetics
• the diversity of your microbiome
• function of your digestive system 
• cancer type and subtype*
• other hormonally driven conditions such as hypo- and hyperthyroidism 

*A note on ER+ and postmenopausal breast cancer:

For years, the standard advice was caution or avoidance here, based mostly on older lab and rodent studies. But the picture has shifted slightly.

A 2024 meta-analysis pooling 32 observational studies found soy isoflavones associated with a 26% reduction in recurrence risk overall, with the strongest benefit seen specifically in postmenopausal and ER+ survivors (van Die et al., 2024). The benefit peaked around 60mg of isoflavones per day, about 2-3 servings of whole soy food, with no added benefit from eating more.

The important distinction is form: this research is about whole, fermented soy foods (tofu, tempeh, miso, edamame), not concentrated isoflavone supplements or soy protein isolates, which remain far less studied.

If you have a hormone-dependent cancer, this is a great topic to bring to a 1:1 consultation, since your specific subtype, treatment (like tamoxifen or an aromatase inhibitor), and equol-producer status all factor into what's right for you.

The Bottom Line on Eating Soy

If you eat soy...

1. Always eat organic, non-GMO, fermented soy products such as tofu, miso, natto, tempeh, and edamame. Non-organic soy is one of the most heavily sprayed crops. 
2. Only eat it 1-3 times per week so it doesn't impact the function of your thyroid gland. 
3. Always eat it with some kind of seaweed. Seaweed's iodine content also helps offset soy's mild goitrogenic effect which can inhibit thyroid peroxidase.

The most effective diet and lifestyle recommendations for those with cancer are personalized. If you have a diagnosis, consider working with me 1:1.

To learn more about oncology support, check out my services page here.

Your first step is to book a Discovery Call.

To adopt an anticancer lifestyle, check out my Basics of Anticancer Nutrition Course.

Citations

1. He FJ, Chen JQ. Consumption of soybean, soy foods, soy isoflavones and breast cancer incidence: Differences between Chinese women and women in Western countries and possible mechanisms. Food Science and Human Wellness. 2013;2(3):146-161. doi:10.1016/j.fshw.2013.08.002
2. Belobrajdic DP, James-Martin G, Jones D, Tran CD. Soy and Gastrointestinal Health: A Review. Nutrients. 2023;15(8):1959. doi:10.3390/nu15081959
3. Spagnuolo C, Russo GL, Orhan IE, et al. Genistein and Cancer: Current Status, Challenges, and Future Directions. Advances in Nutrition. 2015;6(4):408-419. doi:10.3945/an.114.008052

4. Setchell KD, Cole SJ. Method of defining equol-producer status and its frequency among vegetarians. J Nutr. 2006;136(8):2188-2193. https://doi.org/10.1093/jn/136.8.2188 

5. Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002;110(Suppl 3):349-353. https://doi.org/10.1289/ehp.02110s3349 

6. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006;16(3):249-258. https://doi.org/10.1089/thy.2006.16.249 

7. Fritz H, Seely D, Flower G, et al. Soy, red clover, and isoflavones and breast cancer: a systematic review. PLoS One. 2013. 

8. M Diana van Die, Kerry M Bone, Kala Visvanathan, Cecile Kyrø, Dagfinn Aune, Carolyn Ee, Channing J Paller, Phytonutrients and outcomes following breast cancer: a systematic review and meta-analysis of observational studies, JNCI Cancer Spectrum, Volume 8, Issue 1, February 2024, pkad104, https://doi.org/10.1093/jncics/pkad104 

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