“Soy Causes Breast Cancer” & Other Lies

“Soy Causes Breast Cancer” & Other Lies

The humble soybean gets too much of a bad rep. Because of its isoflavone (I’ll explain what that is later) content, many believe that soy disrupts hormone balances in humans and is linked to cancers such as of the breast and prostate, and causes other conditions like ‘man-boobs’. In this blog post I will answer the following questions to help clarify the confusing and conflicting conversation around soy:

  • What is soy?
  • Why do people have concerns about its effect on our hormones?
  • In what ways is soy healthy for you?
  • Who should avoid soy?
  • What sort of soy should I eat?
  • What’s the problem with GMO soy (genetically modified)?
  • Why is misinformation on soy so commonplace and trusted?

What is soy?

Soy in its whole form is the soybean therefore is a member of the legume family. Amongst lentils, chickpeas, peas and every variety of bean, it is difficult to understand why people think soy is something to steer clear of when legumes are among the most nutritious foods on the planet. 

Why do people have concerns about its effect on our hormones?

Most people assume soy consumption is linked to hormone related conditions such as cancer/man-boobs is because research shows that excess of the hormone oestrogen in humans is linked to these (1, 2).

Soy, along with other legumes, contain isoflavones, a subclass of flavanoids and are the major phytoestrogenes found naturally in plants (3). Phytoestrogen is the plant form of oestrogen, which has been shown to be 1000 times weaker than animal oestrogen present in humans (4). Despite having a very similar structure to animal oestrogen and so can bind to the same receptors in humans, they do not cause the same effects.

Phytoestrogens actually in many instances counteract the effect of your own oestrogen as the weaker plant form is binding to the same receptor instead of your own oestrogen (5). Studies have shown that soy actually reduces the risk of conditions associated with oestrogen dominance (too much oestrogen), such as menopause symptoms (6, 7), endometriosis (8) and breast cancer (9).

There are two subtypes of oestrogen receptors in the body: alpha (ERα) and beta (ERβ) receptors. Your own oestrogen normally binds to alpha, and phytoestrogens prefer to bind to beta. Oestrogen in general has positive effects in some tissues and negative effects in others i.e. high oestrogen levels support bone health (10) but increase the risk of developing breast cancer (1).

Therefore it’s crucial that oestrogen only selectively binds to certain receptors so as to have pro-oestrogenic effects in some tissues and anti-oestrogenic effects in others.

Luckily, soy phytoestrogens appear to do just that (11).

Soy has been shown to lower breast cancer risk, an anti-oestrogenic effect, (12) meanwhile alleviating menopausal hot flush symptoms, a pro-oestrogenic effect (13). For this reason, isoflavones are termed more accurately as selective oestrogen receptor modulators, because in clinical trials they have no effects on many biologic processes known to be affected by oestrogen. Has this made you go and grab a glass of soy milk yet or wHaT?? 

Breast cancer 

Researchers have found that women diagnosed with breast cancer and ate the most soya lived significantly longer and had a significantly lower risk of recurrence than those who ate less (14). Amazingly, just the quantity of phytoestrogens present in a 250ml glass of soy milk may reduce the risk of breast cancer returning by 25%! (15) Simply by having soy milk in your breakfast porridge and maybe a glass later, you’ve cut your risk of breast cancer in half.

In another study, 90% of breast cancer patients who ate the most soy phytoestrogens after diagnosis were still alive 5 years later, while half of those who ate little to no soy were dead (16). It is thought that one reason for soy’s anti-cancer effects are due to reactivating BRCA genes, ‘caretaker’ genes which repair damaged DNA, mutations of which can cause a form of hereditary breast cancer (17).

Looking to Asia

Some of the lowest breast cancer rates in the world are in Asia, despite soy products being central to their diets: isoflavone consumption is as high as 50 mg/kg body weight/day in Asia compared to 1 to 3 mg/day for individuals eating a typical “Western” diet (18). This soy consumption could be part of the reason why Asian women are up to 5 times less likely to develop breast cancer than North American women (19).

Regarding other hormone-related conditions, soy consumption has also been shown to reduce the risk of prostate cancer (20), have no effect on testosterone or sperm quality (21) and improve bone density and reduce osteoporosis (22) . The hormonal imbalances related to the growth of gynecomastia (‘man boobs’)  should not be blamed on soy phytoestrogens, but instead excess oestrogen (2) as a result of the consumption of mammalian oestrogens via dairy, which do exert the same effect as human oestrogen. 

Soy has also been proven to lower LDL cholesterol, blood pressure, and the risk of developing cardiovascular disease (23). The mechanisms by which isoflavones can reduce cholesterol include upregulating the expression of cholesterol 7-α-hydroxylase enzyme and LDL receptor genes, to enhance bile acid synthesis and excretion, and to modulate the endocrine system (3). One of the reasons why it is associated with lower rates of chronic disease is because of its very high fibre content with 9g of fibre per 100g serving.

Who shouldn’t eat soy?

The only people who should avoid soy are those with hypothyroidism (24) or is deficient in iodine. Soy appears to have no effects on the thyroid, however, it may reduce the absorption of the drug often taken by people with hypothyroidism, levothyroxine. (25). It is advised that people taking this medication avoid high doses of soy protein, such as soy protein powders, and avoiding eating soy within 2-4 hours of taking your medication.

Soy is healthy for all ages and studies have shown that having a single portion of soya per day in childhood and adolescence can reduce breast cancer risk (26)

What’s the problem with GMO soy?

Soy is undoubtedly one of the healthiest foods you can eat, but ensure you are eating the right forms of it. Make sure you eat minimally processed forms of soy, such as edamame, and tempeh and avoid processed soy in the form of soy mock meats and soy isolates which you normally see in chocolate and energy bars. Unfortunately, half of the nutrients in soybeans are lost when processed into tofu, but soy is so healthy that even if you lose half of the nutrition, it’s still a healthy food. The same goes for other processed soy products like soy milk and yoghurts. Miso is fermented and so provides the benefits associated with probiotics, but many are concerned with the high salt content but many studies have shown that the positive health benefits of the soy cancel out, if not outweigh, the negative impacts from the sodium 27. Finally, make sure you include a variety of other plants in your diet, as eating too much of a good thing can be a bad thing too – if it means you cut out other essential parts of a healthy diet.

Buy organic if you can, but ensure you steer clear of GMO soy as GMO crops can withstand large amounts of pesticide and herbicides. Fortunately most of the GMO soy is used as animal feed and not to make soy products for human food, with ‘two thirds of all protein-based animal feed in the EU coming from soy, of which about 70% is imported, and over 90% of that is produced from GM soybeans’ (28). Approximately 90-95% Soy in the USA is genetically modified, too (29). If animals eat this GMO soy, then humans will end up eating it too, so if you’re concerned about GMO (or soy in general) then the first thing to do would be to stop eating animals and their products. 

Why is there so much misinformation about soy?

Soy is arguably one of the most disputed foods, and so has been researched intensely, with 2000 studies on soy being published every year (26). Between 1990 and 2010, there were over 10,000 peer-reviewed journal articles on soy (30) many of which were either tested in vitro (in a test tube) or in animals who metabolise soy very differently from us, so it is difficult to translate these findings into humans. Also, studies which show side effects as a result of soy consumption have examined subjects eating ridiculously large amounts of soy, up to 14 servings a day. Eating that much of any food is likely to result in some adverse effects, and is unrealistic to the amount of soy normally eaten. As with all science, research funding is also important to note and it is often the animal product industries that like to push this confusion around, as it leaves people so confused about what to eat that they stick with their meat rather than purchasing soy.

I hope this has answered many of your questions about soy and settled any concerns – you can happily eat your tofu and drink your soy milk knowing that it’s doing your body a lot of good!

Worried about the environmental effects of soy?

Read>>Are Tofu and Soya Milk to Blame for our Environmental Crisis?


  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC314432/
  2. https://pubmed.ncbi.nlm.nih.gov/27145756/
  3. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/isoflavones#:~:text=Isoflavones%20are%20a%20type%20of,contain%20varying%20concentrations%20of%20isoflavones.
  4. https://thechart.blogs.cnn.com/2011/06/03/how-much-is-too-much-phytoestrogen/
  5. https://nutritionfacts.org/video/who-shouldnt-eat-soy/
  6. https://pubmed.ncbi.nlm.nih.gov/15243277/
  7. https://nutritionfacts.org/video/soy-phytoestrogens-for-menopause-hot-flashes/
  8. https://pubmed.ncbi.nlm.nih.gov/17474167/
  9. https://pubmed.ncbi.nlm.nih.gov/21113655/
  10. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC381441/
  11. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2587438/
  12. https://pubmed.ncbi.nlm.nih.gov/24453272/
  13. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4389700/
  14. https://pubmed.ncbi.nlm.nih.gov/23725149/#:~:text=lowest%20dose%2C%20soy%20food%20intake,both%20ER%20negative%20(highest%20vs.
  15. https://pubmed.ncbi.nlm.nih.gov/22648714/
  16. https://pubmed.ncbi.nlm.nih.gov/22631686/
  17. https://pubmed.ncbi.nlm.nih.gov/22339411/
  18. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6390141/
  19. https://pubmed.ncbi.nlm.nih.gov/16430400/
  20. https://pubmed.ncbi.nlm.nih.gov/19838933/
  21. https://www.fertstert.org/article/S0015-0282(10)00368-7/abstract
  22. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3383497/
  23. https://pubmed.ncbi.nlm.nih.gov/12597262/
  24. https://pubmed.ncbi.nlm.nih.gov/21325465/
  25. https://pubmed.ncbi.nlm.nih.gov/16571087/
  26. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5188409/
  27. https://pubmed.ncbi.nlm.nih.gov/11195162/
  28. https://royalsociety.org/topics-policy/projects/gm-plants/where-are-gm-crops-being-eaten/
  29. https://phys.org/news/2013-06-gmo-corn-soybeans-dominate.html
  30. https://www.fertstert.org/article/S0015-0282(10)00368-7/fulltext

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