Is Soy Safe to Eat After a Breast Cancer Diagnosis?

Soybeans and soy milk

Today’s blog post is especially for all my readers who have had a breast cancer diagnosis. Out of all the frequently asked questions about nutrition and breast cancer, the question of whether or not to eat soy must be the one I get asked the most! Soy is a known phytoestrogen (plant oestrogen), but does that mean that it is safe to eat, or should it be avoided if you wish to reduce your risk of recurrence, particularly if you had a cancer that was hormone receptor positive? Hopefully you’ll feel a bit clearer by the end of this article. However, in order to be comprehensive, this article is a long one, so if you’re short on time, feel free to skip to the conclusions.

Of course, soy has impacts on other areas of health, but this article is limited to breast cancer.

What is soy?

The soybean is a legume native to East Asia, where it has been grown for thousands of years. The USA began large scale soy farming in the 20th century, and it is now also grown extensively in South America. Much of the worldwide soy crop is used for animal feed. The impact of soy feed on the nutritional quality of meat is another story altogether, but what of soy as a food for humans?

Soy is eaten in a wide variety of forms, some of them traditional and some, by contrast, quite processed. The nutritional benefits of soy vary considerably between forms, so it is important to understand how they are produced. Here are some of the more commonly found forms of soy.

Soy tofu
  • Tofu – originally from China, tofu (also called bean curd) is made by curdling soymilk so that the proteins coagulate and can be pressed into a sliceable “cake”. Most tofu sold in the UK is unfermented. Its bland taste makes it quite versatile, and it comes in different consistencies, from silken to firm. It is thought that, although it may first have been made around 2000 years ago, tofu did not become popular until about 1000 years ago.

  • Soy sauce – another food of Chinese origin. Soy sauce is traditionally made by fermenting soybeans with wheat and cultures of fungi and yeasts, then brewing in salt or brine. Tamari is a soy sauce of Japanese origin that is usually wheat- and gluten-free.
  • Miso – a Japanese fermented soybean and salt paste. It is available in different varieties, such as white and red. Sometimes grains are used in the fermentation process.
  • Natto – a Japanese dish of soybeans fermented with the bacterium Bacillus subtilis. Natto has a strong taste and smell and a rather sticky, stringy texture. It is regarded as a delicacy in Japan and is not so popular or easy to find in the UK.
Buddha bowl containing edamame (soy) beans
  • Edamame – young soybeans, either as individual beans or still in the pod. They are unfermented.
  • Tempeh – a food of Indonesian origin, tempeh is made by fermenting whole soybeans with a fungus starter. The result is a solid “cake”, but with a stronger, nuttier flavour than tofu. Many people, myself included (!), think that tempeh needs to be marinated to disguise its flavour. Tempeh is now available in health food shops as well as in large supermarkets.
  • Textured vegetable protein (TVP) – not a traditional form of soy, TVP was created by a food company in the 1960s. It is used in a variety of shapes and forms as a meat substitute. TVP is made by heating soy protein or soy flour to very high temperatures, then extruding it into the desired shape. It is important to note that TVP is a highly processed food. It may be flavoured with MSG (monosodium glutamate) which some people are sensitive to. Small amounts of the solvent hexane may also be used in the extraction process.
  • Soy milk, flour, yoghurts – soy is made into all sorts of other forms, many of them processed and not traditional.
  • Soy lecithin – a fatty substance extracted from soybeans which is frequently used as an emulsifier. It contains very little soy protein.

You can see from this that there is a wide variation in the amount of processing used in producing different soy foods. Edamame beans are the least processed. Then there are the forms that have been traditionally made for hundreds of years: tofu, fermented soy sauce, tempeh, miso and natto. Soy flour, TVP and other soy meat substitute products are very highly processed, and I recommend avoiding them for the most part, as I recommend minimising the amount of processed foods you eat in general.

Nutrient Benefits of Soy

Many soy products are high in protein, typically higher than the protein content of other legumes. Tempeh and some tofu products can contain a similar amount of protein to meat or fish per 100g. This high protein content is unusual in a plant food. It is one reason why it can be helpful to include good quality soy in vegan diets, as well as being a potentially useful tool in reducing animal protein consumption.

Soy tempeh strips in a wok

But there is more to soy than protein. Soy is high in fibre and low in saturated fat. It is rich in vitamins and minerals, including the B vitamins folate and riboflavin, vitamin K, calcium, magnesium, iron, copper, manganese, molybdenum, phosphorus, and potassium. It is lower in carbohydrate than other legumes. This makes it particularly beneficial in vegan diets, in which protein consumption from other legumes can often lead to a very high carbohydrate intake.

Many of the health benefits ascribed to soy may be related to its high content of isoflavones. Isoflavones are compounds which generally have antioxidant and anticancer effects. The main isoflavones in soy are genistein and daidzein. These may have benefits in normalising cholesterol and increasing bone mineral density. They also have multiple anticancer effects that are not related to oestrogen. Some of these include:

  • Encouraging cancer cells to undergo apoptosis (cell suicide)
  • Inhibiting cancer growth signalling
  • Inhibiting the formation of new blood supplies to tumours (angiogenesis)
  • Inhibiting metastasis (spread)
  • Upregulating the protective BRCA1 tumour suppressor gene(1)

However, the concern often raised about isoflavones is that they behave like oestrogen in the body and might therefore contribute to increased recurrence risk after an oestrogen receptor positive breast cancer diagnosis. Is this concern warranted? Let’s take a closer look.

Soy as a Phytoestrogen

Genistein has a similar chemical structure to oestrogen. Daidzein is a very weak phytoestrogen, but certain gut bacteria can convert it into a compound called equol, which is a stronger phytoestrogen(2). Therefore, the phytoestrogenic effects of soy can vary from person to person, depending on a person’s gut microbiome.

The phytoestrogens in soy bind to oestrogen receptors. There are 2 oestrogen receptors: ERα and ERβ (the “E” is from the US spelling, estrogen). Broadly speaking, ERα promotes breast cancer cell growth and proliferation, whereas ERβ seems to have the opposite effect, putting the brakes on breast cancer cell growth. In normal dietary quantities, soy isoflavones bind to ERβ, meaning that as far as we understand it, they are likely to oppose breast cancer growth and invasion(3), (4).

However, at high concentrations, such as might occur if taking soy supplements, soy isoflavones bind to ERα just as strongly as they bind to ERβ (5). As expression of ERβ on breast cancer cells tends to reduce as breast cancer becomes more advanced(6), (7), high concentrations of soy in supplement form are not recommended after a breast cancer diagnosis.

Woman checking breast

These findings are borne out by research studies that have looked at the impact of soy in both dietary and supplemental forms on breast cancer recurrence risk. While some studies suggest that Western women may not obtain the same reductions in recurrence risk by eating soy as their East Asian counterparts(8), some studies have shown a benefit from eating soy. An analysis of studies of breast cancer survivors living in the US and in China(9) revealed that post-diagnosis consumption of ≥10mg/day of soy isoflavones (a level common in China but not in the US) was associated with reductions in risk of breast cancer recurrence and mortality, regardless of ethnicity (9).

Studies on soy supplements have resulted in contradictory findings, although some reported an increase in the expression of cancer-promoting genes(10), (11). Because high doses of soy can bind to ERα as strongly as they bind to ERβ, it would seem prudent to avoid soy in supplement form if you have had a breast cancer diagnosis.

What about Tamoxifen?

Medication tablets

Most of the epidemiological studies looking at soy in the diet have involved a large proportion of women also taking Tamoxifen. The study of breast cancer survivors living in the US and China showed that among oestrogen receptor positive breast cancers, women taking Tamoxifen experienced the greatest benefit in recurrence risk reduction by eating soy(9).


Perhaps having read this far, you have decided to increase the amount of soy that you eat, but since it is available in a bewildering variety of forms, which should you choose? One consideration is that the availability of nutrients varies considerably between forms of soy.

Soy, like many other plant foods, contains a substance called phytic acid. Phytic acid binds to minerals and reduces our ability to absorb them. Minerals such as zinc, iron and copper may be present in soy, but that’s not much good if we can’t absorb them! This is where the wisdom of ancient traditions of eating soy comes in. The fermentation process used to make tempeh, miso, natto and soy sauce can degrade phytic acid, making minerals more available(12). I would encourage you to choose these fermented forms over unfermented forms as much as possible, and of course avoid highly processed soy.

Other Concerns about Soy

There is some evidence that consumption of soy may have an adverse effect on thyroid function in people who already have thyroid issues or who are deficient in iodine(13). Until the research is clearer, it may be prudent to avoid soy if you have thyroid disease.

Soy is also a very common allergen(14).


Although soy is rich in phytoestrogens, these do not act in exactly the same way as our own oestrogen. The weight of the scientific evidence shows that soy in food form is safe to eat after a breast cancer diagnosis, and may even be protective against recurrence. Soy has anticancer actions that are also protective against oestrogen receptor negative cancers.

There is considerable variation in the amount of processing involved in making different forms of soy. In line with my general guidance on avoiding processed foods as much as possible, I recommend focusing on minimally processed, traditionally fermented forms of soy (such as tempeh, miso and soy sauce/tamari) if you are thinking of increasing the amount of soy that you eat.


  1. Banerjee, S., Li, Y., Wang, Z. et al. (2008). ‘Multi-targeted Therapy of Cancer by Genistein’, Cancer Letters, 269(2), pp226-242. Available at (Accessed 21 July 2021).
  2. Cady, N., Peterson, S.R., Freedman, S.N. et al. (2020). ‘Beyond Metabolism: The Complex Interplay Between Dietary Phytoestrogens, Gut Bacteria, and Cells of Nervous and Immune Systems’, Frontiers in Neurology, v11, 150. Available at (Accessed 21 July 2021).
  3. Spagnuolo, C., Russo, G.L., Orhan, I.E. et al. (2015). ‘Genistein and Cancer: Current Status, Challenges, and Future Directions’, Advances in Nutrition, 6(4), 408-419. Available at (Accessed 16 July 2021).
  4. Zhou, Y. and Liu, X. (2020). ‘The role of estrogen receptor beta in breast cancer’, Biomarker Research, 8: 39. Available at (Accessed 21 July 2021).
  5. Jiang. Y., Gong, P., Madak-Erdogan, Z. et al. (2013). ‘Mechanisms enforcing the estrogen receptor β selectivity of botanical estrogens’, The FASEB Journal, 27(11), 4406-4418. Available at (Accessed 21 July 2021).
  6. Shaaban, A.M., O’Neill, P.A., Davies, M.P.A. et al. (2003). ‘Declining estrogen receptor-beta expression defines malignant progression of human breast neoplasia’, The American Journal of Surgical Pathology, 27(12), 1502-1512. Available at (Accessed 21 July 2021).
  7. Roy, S.S. and Vadlamudi, R.K. (2012). ‘Role of Estrogen Receptor Signaling in Breast Cancer Metastasis’, International Journal of Breast Cancer, 2012: 654698. Available at (Accessed 21 July 2021).
  8. Chen, M., Rao, Y., Zheng, Y. et al. (2014). ‘Association between Soy Isoflavone Intake and Breast Cancer Risk for Pre- and Post-Menopausal Women: A Meta-Analysis of Epidemiological Studies’, PLoS One, 9(2), e89288. Available at (Accessed 21 July 2021).
  9. Nechuta, S.J., Caan, B.J., Chen, W.Y. et al. (2012). ‘Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women’, The American Journal of Clinical Nutrition, 96(1), pp123-132. Available at (Accessed 21 July 2021).
  10. Khan, S.A., Chatterton, R.T., Michel, N. et al. (2012). ‘Soy Isoflavone Supplementation for Breast Cancer Risk Reduction: A Randomized Phase II Trial’, Cancer Prevention Research, 5(2), pp309-319. Available at (Accessed 21 July 2021).
  11. Shike, M., Doane, A.S., Russo, L. et al. (2014). ‘The Effects of Soy Supplementation on Gene Expression in Breast Cancer: A Randomized Placebo-Controlled Study’, Journal of the National Cancer Institute, 106(9). Available at (Accessed 21 July 2021).
  12. Garcia-Mantrana, I., Monedero, V. and Haros, M. (2015). ‘Reduction of Phytate in Soy Drink by Fermentation with Lactobacillus casei Expressing Phytases From Bifidobacteria’, Plant Foods for Human Nutrition, 70(3), 269-274. Available at (Accessed 21 July 2021).
  13. Hϋser, S., Guth, S., Joost, H.G. et al. (2018). ‘Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation’, Archives of Toxicology, 92(9), 2703-2748. Available at (Accessed 21 July 2021).
  14. Seth, D., Poowutikul, P., Pansare, M. et al. (2020). ‘Food Allergy: A Review’, Pediatric Annals, 49(1), e50-e58. Available at (Accessed 21 July 2021).