The research into soy intake and breast cancer risk is very much a grey area and this makes it controversial. As a lifelong people pleaser, sticking my head above the parapet and heading into the domain of controversy is something I would normally avoid. But, after I was diagnosed with breast cancer, this was a question I wanted to know the answer to and I hope that this post is helpful for you.
Soy and oestrogen, what is the link?
Soy foods contain high levels of phytoestrogens (plant oestrogens) called isoflavones. Other foods also contain isoflavones but in vastly smaller quantities. The two main isoflavones are genistein and daidzein.
What is a phytoestrogen?
A phytoestrogen is a plant compound which looks like oestrogen to our cells. For oestrogen to work, it needs to lock onto an oestrogen receptor. These receptors are shaped so that only oestrogen (and not other hormones) fit into them (a bit like a toy where you fit shapes into matched holes). A phytoestrogen can also fit into the oestrogen shaped receptor. When oestrogen locks onto the receptor it exerts a strong effect on the cell (think 10/10 on the volume control). When a phytoestrogen locks onto the receptor it only exerts a very weak effect (less than a 1/10).
Can phytoestrogens increase our oestrogen levels?
No. Phytoestrogens don’t increase or decrease our own natural levels of oestrogen (endogenous oestrogen).
When we have a lot of oestrogen circulating, phytoestrogens may help to keep levels in check by blocking some of the oestrogen receptors. If a phytoestrogen has fixed onto the oestrogen receptor that means oestrogen can’t fit on at the same time. Where as in menopause, when oestrogen levels have crashed; phytoestrogens can fit into the oestrogen receptor and exert a very weak oestrogenic effect. This is why soy isoflavones are often marketed as a natural solution to menopause symptoms.
Can soy prevent breast cancer? (1–4)
A lot of the research which has demonstrated a positive link between eating more soy and a reduced risk of getting breast cancer has been conducted in Asian populations. Many of these women ate soy throughout their lives and it is thought that this long-term exposure to soy phytoestrogens may be an important factor. Consuming lots of phytoestrogens over a lifetime, may mean that the breast tissue has had less exposure to endogenous oestrogen (stronger oestrogen). Eating whole soy (like tofu, tempeh and edamame) may also support other biological pathways that could reduce breast cancer risk.
There is research looking at breast cancer recurrence and soy intake. One study of note looked at Asian women who had previously recovered from breast cancer. The women who ate the most soy were far less likely to have their breast cancer recurr when compared to the women who at e the least soy. While this sounds like great news, we need to consider the type of research we are looking at.
This study (and most others which show a link between soy intake and reduced breast cancer risk) are cohort studies. Cohort studies follow groups of people over a long period of time and track them for various health outcomes. The trouble with these types of studies is that, while they are very interesting, they can’t prove cause and effect. For example, did the women who ate the most soy also eat more cruciferous vegetables and fish and did they eat less red meat and do more exercise. Yes, in this research study they did. So, what actually caused the reduction in breast cancer recurrence? Was it the consumption of soy foods or these other factors? From this type of study, we can’t answer this question.
When we see statements like “eating more soy reduces your risk of breast cancer” or “there is no evidence that soy causes breast cancer”, we need to question the type of research behind the statement, especially before we start adding large amounts of soy foods into our diet. This is because some research has shown a potential negative effect of soy on breast cancer risk.
Can soy cause breast cancer?
The controversy around soy began with animal studies where research in rodents showed that genistein (one of the soy isoflavones) could increase oestrogen fed tumours. However, it was later proven that rodents and humans metabolise soy isoflavones differently, meaning that rodents are exposed to much higher levels of genistein than a human would be. Research at a cellular level (think test tubes and petri dishes), has also produced conflicting results. In summary when combining animal and cellular level research, we have a very mixed picture of how soy isoflavones might interact with breast tissue and breast cancer cell growth (5–8).
In humans two randomised controlled trials (the type of study which can determine cause and effect) have flagged a potential negative effect of high levels of soy isoflavones (9,10). One trial looked at women at increased risk of breast cancer, and the other trial at women with a recent breast cancer diagnosis who were awaiting surgery. In some women (but not all women), high doses of soy isoflavones switched on genes that could make tumour cells grow. Another randomised controlled trial looked at high dose soy isoflavones on breast density (a risk factor for breast cancer) in post-menopausal Dutch women (11). The soy group took the supplements for one year, so a good amount of time. The results showed no significant change to breast density either positively or negatively. So, once again we have a mixed picture
In summary, we don’t have a black or white answer and much more research is needed. In the meantime, we need to think safety first and to be mindful of the fact that for SOME women, eating LARGE amounts of soy isoflavones (four or more servings per day) MAY increase their risk for breast cancer recurrence.
After breast cancer should we eat soy or not?
Firstly, we don’t need to be afraid of eating soy foods. There is no evidence to suggest that eating whole soy foods in moderation, will increase our risk of breast cancer recurrence. But neither is there good evidence to suggest that increasing soy intake will reduce our risk. So, if you like soy foods – eat them! But, as with all things, moderation is key, and I recommend sticking to one or two portions per day. Ultimately the best diet, is a varied diet and too much of anything isn’t good for us.
If you like soy choose good quality whole soy foods such as tofu, tempeh, edamame beans and miso. For soy milk choose an unsweetened brand, preferably with nothing else added. Aim to avoid highly processed soy (like meat substitutes) and soy isolates (which we might find in vegan protein bars).
Are soy isoflavone supplements safe after breast cancer?
I would not recommend anyone used supplements containing soy isoflavones after a breast cancer diagnosis. At concentrated levels, in some genetically predisposed women, soy isoflavones may promote tumour growth. Soy isoflavones are often found in menopause supplements so check labels carefully. Also avoid soy protein powders or soy isolates which might be found in sports and protein drinks and bars or which you might be adding to smoothies.
Do I eat soy?
Yes, sometimes! As a breast cancer survivor (oestrogen receptor positive), I enjoy good quality soy foods as and when I want to eat them. I sometimes eat organic tofu, I regularly use miso paste in my cooking (it is delicious stuff) and I eat edamame beans because I like them. But I don’t eat soy because I think it will be offering me any protection against breast cancer recurrence, and I tend to keep my intake fairly low.
More importantly I aim for a diverse diet packed full of colour, taste and nourishment, and I choose to eat a wide variety of delicious natural foods which make me feel great.
I hope you find this helpful. Feel free to get in touch with any questions and best of luck with your breast cancer recovery.
1. Shu, X. O. et al. Soy food intake and breast cancer survival. JAMA – Journal of the American Medical Association 302, 2437–2443 (2009).
2. Wei, Y. et al. Soy intake and breast cancer risk: a prospective study of 300,000 Chinese women and a dose–response meta-analysis. European Journal of Epidemiology 35, 567–578 (2020).
3. Guha, N. et al. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: The Life after Cancer Epidemiology study. Breast Cancer Research and Treatment 118, 395–405 (2009).
4. Caan, B. J. et al. Soy food consumption and breast cancer prognosis. Cancer Epidemiology Biomarkers and Prevention 20, 854–858 (2011).
5. Allred, C. D., Allred, K. F., Ju, Y. H., Virant, S. M. & Helferich, W. G. Soy Diets Containing Varying Amounts of Genistein Stimulate Growth of Estrogen-dependent (MCF-7) Tumors in a Dose-dependent Manner 1. CANCER RESEARCH vol. 61 (2001).
6. Young, H. J., Doerge, D. R., Allred, K. F., Allred, C. D. & Helferich, W. G. Dietary Genistein Negates the Inhibitory Effect of Tamoxifen on Growth ofEstrogen-dependent Human Breast Cancer (MCF-7) Cells Implanted inAthymic Mice. Cancer Research 62, 2474–2477 (2002).
7. Liu, B. et al. Low-Dose Dietary Phytoestrogen Abrogates Tamoxifen-Associated Mammary Tumor Prevention. www.aacrjournals.org (2005).
8. Setchell, K. D. R. et al. Soy isoflavone phase II metabolism differs between rodents and humans: Implications for the effect on breast cancer risk. American Journal of Clinical Nutrition 94, 1284–1294 (2011).
9. Khan, S. A. et al. Soy isoflavone supplementation for breast cancer risk reduction: A randomized phase ii trial. Cancer Prevention Research 5, 309–319 (2012).
10. Shike, M. et al. The effects of soy supplementation on gene expression in breast cancer: A randomized placebo-controlled study. Journal of the National Cancer Institute 106, (2014).
11. Verheus, M. et al. Soy protein containing isoflavones and mammographic density in a randomized controlled trial in postmenopausal women. Cancer Epidemiology Biomarkers and Prevention 17, 2632–2638 (2008).