Discover Now if Soy is Good or Bad For You?

Is soy good or bad for you and is it safe?

Is soy good or bad for you and is it safe? So many headlines are written about soy, for example as a saviour for menopausal hot flushes, or does soy protect or cause breast cancer and is eating soy safe with breast cancer? But unpick the evidence and the truth about soya is a lot more nuanced.

What are phytoestrogens?

Phytoestrogens are plant derived compounds, that have a similar structure to the human hormone estrogen, and similar but weaker activity. They are found in foods and in a more concentrated form in supplements. Phytoestrogens supplements are frequently marketed and chosen to target hot flushes. 

Isoflavones found in soy products are a type of phytoestrogen. The two major isoflavones found in soy are called genistein and daidzein.

soy menopausal hot flushes

Soy and menopausal hot flushes

Can soy help with menopausal hot flushes? Isoflavone supplements contain much higher concentration phytoestrogens than are naturally found in food. There is mixed evidence that isflavone supplements may reduce menopausal hot flushes (Lethaby et al., 2013). Given the mixed evidence about efficacy of the supplements, is therefore unlikely that consuming these food items, will have a significant benefit on symptoms. If you are menopausal or peri-menopausal, given the mixed evidence, it is worth trying soy to see if it helps your from menopausal hot flushes. They are also a great source of lean plant-based protein.

Read more about dietary changes for the menopause here and get your free menopause nutrition planner here.

Metabolism of soy

Soy is metabolised into different active compounds and this depends on a number of factors including genes, diet and microbiota (Mace et al., 2019).

Daidzein, an isoflavone phytoestrogen found in soy, is metabolized to equol and O-desmethylangolensin (O-DMA) by intestinal bacteria (Mace et al., 2019)(Atkinson, Frankenfeld & Lampe, 2005). Cellular and animal studies have suggested that equol and O-DMA are more biologically active than their precursor daidzein (Atkinson, Frankenfeld & Lampe, 2005). The specific bacteria responsible for equol and O-DMA production in humans have yet to be identified, but only about one third of people have the ability to produce equol, while the majority (80-95%) of people can produce O-DMA (Mace et al., 2019)(Atkinson, Frankenfeld & Lampe, 2005).

Studies in humans have suggested that the ability to metabolise daidzein to equol may be associated with reduced risk of certain diseases including breast and prostate cancers (Atkinson, Frankenfeld & Lampe, 2005). With a woman’s ability to produce equol influencing the extent to which soy consumption in her diet, can impact on her risk of developing breast cancer (Setchell, Brown & Lydeking-Olsen, 2002)(Wu et al., 2008).

is eating soy safe with breast cancer?

Is eating soy safe with breast cancer?

Answering if eating soy is safe with breast cancer isn’t straight forwards. First some back ground about the rates of breast cancer in different countries and their diets. Rates of breast cancer in Asia are about one-sixth of those seen the Caucasian population in the USA (Wu et al., 2000). This lower risk has been linked with consumption of soy in Asian populations with an approximately 16% risk reduction per 10 mg of isoflavones intake per day. Age at exposure may be a co-determinant of risk; adolescent, pre-pubertal intake showing a stronger effect on risk than intake during adulthood (Wu et al., 2008).

Another difference is the type of soy consumed in Asia compared with that in Western countries. In Asia, it is usually tofu, soybeans, soy milk, and miso with an average intake of approximately 25 and 50 mg of isoflavones per day (Messina et al, 2006). In contrast, intake of soy isoflavones in Western populations is usually less than 1.0 mg of isoflavones per day and Asian type soy foods are rarely consumed (Messina et al, 2006). The source of Western soy isoflavones is also different, with the majority from legumes, sprouts and vegetables containing small amounts of isoflavones, and soy flour and soy protein that are commonly added as extenders and fillers in different bakery and canned goods (Horn-Ross et al, 2001)(Wu et al., 2008).

Additionally, the protective effect of soy in Asian populations may be partly due to soy metabolism. As mentioned above, the gut microbiota play an important role in the metabolism of soy to the active metabolite equol. Only 30% of Western microbiomes are associated with this capacity while 60% of Asian’s are (Rafii, 2015).

Breast cancer recurrence

Fears that eating soy might increase the risk of breast cancer recurrence or formation, originate from laboratory studies. In the laboratory, exposure to soy has been associated with stimulation of growth of breast cancer cells in cellular models, and also in mice that have had their ovaries removed, simulating the menopause (Messina, McCaskill-Stevens & Lampe, 2006).

How can soy stimulate growth of cancer in these models, but appear to be associated with a lower risk of developing breast cancer in human studies? One theory is that the laboratory and animal models are “low estrogen environments” and are essentially missing the naturally occurring estrogen seen in women (Stubert & Gerber, 2009).

Another is that the type of soy metabolite matters, as administration of equol in cellular and mouse models has not been associated with increased tumour growth (Stubert & Gerber, 2009). This could be another explanation that fits in with the protective effects seen with equol (Atkinson, Frankenfeld & Lampe, 2005), that the effects of soy are related to specific metabolites. These effects would not be seen in cellular models that lack the gut microbiota, and potentially animal models as well as.

Soy and tamoxifen

Drugs that inhibit estrogen such as tamoxifen, are used to prevent or slow the growth hormone receptor positive breast cancer. The results of studies investigating if soy affects the efficacy of these drugs are mixed. Some have found that genistein prevented the anti-cancer effects of tamoxifen, while only extremely high doses had the reverse effects, preventing cancer growth (Stubert & Gerber, 2009). The effects are therefore unknown, and despite two trials of over 2,500 women that did not demonstrate a negative effect on tamoxifen efficacy, there is still considerable caution (Stubert & Gerber, 2009). The German recommendations are for women taking anti-estrogen drugs such as tamoxifen to avoid high dose soy or isoflavone supplements (Stubert & Gerber, 2009). While we don’t fully know the effects of soy on tamoxifen, it seems sensible to avoid high doses or isoflavone supplements if you are taking tamoxifen.

Why when you started eating soy matters

There are often worries about giving children soy milk, but instead early and sustained introduction of products containing soy look to be beneficial, associated with a lower risk of breast cancer (Shu et al, 2001; Wu et al, 2002).

Animal and human observational studies suggest that the timing of soy in our diet matters (Shu et al, 2001; Wu et al, 2002)(Warri et al., 2008). Soy consumption before puberty is associated with a decreased long-term risk of breast cancer (Shu et al, 2001; Wu et al, 2002).

The mechanism of how this timing impacts risk is currently unknown. It is well established that pregnancy before 20 years of age reduces the risk of breast cancer, while women undergoing their first pregnancy after aged 35 are at an increased risk (Warri et al., 2008). One theory for soy consumption, is that it is linked with early development of the mammary glands and potentially acts in a way that is similar to pregnancy (Warri et al., 2008). This would fit with the recent hypothesis of a time sensitive window of pubertal breast development related to long-term risk of breast cancer (Stubert & Gerber, 2009).

The density of breast tissue on mammograms is a marker of increased breast cancer risk, with high density associated with a four-to-six-fold increase (Warri et al., 2008). In a study of pre-menopausal women, supplementation with 100mg isoflavones for one year, did not reduce the density of breast tissue on mammogram (Maskarinec et al, 2003), in keeping with the importance of eating soy earlier in life to have a beneficial effect.

Wider importance of soy

Soy may also have a role in modulating inflammation and the tumour micro-environment (the area immediately surrounding the tumour) (Mace et al., 2019). The role of the tumour micro-environment is very important in either shielding cancer cells from recognition and destruction by the immune system.

In cellular models genistein slows cancer growth by promoting cell death, stops cancer reproduction, and prevents growth of new blood vessels (angiogenesis) (Nagaraju, Zafar & El‐Rayes, 2013). These findings highlight the multiple roles of soy, and the complexity of cancer formation. In addition to just cellular cell growth, cancers also need to evade controlled cell death, poses the ability to metastasise and grow at distant sites, and evade the immune system. More research is needed to understand the role of soy not just in cellular and animal models but human studies.


I hope you have found this article about if soy i good or bad informative. Answering the question if eating soy is safe with breast cancer is nuanced. There is still much to understand about soy, for example the interplay between metabolism to different metabolites and age at consumption, along with the exact role of the microbiota.

While enjoying moderate consumption of soy foods appears generally safe, it seems sensible to avoid isoflavone supplements or high dose soy while taking tamoxifen.

More research might help us understand how to cultivate a microbiota that possess the ability to metabolise soy to equol, but at the moment, swapping from a Western diet, high in saturated fat, sugar and processed food, to a more whole food, low saturated fat, high fibre diet may help, and has other help benefits as well.

Aim to eat soy that is in the form of tofu, soybeans, soy milk, and miso, instead of processed soy in the form of food additives. If you are menopausal or peri-menopausal, given the mixed evidence, it is worth trying soy to see if it helps your from menopausal hot flushes. They are also a great source of lean plant-based protein.

What next?

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Atkinson, C., Frankenfeld, C.L. & Lampe, J.W. (2005) Gut Bacterial Metabolism of the Soy Isoflavone Daidzein: Exploring the Relevance to Human Health. Experimental Biology and Medicine. [Online] 230 (3), 155–170.

Lethaby, A., Marjoribanks, J., Kronenberg, F., Roberts, H., et al. (2013) Phytoestrogens for menopausal vasomotor symptoms. Cochrane Database of Systematic Reviews.

Mace, T.A., Ware, M.B., King, S.A., Loftus, S., et al. (2019) Soy isoflavones and their metabolites modulate cytokine-induced natural killer cell function. Scientific Reports. [Online] 9 (1), 5068.

Messina, M., McCaskill-Stevens, W. & Lampe, J.W. (2006) Addressing the Soy and Breast Cancer Relationship: Review, Commentary, and Workshop Proceedings. JNCI: Journal of the National Cancer Institute. [Online] 98 (18), 1275–1284.

Nagaraju, G.P., Zafar, S.F. & El‐Rayes, B.F. (2013) Pleiotropic effects of genistein in metabolic, inflammatory, and malignant diseases. Nutrition Reviews. [Online] 71 (8), 562–572.

Rafii, F. (2015) The Role of Colonic Bacteria in the Metabolism of the Natural Isoflavone Daidzin to Equol. Metabolites. [Online] 5 (1), 56–73.

Setchell, K.D.R., Brown, N.M. & Lydeking-Olsen, E. (2002) The Clinical Importance of the Metabolite Equol—A Clue to the Effectiveness of Soy and Its Isoflavones. The Journal of Nutrition. [Online] 132 (12), 3577–3584.

Stubert, J. & Gerber, B. (2009) Isoflavones – Mechanism of Action and Impact on Breast Cancer Risk. Breast Care. [Online] 4 (1), 22–29.

Warri, A., Saarinen, N.M., Makela, S. & Hilakivi-Clarke, L. (2008) The role of early life genistein exposures in modifying breast cancer risk. British Journal of Cancer. [Online] 98 (9), 1485–1493.

Wu, A.H., Stanczyk, F.Z., Hendrich, S., Murphy, P.A., et al. (2000) Effects of soy foods on ovarian function in premenopausal women. British Journal of Cancer. [Online] 82 (11), 1879–1886.

Wu, A.H., Yu, M.C., Tseng, C.-C. & Pike, M.C. (2008) Epidemiology of soy exposures and breast cancer risk. British Journal of Cancer. [Online] 98 (1), 9–14.

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