Updated: Jul 21
Soy has received an incredibly bad reputation even among health professionals, doctors and nutritionists for its phytoestrogen effect. A quick google search will reveal anything from "causes cancer", "steals testosterone" to "causes man-boobs" and "turns men into women". But are any of these claims really true? Does soy cause cancer? Does soy reduce male testosterone? Does soy make men feminine? Does soy reduce thyroid function? Let's find out. Over these 2 articles, we'll explore some of the popularised claims and myths and see whether they are based on some good quality research on mostly just hearsay.
SOY & OESTROGEN - What is the connection?
The reason why soy has become so controversial are phytoestrogens. Phytoestrogens are components in soy products that resemble the biochemical structure of natural mammalian oestrogens, the primary sex hormones in women and secondary sex hormones in men. (Yes, men also have some natural oestrogens). In women, oestrogen is responsible for the regulation of the menstrual cycle and reproduction, bone density, brain function, cholesterol mobilisation, development of breast tissue and sexual organs as well as control of inflammation. In men, oestrogens work closely with testosterone in the development & maturation of sexual organ, production of sperm and libido regulation.
Let's take a step back and look at natural oestrogen. In order for it to be produced, a signal has to be sent from the brain into the ovaries (in females) or the testis (in males). Once the oestrogen has been produced, it needs to dock in to Oestrogen Receptor.
There are two types of Oestrogen Receptors:
Oestrogen Receptor Alpha – ERα – found in breasts, liver, ovaries, testes, bones and central nervous system. Excessive docking of Oestrogen into ERα has been linked to cancer.
Oestrogen Receptor Beta – ERβ – all above + digestive system, prostate, bladder & lungs. Docking of Oestrogen into ERβhas been linked to cancer prevention.
Phytoestrogens in soy ARE ABLE to dock into Oestrogen Receptors…..and here comes the most significant confusion. It is known that excess of Oestrogen Receptor stimulation (especially ERα) by having too much Oestrogens in the body may increase the risk of certain cancers. It is commonly thought that eating soy increases the amount of oestrogen in the body because of phytoestrogens, and thus it is assumed that eating soy may contribute to cancer. But there are several reasons why this theory is wrong:
Phytoestrogens cannot increase the production of natural oestrogens. The biological hormonal system is a closed and tightly controlled system. This means that phytoestrogens in soy DO NOT make men feminine. They have nothing to do with real hormones. Men who produce too much oestrogen often have a serious hormonal disruption that could be coming from impaired liver, kidneys, thyroid, testicles or inherited genetic disease. Soy has no effect here.
Soy phytoestrogens can indeed dock into oestrogen receptors, but their affinity (strength of connection) and the duration on how long they are docked in is tiny compared to real biological oestrogen.
Phytoestrogen preferentially docks to Oestrogen Receptor Beta (ERβ), which is associated with cancer prevention. We have in-vitro studies, animal studies and human studies to show for this.
Phytoestrogens are not found only in soy but in certain mushrooms (as mycoestrogens), fruits, vegetables and legumes, all of which have consistently been shown to reduce the risk of hormone-driven cancers.
Genistein, the most prevalent and most abundant phytoestrogen in soy, has consistently been associated with a reduced risk of various cancers in both animal and human studies - A fascinating fact is that genistein may even increase the response of cancerous cells to medical chemotherapy and can significantly slow down the cancer process in the cell.
WHAT ABOUT MAN-BOOBS?
Gynaecomastia, often referred to as "man-boob" is a condition caused by hormonal disruption, by excess oestrogens and lack of androgens (DHEA & testosterone), the most common causes are liver or kidney problems, hypothyroidism (decreased function of thyroid gland), genetic predisposition or hypogonadism (physiological failure in maturation of testicles and decline in sex hormones). There is no evidence to show that this condition is caused by soy. However, it is known that obese men are more likely to develop gynaecomastia. This is because excess fat on male body (especially the fat around belly) actively produces estrogen and also increases conversion of natural testosterone into oestrogen through a process called Aromatisation. Statistically speaking, men who consume more soy products like tofu or tempeh are often less likely to be obese as well and generally have a healthier diet and lifestyle.
So what's the best way to avoid getting man-boobs? Stay lean, maintain good level of muscle mass and exercise regularly, don't eat junk and don't overeat. For those men who are unfortunate to have developed gynaecomastia anyway, they need to work with endocrinologist (hormone doctor) to find the root cause and treat it.
BUT I HEARD SOY KILLS TESTOSTERONE IN MEN!
There is not a lot of research data looking at soy and testosterone levels, but the small amount of data we have, show the following:
Soy consumption does not seem to affect healthy testosterone levels although we do not know whether soy consumption affects men with already low testosterone levels
There is no evidence that shows a reduction of total testosterone in healthy men after eating soy
There was, however, one odd study (Tanaka et al. 2009) that actually showed a decrease in free testosterone
WHAT HAPPENED IN THE ODD STUDY?
First of all, this study was tiny (n=28), and the participants were not given soy as a whole food but an extract. Previous research has shown that soy is most effective when it is consumed whole as a soybean, not as an extract, so this is already a limitation. In this study, a free testosterone decreased as well as a more potent form called Dihydrotestosterone (DHT) decreased. But this was actually a good thing; let me explain why.
First of all, we have to mention that in this study, a marker called Sex-Hormone Binding Globulin (SHBG) increased. SHBG is a molecule that sticks to any sex hormones that are roaming around the blood freely. This is good; we don't want a lot of unbound (free) testosterone running around. The reason is that excess of free testosterone (not bound to SHBG) increases its conversion to Dihydrotestosterone (DHT) and too much DHT may stimulate growth of the prostate which is not something we want. However, there is a chance that this may not be entirely true as the link between too much free testosterone and prostate cancer has not yet been established (studies are too conflicting on this point).
This is what happened in the end
DHT went down on average by around 20% - this is good, DHT is more aggressive towards the prostate and having less DHT is generally more favourable in adult males (young growing males need more DHT during their main growth periods). Unlike free testosterone, we know that too much DHT can be problematic
Free testosterone went down by around 20% - this is also good; despite there being conflict in studies, there is little benefit of having too much free testosterone running around. The important marker is total testosterone
There were no statistically significant changes to total testosterone
Based on the available evidence, we can say that soy does not increase oestrogen in men nor does it cause man-boobs. Neither does soy reduce total testosterone although it has been shown to reduce dihydrotestosterone, a more potent form of testosterone. This is however a positive finding because too much dihydrotestosterone can stimulate growth of prostate gland in men.
In part 2 we will explore the link between soy & cancer and see whether soy is causative or protective. We will also look at soy in relation to thyroid hormones and whether there is anything to be vary about.
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Desmawati, D., & Sulastri, D. (2019). Phytoestrogens and their health effect. Open Access Macedonian Journal of Medical Sciences, 7(3), 495–499.
Hooper, L., Ryder, J. J., Kurzer. et al. (2009). Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: A systematic review and meta-analysis. Human Reproduction Update, 15(4), 423–440.
Jargin, S. V. (2014). Soy and phytoestrogens: Possible side effects. German Medical Science : GMS e-Journal, 12, Doc18–Doc18. https://doi.org/10.3205/000203
Kraemer, W. J., Solomon-Hill, G., Volk, B. M. et al. (2013). The Effects of Soy and Whey Protein Supplementation on Acute Hormonal Responses to Resistance Exercise in Men. Journal of the American College of Nutrition, 32(1), 66–74.
Lecomte, S., Demay, F., Ferrière, F. et al. (2017). Phytochemicals targeting estrogen receptors: Beneficial rather than adverse effects? International Journal of Molecular Sciences, 18(7), 1–19.
Tanaka, M., Fujimoto, K., Chihara, Y. et al. (2009). Isoflavone supplements stimulated the production of serum equol and decreased the serum dihydrotestosterone levels in healthy male volunteers. Prostate Cancer and Prostatic Diseases, 12(3), 247–252.