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Soy Foods - do they affect soy hormones? Debunking myths

Debunking Myths:

Soy and Hormones

A while back I wrote an article on soy foods, setting the record straight and separating fact from fiction. This is because a lot my patients come to see me with fears around eating too much soy foods or want to avoid them completely. Typically, I recommend including (non-GMO) soy bean products such as fortified soy milk, tofu, tempeh/natto, edamame, miso paste and soy beans as well as their sprouts. When I say this, the immediate questions raised are, “Do Soy foods cause cancer?”, “Will they make me gain weight” and “Will I (or my partner) develop bigger breasts or man boobs if we eat too much soy?”…the list goes on.


So as a response to this I’m sharing with you the latest studies and evidence on soy piece by piece, delving a little more in depth into the facts about soy I summarised in the previous article. Let’s start with the big one:

Do soy foods impact our hormone (endocrine) system? In other words, does eating lots of soy based foods cause breast or ‘manboob’ growth?

Whole soy foods are beneficial for health, the evidence is overwhelming. They are minimally processed, meaning they retain a lot of their nutrition including essential nutrients (meaning we must get them from our diet) such as iron, calcium, zinc and protein to name a few.

Soy beans Edamame Debunking myths soy and hormones

How do soy foods affect our hormonal systems?

The key concern stems from a naturally occurring compound in soy called isoflavones. Isoflavones are oestrogen like compounds, otherwise known as “phytoestrogens”.

Effects of estrogen; Debunking soy and hormone myths

Image source Mahan L, Escott-Stump S. Krause’s 2004

  • Oestrogen is a female hormone found in both men and women. It’s helps you maintain a healthy reproductive system, and also plays a key role in strong bones, healthy skin, heart health, your central nervous system and healing injuries such as bruising. (1-2)
  • Phyto is a Greek word meaning “plant” aka plant oestrogen.
  • Phytoestrogens: put these together and the “isoflavones” in soy are a naturally occurring plant compound that looks similar to and can behave like human oestrogen.

But will eating lots of isoflavones from soy affect or disrupt our hormone systems? The evidence seems to say it does not.


Soy, Fertility and Reproductive Health

Studies conducted in both men and women show that soy foods and soy isoflavones do not affect or hinder growth, fertility or reproductive health. (3-6)

In fact, a large American study raised 248 baby boys and girls on soy formula (with lots of isoflavones) and compared this to 563 baby boys and girls raised on cow milk formula. The study then followed these babies over 20-34 years when they were grown adults to see if the formula had any effect on their development.

The verdict: the men and women raised on soy formula weren’t any different and the soy formula had no apparent negative effects on their growth rate, development in puberty, reproductive health or fertility compared to those raised on the cow milk formula.

In fact, the study also suggests that the soy formula did not increase the chance of developing hormonal or reproductive system disorders, cancer, libido dysfunction, sexual orientation, and birth defects in their own children. (5-6)

soy infant formula; debunking soy and hormone myths

Summary: soy isoflavones do not impair fertility or reproductive health. Soy formulas are safe to feed infants and can help them to grow into healthy adults.

Soy and Male Hormones

Thus far, evidence suggests phytoestrogens in soy foods do not affect male hormone systems or testosterone levels. A recent meta-analysis of 36 placebo controlled treatment groups fed men soy products including soy milk, soy grits, soy flour, tofu and isolated soy protein. These studies ran anywhere from 1 week to 4 years, providing pretty reliable longer term data.

The meta-analysis concluded that neither soy foods nor isoflavone supplements affected testosterone levels in men. (7) This is pretty sound evidence considering these men where fed larger amounts of isoflavones than you would typically get from a vegetarian or vegan diet. Several other studies also support that soy phytoestrogens do not effect or lower male sex hormones. (8-9).

Soy isoflavones have also been shown to be protective against prostate cancer…but more on that in my next soy article ‘Does eating Soy foods cause Cancer?‘.

soy male hormones; debunking soy myths

Summary: eating tofu or soy milk or any other soy products for that matter will not to make you more feminine!

Soy and Fibroids

Soy products may actually be protective for the female reproductive organs. One example is evidence around soy and fibroids. Fibroids are knots of muscle tissue that develop in the thin muscle layer under the lining of women’s uterus’. (2) Fibroids can be extremely painful to say the least and can often lead to having a hysterectomy (surgically removing the uterus). The good news…

Soy Fibroids; debunking soy mythsSoy Fibroids; debunking soy myths

soy products may reduce the risk of fibroids!!

A study  of 1172 randomly selected Japanese women found those who ate the most soy were 35% less likely to need a hysterectomy, suggesting fibroids were less frequent. (10) These results are true for Western populations too! A study of American women showed soy foods did not improve but did not worsen fibroids. These results are most likely because the American women only consume very small amounts of soy compared with the Japanese women, thus not enough soy to have any positive impacts. (11)

soy reproduction; debunking soy myths

Summary: high intakes of soy isolavones and soy foods could protect female reproductive organs and reduce the chance of getting fibroids and/or having a hysterectomy.

Soy foods and breast growth

Finally, the big one on everyone’s minds, can soy foods make women’s (and men’s) chest tissue grow larger? Aka can you get bigger breasts or man boobs if you eat too much soy?

soy beans, soy milk, tofu, edamame; debunking soy myths

To answer this, I need to explain some biochemistry. Warning this is going to get a little scientific.

Our body has two predominant oestrogen receptors; oestrogen receptor alpha and oestrogen receptor beta.You may find it help to think of these as receptors A and B or ER alpha and ER beta. The alpha receptors are mostly found in endocrine tissue such as breast and reproductive organs as well as liver cells, while the beta receptors are typically found in your bones (think B for bones).

Laboratory studies show that soy phytoestrogens mostly affect the ER beta receptors in your bones but rarely affect the ER alpha receptors in your breast or genital tissue. (12)

This is because soy isoflavones are selective oestrogen receptor modulators: meaning something that has pro-estrogenic effects in some tissues (like bone), but anti-estrogenic effects in other tissues (like breasts). (12-13)

The physicians committee of responsible medicine (PCRM) have a great metaphor for explaining this

 Think of it this way: An estrogen molecule is like a an aeroplane that attaches to the runway of an airport. It discharges passengers (oestrogen hormones) into the terminal, which is suddenly a very busy, bustling place. Phytoestrogens, are weak oestrogens, like little private planes with only a few passengers and no cargo. But when they land they still reside in the runway after landing and block it so that normal bigger oestrogen aeroplanes cannot land. The effect is that plant oestrogens don’t block all of oestrogen’s effects, but can block some of them/minimise them, apparently reducing breast cancer risk and menstrual symptoms. (14)

Oestrogen molecule; debunking soy hormone myths

What does this mean? Basically, it takes a lot of soy foods to have any effect on your alpha receptors. If you ate roughly 1 cup of cooked whole soybeans your blood levels of genistein (a phytoestrogen that attaches to alpha receptors) would be around 20 to 50 nanomoles, and only half of this is actually available to bind to oestrogen receptors. That’s barely anything! So that cup of soy beans mostly activates beta cells and barely effects the alpha cells. In fact you would need to eat around 30 cups of soybeans a day to effect your alpha receptors. Pharmaceutical oestrogen’s (aka hormone replacement therapy) can do this, but soy phytoestrogens are unlikely to reach these levels. (18-17)

hormone replacement therapy; debunking soy hormone myths

The wrap up

So there you have it! The latest science and research into how soy affects your hormones. There is also some evidence around soy and menopause…but I’ll save that for another post.

soy beans; debunking soy hormone myths

If you’re looking for tips and tricks on plant based eating and how to include whole soy foods more in your diet, please book an appointment via my booking page here.

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Post by plant-based vegan dietitian Amber Sewell-Green

Accredited Practicing Dietitian and Nutritionist


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  1. Mahan L, Escott-Stump S. Krause’s food, nutrition, & diet therapy. Philadelphia: Saunders; 2004.
  2. Raman R. Phytoestrogens: Benefits and Risks [Internet]. 2017 [cited 27 November 2017]. Available from:
  3. Mitchell JH, Cawood E, Kinniburgh D, Provan A, Collins AR, Irvine DS. Effect of a phytoestrogen food supplement on reproductive health in normal males. Clin Sci (Lond). 2001;100:613-618.
  4. Kurzer MS. Hormonal effects of soy in premenopausal women and men. J Nutr. 2002;132:570S-573S.
  5. Strom BL, Schinnar R, Ziegler EE, et al. Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. JAMA. 2001;286:807-814.
  6. NTP-CERHR Monograph on soy infant formula No. 10-5995 [Internet]. National Toxicology Program: Center for the Evaluation of Risks to Human Reproduction; 2010 [cited 23 October 2017].
  7. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94:997-1007.
  8. Habito, R. C., Montalto, J., Leslie, E.& Ball, M. J.(2000) Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males. British Journal of Nutrition 84, 557–563.
  9. Nagata, C., Takatsuka, N., Shimizu, H., Hayashi, H., Akamatsu, T. & Murase, K.(2001) Effect of soymilk consumption on serum estrogen and androgen concentrations in japanese men. Cancer Epidemiology, Biomarkers and Prevention 10, 179–184.
  10. Nagata C, Takatsuka N, Kawakami N, Shimizu H. Soy product intake and premenopausal hysterectomy in a follow-up study of Japanese women. Eur J Clin Nutr. 2001;55:773-777.
  11. Atkinson C, Lampe JW, Scholes D, Chen C, Wahala K, Schwartz SM. Lignan and isoflavone excretion in relation to uterine fibroids: a case-control study of young to middle-aged women in the United States. Am J Clin Nutr. 2006;84:587-593.
  12. Morito K, Hirose T, Kinjo J et al (2001). Interaction of Phytoestrogens with Estrogen Receptors .ALPHA. and .BETA.. 2001. Biological and Pharmaceutical Bulletin, 24;4: 351-356.
  13. Brzezinski A, Debi A. Phytoestrogens: the “natural” selective estrogen receptor modulators? Eur J Obstet Gynecol Reprod Biol. 1999; 85:47-51.
  14. McCarty MF. Isoflavones made simple – genistein’s agonist activity for the beta-type estrogen receptor mediates their health benefits. Med Hypotheses. 2006;66(6):1093-114.
  15. Ask the Expert: Soy [Internet]. The Physicians Committee. 2016 [cited 29 November 2017]. Available from:
  16. Oseni T, Patel R, Pyle J, Jordan VC. Selective estrogen receptor modulators and phytoestrogens. Planta Med. 2008 Oct;74(13):1656-65.
  17. Mueller SO, Simon S, Chae K, Metzler M, Korach KS. Phytoestrogens and their human metabolites show distinct agonistic and antagonistic properties on estrogen receptor alpha (ERalpha) and ERbeta in human cells. Toxicol Sci. 2004 Jul;80(1):14-25.
  18. Taylor AH, Al-Azzawi F. Immunolocalisation of oestrogen receptor beta in human tissues. J Mol Endocrinol. 2000 Feb;24(1):145-55.
Amber Sewell-Green

Author Amber Sewell-Green

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