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Soy and Hormones: Separating Fact from Fiction

Soy is often a protein-rich staple of plant-based diets, but some say it affects your hormones. Here, we set the record straight.


Introduction

Soy foods like tofu have been staples in Asian diets for centuries. However, in more recent times, soy intakes have been on the rise in Western countries—and especially in vegetarians and vegans. Soy has a lot of things going for it: it’s a high-quality protein source, it’s a source of heart-healthy (unsaturated) fats as well as fibre, and it contains bioactive compounds that may improve aspects of health (e.g., plant sterols, which lower cholesterol (1)). Despite this, some claim that certain polyphenols in soy, called isoflavones, can impact sex hormones (e.g., oestrogen, testosterone). Luckily for us, this has been well studied.


sesame tofu

Sesame tofu.


Soy is Feminising?

The basic argument is that soy is feminising; meaning, it increases the (primarily) female sex hormone oestrogen, while decreasing the (primarily) male sex hormone testosterone. Because isoflavones—also known as phytoestrogens (or plant oestrogens)—can bind to oestrogen receptors in the human body and exert oestrogenic effects, this claim seems, on the surface, quite plausible. However, we must consider that these isoflavones are ~100–1,000 times weaker than human oestrogen, and can sometimes even block the effect of human oestrogen (2).


The Evidence

In a meta-analysis that combined the results from 41 different studies including 1,753 males, soy had no significant effect on testosterone or oestrogen levels (3). Even when the researchers just looked at studies where people ate at least three servings of soy per day (i.e., ~75+ milligrams of soy isoflavones per day), soy had no significant impact on testosterone or oestrogen levels (3). The story is similar for females; in a meta-analysis of 47 studies including 1,813 females, soy had no significant impact on oestrogen levels (4).


Does soy affect hormones?

Watch this short video from Simon Hill, of The Proof, who dispels the myth that soy foods impact male hormones. Simon is a great source of nutrition information, so check out his YouTube channel and podcast.


Summary

The best quality evidence suggests that soy is not feminising, so if you have heard this claim, rest assured that it’s contradicted by strong evidence. Considering that soy intake is associated with a reduced risk of many chronic diseases like heart disease and certain cancers (5), it would be unwise to avoid it based on an unsupported claim. Finally, if you want to learn more about this topic, I have written about this in more detail elsewhere.


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Thank you very much for reading. Enjoy your weekend!


Patrick Elliott, BSc, MPH

Health and Nutrition Science Communication Officer at Training121


References

(1) Abumweis SS, Barake R, Jones PJ. Plant sterols/stanols as cholesterol lowering agents: A meta-analysis of randomized controlled trials. Food Nutr Res. 2008;52:10.3402/fnr.v52i0.1811. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596710/


(2) Mostrom M, Evans TJ. Chapter 60 – Phytoestrogens, in Veterinary Toxicology, 3rd edition, Academic Press; 2018. Available at: https://www.sciencedirect.com/science/article/pii/B978012811410000060X


(3) Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60–7. Available at: https://www.sciencedirect.com/science/article/pii/S0890623820302926


(4) Hooper L, Ryder JJ, Kurzer MS, et al. Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis. Hum Reprod Update. 2009;15(4):423–40. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2691652/


(5) Li N, Wu X, Zhuang W, et al. Soy and Isoflavone Consumption and Multiple Health Outcomes: Umbrella Review of Systematic Reviews and Meta-Analyses of Observational Studies and Randomized Trials in Humans. Mol Nutr Food Res. 2020;64(4):e1900751. Available at: https://onlinelibrary.wiley.com/doi/10.1002/mnfr.201900751


Technical Terms

Polyphenols: These are a group of phytochemicals—or plant nutrients—that act as antioxidants and improve our health. Health benefits from polyphenol intake include reductions in LDL cholesterol, improved cognition, and other metabolic improvements.


Isoflavones: These are a type of polyphenol found in plant foods, and are also known as phytoestrogens (or plant oestrogens).


Oestrogen: This refers to a group of sex hormones that promote the development and maintenance of female characteristics in the body. The main type is called 17β-estradiol.


Testosterone: This is the main male sex hormone in the human body, and it stimulates the development of male secondary sexual characteristics. More than 95% is secreted by the testis.


Meta-analysis: This is a type of study that combines the results of a number of other studies that look at the same thing, and produces an overall estimate of the effect or association. For example, a meta-analysis of five studies looking at smoking and lung cancer would combine the results from all five studies into one overall result, with the intention of providing a better estimate of the true effect of smoking on lung cancer.

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