I originally published a blog post about the safety of soy consumption back in 2014, but as I am still asked about it on a daily basis, I figured it was high time I gave it a revamp and re-posted it, with more of a focus on the effect of soy consumption on hormones.

In the original post, I also looked at various anti-nutrients in soy products (including saponins, phytate, enzyme inhibitors and lectins) in addition to the effect of soy on hormones, so if you have any concerns about these, please read the 2014 post here. Concerns about lectins in particular have resurfaced in the past year or so in the media, and what I wrote back in 2014 is certainly still relevant.

I was originally motivated to write this post as many clients I see in my clinical practice deliberately avoid soy products, with the most common reason stated for doing so being 'I have heard that soy is bad for hormones'. As such, this post will delve into the science of soy consumption and its effect on sex hormones (estrogen and testosterone).

Firstly, soy products refers to foods made from soybeans, and includes:

Soy milk
Tempeh - this is a product not many people are familiar with - it is a traditional Indonesian food, made by forming whole soy beans into a cake and then fermenting it with a mold
Tofu
Edamame
Soy yoghurt
Textured vegetable protein (TVP)
Isolated soy protein powders (found in processed foods such as faux meats)
Miso paste
Natto
Soy sauce
Tamari

Does soy contain estrogen?

Soybeans contain a class of compounds known as isoflavones, which are characterised as phytoestrogens (‘phyto’ meaning plant-derived). Many other plant foods also contain isoflavones, including flaxseeds and whole grains, though soy is the most significant dietary source.

The isoflavones in soy (daidzein, genistein and glycitein) are structurally similar - although not identical - to human estrogen, and as such, can bind to estrogen receptors in the body. However, compared with human estrogen, phytoestrogens are known to exert a much weaker effect in the body.

There are also two types of estrogen receptors (ER): ER-alpha and ER-beta, which are found in different tissues in the body. Isoflavones have been shown to be selective in their binding to these receptors, choosing to bind to ER-beta in preference to ER-alpha, and hence are often referred to as selective estrogen receptor modulators (SERMs). This selective binding affinity helps to explain how they can both enhance and inhibit the action of estrogen depending on the tissue type.

Soy consumption and breast cancer

Many people believe that the isoflavones in soy products may increase the risk of breast cancer, however, data from population studies (primarily conducted in Asia, where people regularly consume soy products) actually show that moderate consumption of soy products (1-2 servings per day) may reduce breast cancer risk in women (1).

Recent results by Baglia et al from the Shanghai Women's Study (a cohort of over 70,000 women) found that those who had the highest intake of soy products during childhood and adulthood had a reduced risk of breast cancer (2).

Soy consumption in patients with breast cancer

Moderate consumption of soy products is known to be safe for women with breast cancer and those with a history of breast cancer, and may also reduce the risk of breast cancer recurrence (1).

Soy consumption and men’s health

Prostate Cancer

A 2009 meta-analysis (which pools a number of studies) found that men with the highest intake of soy had a statistically-significant reduction in prostate cancer risk, compared to the men who consumed the least soy (3).

Testosterone levels

A 2010 review on the topic found that neither isoflavone supplements nor soy products affected total or free testosterone levels, or estrogen levels in men (4).

Excessive consumption of soy products in men and feminising effects

I was able to find two journal articles describing feminising effects relating to excessive soy consumption in men.

The first was a case report of a 60 year old man, who had been habitually consuming three quarts (2,840mL) of soy milk every day, which is just over 11 cups. The man developed gynaecomastia, or growth of breast tissue, which resolved on discontinuation of the soy milk consumption (5).

The second case was a 19 year old vegan man who was consuming large quantities of soy products (>14 serves) on a daily basis, which resulted in hormonal dysfunction, loss of libido, and erectile dysfunction, which resolved following discontinuation of his extremely high soy, vegan diet (6).

How much is safe to have each day?

1 serve of a soy product contains around 7g of protein and 25mg of isoflavones, and is equal to:

100g firm tofu
50g tempeh
1 cup soy milk

For reference, the mean (average) soy product consumption in Japan is equal to 1-2 servings per day (30-50mg isoflavones) (7). This level of intake is described as a 'moderate' intake.

In the interest of dietary variety, I would advise against the consumption of more than four servings of soy products per day.

My tips for healthy soy consumption

Choose whole food soy products such as tofu, tempeh, and soy milk made from whole soybeans (rather than from soy protein or soy protein isolate)
Avoid highly-refined soy products such as Textured Vegetable Protein (also known as TVP, which is a product made from defatted soy flour). Choose whole legumes (beans, peas, chickpeas and lentils) as a protein-rich option with main meals instead
Avoid soy protein bars, powders and other products in the interests of eating a whole food, plant-based diet
If you have elevated cholesterol levels, include three servings of soy products daily - this is a well-studied intervention for reducing cholesterol levels
If you'd prefer to be conservative with your soy product intake, don't exceed 1-2 servings per day (known as a 'moderate' intake, and in line with typical intakes in Asian countries such as Japan)

References

1. Soy Research | American Institute for Cancer Research (AICR) [Internet]. [cited 2018 Feb 28]. Available from: http://www.aicr.org/foods-that-fight-cancer/tab-content/soy-research-1.html?referrer=https://www.google.com/

2. Baglia ML, Zheng W, Li H, Yang G, Gao J, Gao Y-T, et al. The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status: Soy intake and breast cancer risk by receptor status. International Journal of Cancer. 2016 Aug 15;139(4):742–8.

3. Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr. 2009 Apr 1;89(4):1155–63.

4. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010 May;93(7):2095–104.

5. Martinez J, Lewi J. An Unusual Case of Gynecomastia Associated with Soy Product Consumption. Endocrine Practice. 2008 May 1;14(4):415–8.

6. Siepmann T, Roofeh J, Kiefer FW, Edelson DG. Hypogonadism and erectile dysfunction associated with soy product consumption. Nutrition; Kidlington. 2011 Jul;27(7/8):859–62.

7. Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016 Nov 24;8(12):754