The Science of Soy – Episode 118

With the recent explosion of plant-based diets, there has been a resurgence of people consuming soy as a main protein source.  In the episode, we explore what soy is, what the research says regarding health and hormones, and what the upper limit of intake should be.

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With the recent explosion of plant-based diets, there has been a resurgence of people consuming soy as a main protein source.  In the episode, we explore what soy is, what the research says regarding health and hormones, and what the upper limit of intake should be.  

What is Soy?

Soy is found in many, many processed foods (such as tofu and Soymilk). It should be no surprise to learn that the soy is isolated from the soybean. Fairly simple and straightforward.

Soy + Phytogestrogens

Soy naturally contains isoflavones. When consumed, isoflavones are converted into phytoestrogens, such as Biochanin A, Clycitein, Daidzein, Formononetin, and Genistein (1-2). Phytoestrogens are structurally similar to the estrogen our bodies make, and can weakly bind estrogen receptors (3). When bound to an estrogen receptor, phytoestrogens can either have a pro-estrogen or an anti-estrogen effect in a specific muscle, tissue, or organ (3). 

Think of this this way: phytoestrogens fight estrogen for their receptors (even though they are very weak in comparison). Fight-o-estrogens, got it? Okay. Moving right along.

Brief Introduction to Estrogen and Estrogen Receptors (ERs) (4)

Estrogen is the primary female sex hormone, and is important for reproductive and non-reproductive health. In pre-menopausal women, circulating estrogen (estrogen found in the blood) is synthesized in the ovaries, adrenal glands, fat cells, and the placenta in pregnant women. 

There are three main forms of estrogen: estron, estradiol and estriol. Estradiol is the most prominent form of estrogen in premenopausal women, whereas estriol plays a major role after menopause. Estradiol (which we will refer to as estrogen for the remainder of the article) is made in the body in a phase-specific manner during a woman’s menstrual cycle, with peak concentrations found in the ovulatory phase and lower concentrations in the follicular (before ovulation occurs) and luteal (after ovulation occurs) phases.

So how does estrogen affect the body? It acts through binding receptors called estrogen receptors (ERs). ERs come in two forms: ER-alpha(ɑ) and ER-beta(β). Depending on what tissue or organ you are looking at, there will likely be different amounts of ER-alpha and ER-beta present. For example, ER-alpha can be found in organs such as the liver and kidney, whereas ER-beta can be found in other systems such as blood vessels and bone marrow. 

When estrogen binds to either ER-alpha or ER-beta, changes occur in the body that are important to maintaining menstrual, cardiovascular, and bone health.  This is imporatant becasue reductions in levels of estrogen increases the risk of bone fractures, cardiovascular disease, and Alzheimer’s in postmenopausal women (5 – 8).

Phytoestrogens & Estrogen Signaling

Remember, phytoestrogens fight estrogen, meaning they bind ERs, but more specifically, they preferentially bind ER-beta. This doesn’t mean they don’t bind ER-alpha, it simply means they have a preference for ER-beta. Phytoestrogens can weakly bind ER-beta, and therefore have a much lesser effect than estrogen(9). 

Some phytoestrogens have a pro-estrogenic effect and some have an anti-estrogenic effect. This means that some phytoestrogens have an effect similar to estrogen or have an opposite to estrogen. Complicated, right? And to make things even more interesting/confusing, researchers have discovered that phytoestrogens can actually have a heart-protective and bone-protective effect (9). Since there is a higher concentration of ER-beta in the bone marrow and blood vessels, phytoestrogens likely can enhance the protective effects of estrogen.

Soy and Menstrual Health (10)

Remember, the normal fluctuation of estrogen is as follows: 

• High levels a few days before ovulation
• Levels fall, but are not reduced to zero, after ovulation
• Levels increase again right before menstruation

If consumed in high doses, phytoestrogens can likely lead to menstrual dysfunction as they directly compete with estrogen (one of the main components of menstrual cycles). But in moderate to low doses, soy/phytoestrogens can actually benefit period health by reducing the effects of estrogen, and will therefore let progesterone (the other vital hormone in menstrual health) shine, assuming you do not have low estrogen to begin with.

If we had a preference, we want more progesterone. When issues occur with our periods or we are taking hormonal birth control, we lose progesterone, and progesterone is beneficial for so many things including mental health and muscle building! (10)

Soy Consumption Recommendations

So the lingering question still remains: how much soy is too much soy?

For that, let’s take a step back. What is a serving of soy? The general consensus seems to be that 1 cup of Soymilk, ½ cup tofu or tempeh equals about 8-10 grams of soy.

In a 2 year study, people were given 3 servings (or 50mg of isoflavone and 5-22g of soy) per day for 2 years. Researchers found no increases in certain markers or cancer (11).

Another study assessed the risk of certain cancers in participants consuming 58g of soy per day (5-10 servings) and found that after 1 year, certain cancer-related proteins were elevated in participants (12). This study along with another study on the effects of soy agree that too much soy (7-18 servings or 70-180g of soy) negated the positive benefits of soy (13-14).

Summary

From the research, we can conclude that soy is not inherently bad for you. In fact, moderate to low doses of soy actually can have positive benefits to overall health and longevity.

The issue with soy is two-fold: 1) eating more the 5-7 servings per day and 2) eating highly processed foods containing soy. Now the second issue is not primarily because the food has soy, but because a lot of highly processed food has soy in it. So we could be eating a lot more soy than we realize when we consume processed foods. 

So to keep is brief, eat your soy, but keep your servings to less than 5 per day (or 45g of soy max) and try to eat mainly whole, unprocessed foods.

If you missed last week’s episode, you can listen to it HERE.

Resources: 

1. Moutsatsou, P. The spectrum of phytoestrogens in nature: our knowledge is expanding (2007). HORMONES, 6(3):173-193 https://www.ncbi.nlm.nih.gov/pubmed/17724002
2.  Patisaul, H.B. and Jefferson, W. The pros and cons of phytoestrogens (2010). Front Neuroendocrinol (4): 400–419.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3074428/
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3074428/
4. Cui,J., Shen, Y., and Li, R. Estrogen synthesis and signaling pathways during ageing: from periphery to brain (2013). Trends Mol Med. 19(3): 197–209.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3595330/
5. Ghosh S,and Thakur MK. Interaction of estrogen receptor-alpha transactivation domain with nuclear proteins of mouse brain: p68 RNA helicase shows age- and sex-specific change (2009). J Neurosci Res.87(6):1323-8. https://www.ncbi.nlm.nih.gov/pubmed/19025768/
6. Novensà L, Novella S, Medina P, Segarra G, Castillo N, Heras M, Hermenegildo C, and Dantas AP. Aging negatively affects estrogens-mediated effects on nitric oxide bioavailability by shifting ERα/ERβ balance in female mice (2011). PLoS One. 2011;6(9):e25335. https://www.ncbi.nlm.nih.gov/pubmed/21966501/
7. Nelson, H.D. Menopause (2008). Lancet. 371(9614):760-70 https://www.ncbi.nlm.nih.gov/pubmed/18313505/
8. Stevenson J.C.; International Consensus Group on HRT and Regulatory Issues. HRT, osteoporosis and regulatory authorities Quis custodiet ipsos custodes? (2006). Hum Reprod. 21(7):1668-71. https://www.ncbi.nlm.nih.gov/pubmed/16556675/
9. Kuiper GG, Lemmen JG, Carlsson B, Corton JC, Safe SH, van der Saag PT, van der Burg B, Gustafsson JA. Interaction of estrogenic chemicals and phytoestrogens with estrogen receptor beta (1998). Endocrinology. 139(10):4252-63. https://www.ncbi.nlm.nih.gov/pubmed/9751507/
10. Briden, Lara (2018). Period repair manual : every woman’s guide to better periods (Revised edition). Pan Macmillan Australia, Sydney, New South Wales
11. Maskarinec G, Takata Y, Murphy SP, Franke AA, and Kaaks R. Insulin-like growth factor-1 and binding protein-3 in a 2-year soya intervention among premenopausal women (2005). Br J Nutr. 94(3):362-7. https://www.ncbi.nlm.nih.gov/pubmed/16176606
12. Dewell A, Weidner G, Sumner MD, Barnard RJ, Marlin RO, Daubenmier JJ, Chi C, Carroll PR, Ornish D. Relationship of dietary protein and soy isoflavones to serum IGF-1 and IGF binding proteins in the Prostate Cancer Lifestyle Trial (2007). Nutr Cancer 58(1):35-42. https://www.ncbi.nlm.nih.gov/pubmed/17571965
13. Frattaroli J, Weidner G, Dnistrian AM, Kemp C, Daubenmier JJ, Marlin RO, Crutchfield L, Yglecias L, Carroll PR, Ornish D. Clinical events in prostate cancer lifestyle trial: results from two years of follow-up (2008). Urology. 72(6):1319-23. https://www.ncbi.nlm.nih.gov/pubmed/18602144
14. Dean Ornish, Mark Jesus M. Magbanua, Gerdi Weidner, Vivian Weinberg, Colleen Kemp, Christopher Green, Michael D. Mattie, Ruth Marlin, Jeff Simko, Katsuto Shinohara, Christopher M. Haqq, Peter R. Carroll. Changes in prostate gene expression in men undergoing an intensive nutrition and lifestyle intervention (2008). Proceedings of the National Academy of Sciences. 105 (24) 8369-8374 https://www.pnas.org/content/105/24/8369