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The Real Truth About Soy

The Real Truth About Soy, Naturopath Kirily Thomas, Soy Myths

Over the last few weeks, I have had several patients asking me about the safety of including soy foods in their diet. They have heard that soy is not that great for you, and can increase the risk of cancer, hence my decision to write this blog and share with you the real truth about soy.

Let me start by saying that there is a lot of misinformation regarding soy on the internet, and most of it comes from unsubstantiated “evidence” that’s recycled on various blog posts. This often creates unnecessary panic, especially when it comes to oestrogen dependent conditions.

Is soy safe to eat?

If you do a proper literature review, and read up on the current evidence, like I commonly do in my work, you’ll see that there’s quite a lot of evidence to the contrary (1,2). In fact, a very recent systematic review and meta-analysis (which is one of the highest levels of evidence) in May 2023 (3) found that there was evidence of a reduction in breast cancer risk with 10mg per day intake of isoflavones (one of the active compounds in soy).

Many people are concerned with the risk of soy due to a mistaken belief about soy increasing oestrogen by binding to oestrogen receptors in the body. However, what is usually completely misunderstood is how oestrogen receptors work, that there is more than one type of oestrogen receptor, and which type of oestrogen receptor the isoflavones, in soy, like genistein and daidzein, bind to.

We all have alpha-oestrogen receptors, which when bound may stimulate proliferative oestrogen action (if conditions are right), however, we also have a second type of oestrogen receptor known as beta-oestrogen receptors that do not exhibit this activity and are antiproliferative. Genistein from soy preferentially binds to BETA-oestrogen receptors and has a much lower affinity for ALPHA-oestrogen receptors (4, 5).

What type of soy is best to eat?

Of course, this doesn’t mean you should drink litres of soy milk a day, as you can overdo anything! However, a balanced intake of TRADITIONAL soy foods is generally okay for most people to include in their diet (there can be exceptions to this regarding certain thyroid conditions), especially if you are perimenopausal and menopausal, as soy isoflavones can help improve cognition (4,6), reduce hot flushes, improve bone density, and reduce risk of diabetes and cardiovascular disease. (4) 

By traditional soy I mean tofu, tempeh, miso, whole soybeans and good quality soy milk. I don’t recommend highly processed soy, soy fillers and soy oil which we see in many highly processed foods, which includes things such as vegan sausages and some vegan meat substitutes. Up to 70% of isoflavones in soy are lost when making highly processed soy products like soy protein isolate (7).

And because I’m a complete gut nerd and can’t help but tie the gut into this!

Did you know our beneficial gut flora are responsible for transforming the phytoestrogens we eat into forms that we can absorb and utilise biologically (8)? Also, if we don’t have healthy populations of beneficial flora to do this job for us then these phytoestrogens may not be absorbed and simply be excreted unutilised into the toilet when we have a bowel motion? So, this is another reason to add to the list as to why it is so important to look after the beneficial flora in our gut microbiota!

I hope this information has helped you understand the nuances of consuming soy. The real truth about soy is that it’s generally okay for most people to consume traditional, unprocessed soy.

Interested in food and hormone health? Read all about flaxseeds and their role in hormone health.

References:

  1. Okekunle AP, Gao J, Wu X, Feng R, Sun C. Higher dietary soy intake appears inversely related to breast cancer risk independent of estrogen receptor breast cancer phenotypes. Heliyon. 2020 Jul 2;6(7):e04228. doi: 10.1016/j.heliyon.2020.e04228. PMID: 32642579; PMCID: PMC7334424.
  2. Ho SC, Yeo W, Goggins W, Kwok C, Cheng A, Chong M, Lee R, Cheung KL. Pre-diagnosis and early post-diagnosis dietary soy isoflavone intake and survival outcomes: A prospective cohort study of early stage breast cancer survivors. Cancer Treat Res Commun. 2021;27:100350. doi: 10.1016/j.ctarc.2021.100350. Epub 2021 Mar 12. PMID: 33770661.
  3. Yang J, Shen H, Mi M, Qin Y. Isoflavone Consumption and Risk of Breast Cancer: An Updated Systematic Review with Meta-Analysis of Observational Studies. Nutrients. 2023 May 21;15(10):2402. doi: 10.3390/nu15102402. PMID: 37242286; PMCID: PMC10224089.
  4. Kim IS. Current Perspectives on the Beneficial Effects of Soybean Isoflavones and Their Metabolites for Humans. Antioxidants (Basel). 2021 Jun 30;10(7):1064. doi: 10.3390/antiox10071064. PMID: 34209224; PMCID: PMC8301030.
  5. Oseni T, Patel R, Pyle J, Jordan VC. Selective estrogen receptor modulators and phytoestrogens. Planta Med. 2008 Oct;74(13):1656-65. doi: 10.1055/s-0028-1088304. Epub 2008 Oct 8. PMID: 18843590; PMCID: PMC2587438.
  6. Cui C, Birru RL, Snitz BE, Ihara M, Kakuta C, Lopresti BJ, Aizenstein HJ, Lopez OL, Mathis CA, Miyamoto Y, Kuller LH, Sekikawa A. Effects of soy isoflavones on cognitive function: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2020 Feb 1;78(2):134-144. doi: 10.1093/nutrit/nuz050. PMID: 31504836; PMCID: PMC7808187.
  7. Messina M. Soy foods, isoflavones, and the health of postmenopausal women. Am J Clin Nutr. 2014 Jul;100 Suppl 1:423S-30S. doi: 10.3945/ajcn.113.071464. Epub 2014 Jun 4. PMID: 24898224.
  8. Kwon C, Ediriweera MK, Kim Cho S. Interplay between Phytochemicals and the Colonic Microbiota. Nutrients. 2023 Apr 20;15(8):1989. doi: 10.3390/nu15081989. PMID: 37111207; PMCID: PMC10145007.
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