The Great Soy Debate: Good or Bad

Is soy evil or healthy? We know vegans love it, we know lots of people hate it but lets actually look at the science.

I must admit this one shocked me a lot in many ways. The differing research, the amount of healthy and harmful claims, the confusion (for myself included) was real! Researching this topic took some time and I am so glad it did and you will see why if you keep reading. 

Okay so when we talk about soy, what are we talking about? 

The most wholefood source of soy would be soy beans (or edamame as you would know it from Japanese restaurants). From this soy bean many products have been created including tofu, tempeh, miso, soy sauce, isolated soy protein, plus a whole bunch of other soy substrates that would barely be recognisable as soy. 

The compound that is usually looked at when it comes to the health risks and benefits of soy are the isoflavones in soy. These compounds are known for their oestrogen-like (a hormone) chemical structure and potentially their oestrogen like role in the body (we will discuss this later in the article).

The Research:

I wanted to point out a few incredibly important key points about SOY science before we delve into it. These are just some of the things I look into when I am looking at the evidence for and against products/food. 

  • Research on soy and soy products has been going on for decades and like all research is ever evolving as our technologies advance in the science world. Meaning what is true today, may not be true in 5, 10 or 20 years.

  • MUCH of the research done on soy and soy products is done in vitro (in a test tube/culture dish or outside any living organism), which obviously makes it almost irrelevant to what happens in the human body. FYI in vitro studies usually take place before human because we don't want to harm humans if we can avoid it, but translating it to humans is very difficult then.

  • The other large portion of the studies are done in animals (rodents and primates). Again, not humans. Isoflavones, that compound of interest in soy, is metabolised VERY differently in animals as it is in humans. Even our closest animal relatives (primates) metabolise isoflavones in a completely different manner to us, making much of the animal studies redundant in their application to human health.

  • The final thing I had to be mindful of was who was sponsoring the research, who was funding it, who has an invested interest in the outcome. Some studies were funded by the soy industry which duh of course they will do their best to make soy look good and some were funded by the dairy industry which obviously want soy to look bad so you drink dairy milk.

  • Much of the research looks at the isolated compounds in soy (isoflavones) rather than the soy products or soy beans that we eat. We must remember that nutrients or compounds consumed in isolation act differently to when they are consumed as a whole food with other nutrients and naturally occurring compounds.

So this is why it takes me a full day to debunk a topic like this, alllll the variables need to be considered. Anyway if I still have your interest, despite the nerd talk let's look at what the evidence says (the evidence I deemed acceptable to consider and was translatable to humans).


The Oestrogen Story: 

Isofavones have a similar chemical structure to the hormone oestrogen which allows them to bind to oestrogren receptors in the body. Many people will avoid soy for this very reason as they are worried that soy and soy products will act like the hormone oestrogen in our bodies. So is this true?

I'll be honest here and when I started doing this research I thought it was true purely because I hadn't reallllly looked at what happens in the human body. So you are not wrong for thinking this because it is true (kind of)...  

  • there are different oestrogen receptors in the body and isoflavones bind to one type of receptor this receptor for the nerds like me is transactivates ER-beta (not important to know the name of this only important to know there are different oestrogen receptors in the body and isoflavones doesn't bind to them all)

  • Because they bind to slightly different receptors than the actual hormone oestrogen, they can have different and sometimes the opposite effects of oestrogen binding. #mindblown

What are the HEALTH facts on SOY: 

Because I know I am going to have people argue with me on this let me again disclaim - this is based on the evidence as it stands at this time, it may change in future as science is ever evolving. This is the facts based on my interpretation of the science taking into consideration all the above points (who funded the research, what soy products were used, was it done in humans ect ect)

  • Soy beans are a great source of vegan protein

  • Soy is lower in carbohydrate and higher in protein than many other vegan sources of protein

  • The type of carbohydrate in soy is one that is not easily digested by humans, thus goes to our large bowel for bacterial fermentation to feed our gut bacteria and help grow our gut ecosytem

  • Soy may reduce LDL cholesterol in those with high LDL cholesterol, particularly when soy products are used in place of high saturated fat animal protein.

  • Soy may reduce the risk of cardiovascular disease

  • Moderate soy consumption may be protective against breast cancer

  • In those breast cancer patients receiving tamoxifen there is no strong evidence that suggests any adverse effects, however the Cancer Council still cautions use and states not to have high dose soy supplements if receiving Tamoxifen breast cancer therapy.

  • Soy does not effect testosterone levels in men and has no effect on sperm concentration or quality

  • Soy may interfere with the absorption of levothyroxine (a drug for hypothyroidism). If you take this drug s of soy t is suggested not to consume high doses of soy protein (e.g. soy based protein powders). Make sure your other soy foods are not consumed within 2-4 hours of taking your medication

  • Soy products do not appear to have any other impacts on the thyroid. The case of Bonsoy a few years back was due to the seweed isolate they used and an excessive iodine intake causing thyroid issues, not the soy.

  • There is no strong evidence that soy can effect endometrial thickness and soy may actually lead to a reduced risk of endometrial cancer.

  • Soy contains phytic acid which is an anti-nutrient (aka it inhibits absorption of other nutrients like calcium/iron ect). This phytic acid is deactivated when soy is cooked or fermented (like miso, natto or tempeh).


Wooaaaah. How's that for some science haha! I am still blown away by what i've read and how different things appear in animals vs humans too. To summarise below this is my suggestion when it comes to soy:

  • As with ALL foods avoid excessive intake (even excessive intake of water can be harmful). Moderation is key. It is recommended 25g soy protein per day, which is about the equivalent of 100g soy beans or tofu is a good amount to consume for health benefits and minimising risks.

  • Cook your soy products to destroy the phytic acid or use fermented soy products

  • AVOID heavily refined and processed soy products and soy compounds. Many products especially vegan faux meat alternatives contain textured soy protein an other processed soy by products.

  • CHOOSE natural soy products - stick to soy beans, tofu, tempeh, miso or soy milk (the most minimally processed one you can find)

  • In America I understand there is a big issue with GMO soy - you are best to choose organic there, and elsewhere organic is probably a better choice too.

  • Consume fermented soy like tempeh, natto and miso - great for the gut! Double bonus.

To be honest I did not think that was going to be the recommendations I would be suggesting before I started doing the research. Previously I would have said caution use a lot more because I hadn't looked into the science and the quality of the evidence. So I am happy to say that soy is NOT AS BAD as what some people make it out to be. 

If you found this article interesting please share it with your family or friends or on social media and leave me a comment below if you have any questions. 

Marika x