Is Soy Safe? Ask a Plant-Based Dietitian

January 4, 2023

“Is soy safe?” is without a doubt one of the most common questions we get from clients, particularly as plant-based dietitians. 

From causing breast cancer and wreaking havoc on your hormones, soy has been getting a lot of bad press over the years. But are any of these claims backed by evidence? 

To clear things up and give you some peace of mind, this blog post will take you through what the science actually says about soy and how it affects our health. 



What is soy and where do we find it? 

First, let’s start with discussing what soy actually is. Soy foods are products that have been made from soybeans. Soy is a high-quality plant-based source of protein, making it a popular option for those following a vegan or vegetarian diet. Not only do soybeans come packed with protein, but they also contain other important nutrients such as calcium, iron, zinc and fibre (1)


What foods contain soy? 

The most wholefood source of soy in the diet is soybeans and edamame, which are young soybeans. 

But this isn’t the only way we consume soy. Other soy-based products include:

  • Tofu 
  • Tempeh 
  • Miso 
  • Textured vegetable protein (TVP) 
  • Soy milk 
  • Soy sauce 
  • Some mock meats are soy based 
  • Soy protein powder and protein bars 
  • Soybean oil – often included in vegetable oils 
  • Soy protein powder 

These are the most well-known sources of soy, but soy protein is also hidden in a lot of common packaged foods as a cheap and convenient way to improve the shelf life, texture and protein content.



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Where does the controversy surrounding soy come from? 

Essentially, much of the debate surrounding the negative health effects of soy comes down to a component it contains called isoflavones. 


What are isoflavones and why do we care about them? 

Isoflavones are a type of phytoestrogen found in plants. Phytoestrogens are essentially plant-based chemicals that have a similar structure to oestrogen, which is a hormone our bodies (male and female) produce. 

Isoflavones are found in other foods such as many seeds, grains and legumes, but soy products are the richest source. 

Isoflavones have a similar structure to oestrogen, which means they can bind to oestrogen receptors in the body and exert oestrogen-like effects. This is where a lot of the controversy stems from, as many people believe that consuming soy will mimic the effects of having too much oestrogen in the body and disrupt normal hormonal balance (1). 

However, the effects of isoflavones are not that simple. In fact, sometimes they can even exert opposite effects to oestrogen (1)



So, what does the science actually say about soy and health? 

The impact of soy on our health has been a large area of research for over 25 years, and can be broken down into a few main topics (1)


Soy and breast cancer risk 

The concern between soy and breast cancer stems from the idea that as breast cancer is strongly associated with oestrogen, the phytoestrogens contained in soy products will increase your risk of developing it. 

However, the research tells a very different story. The Shanghai Women’s Health Study is currently the most in-depth study available on soy and breast cancer. It followed 70 000 Chinese women for just over 7 years and its findings were very much in favour of soy. 

In fact, women with high soy intakes had 59% reduced risk of breast cancer compared to those with low soy intake (2)

When it comes to women who have already been diagnosed with cancer, the research remains

positive for soy. Another large study in Shanghai that followed 5 000 breast cancer survivors over numerous years concluded that soy food consumption was significantly associated with decreased risk of breast cancer death and recurrence (3)

In terms of soy and cancer risk more broadly, outcomes are quite mixed. However, a review of the available research found that consumption of soy products, such as soymilk and tofu are either protective against, or not associated with, cancer risk (4). 

Although the research provides promising evidence that soy intake may be protective against breast cancer, the current consensus from the Cancer Council is that there is still insufficient evidence to make a recommendation on consuming soy foods for cancer prevention. However, by no means should these foods be avoided (5)! 



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Soy and heart health 

Moving onto heart health, research suggests that soy products may reduce LDL cholesterol (the bad cholesterol) in those with high levels. 

This is particularly true when soy foods such as tofu, tempeh, edamame or soy milk are being used in replacement of other protein-rich foods high in saturated fats. Saturated fats can increase cholesterol levels and are found in products such as full-cream dairy, coconut as well as red and processed meats. 

Research suggests that a few other compounds contained in soy may also contribute to their cholesterol-lowering effect, but there is insufficient data for any conclusive links (6). 


Soy and menopause 

There is some promising research suggesting that the phytoestrogens in soy products may also have some benefits during menopause. In particular, they may help with reducing the frequency of hot flushes (7). 


Soy, thyroid health and hypothyroidism 

In general, soy does not appear to impact your thyroid negatively nor positively. However, if you have hypothyroidism (a condition characterised by low thyroid function), you do need to be a bit more conscious of your soy intake – particularly the timing of it. 

This isn’t because soy is directly harmful to the thyroid, but rather because soy may interfere with the body’s ability to absorb levothyroxine which is a common medication used in hypothyroidism (8)

Luckily, this doesn’t mean you need to stay away from soy products completely! Instead, it is recommended you just need to avoid consuming soy products within a 3-4 hour window of your medication (9).


Does soy cause feminisation in men? 

The effect of soy on testosterone levels, sperm quality, and the potential development of ‘man boobs’ has long since been a big topic of debate. However, if we look at the research, there is pretty good evidence that soy intake doesn’t affect any of these things (10)

Most of this speculation sparked after one case where a 60-year-old man experienced a collection of side effects such as breast tenderness, elevated oestrogen levels, erectile dysfunction, and decreased libido after consuming a diet rich in soy. 

However, a big point that is often left out when this study is cited, is that he was drinking almost three litres of soy milk per day which even the authors agree is very out of the realm of normal consumption (11)

So as long as your consuming a few soy serves of soy per day, it is very unlikely any of these side-effects will start to appear! But more on how much soy is safe later. 



Other benefits of soy 

To keep on with the many benefits of soy foods, there are a few other notable things that soy can add to your health that we didn’t want to leave out. 

Soy products such as tofu, tempeh, edamame, soy milk and TVP are the highest and best quality sources of protein for those following a plant-based diet (or even just looking to eat less meat). 

They are cheap, convenient and so versatile to use too (we’ve added a few tips later on). 

Soy foods are also rich in other essential nutrients such as iron, zinc and calcium all of which can be a bit tricky to get enough on a plant-based diet. 

Soy milk and soy yoghurt make great dairy-free alternatives rich in both protein and calcium (when fortified)
But make sure your soy milk is calcium fortified! – Look for 120mg of calcium per 100mL on the nutrition panel! 

Fermented soy products such as miso and tempeh are also great for promoting a healthy gut!



How much soy is safe to eat per day? 

The current Australian Dietary Guidelines promote soy as a safe and healthy addition to the diet (12).

In terms of how much is safe to consume, in order to reap the health benefits of soy, the current consensus from the available research suggests including about 2-4 serves per day (13). However, in order to get the most benefit from your soy products, as dietitians we recommend opting mostly for the lesser processed varieties of soy products. These include tofu, tempeh, edamame, miso and soy milk over more heavily processed items such as mock meats. 

Whilst these foods can still fit into a healthy diet, they are higher in saturated fats and sodium which negate some of the health benefits associated with soy such as reducing cholesterol levels. 

One serve of soy is about: 

  • 1 cup of soy milk 
  • 100g-150g of tofu or tempeh 
  • 1 cup of edamame 
  • 50g dry textured vegetable protein (TVP) 
  • 30g soy protein powder 

It should be noted that there are some people who are allergic to soy. In these cases, no amount of soy is safe and we recommend eliminating it from the diet completely. 



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How to include soy in the diet 

As we mentioned previously, soy products are some of the most versatile plant-based protein options and can be enjoyed in a myriad of ways. Here’s a few of our favourite ways to add soy foods into your diet! 

  • Swap out your current dairy free milk of choice for soy milk for added protein (TIP: we recommend choosing a calcium fortified variety – look for one with at least 120mg of calcium per 100mL on the nutrition label) 
  • Use TVP in replacement of mince for a protein rich plant-based meatball, Bolognese sauce or taco filling 
  • Add marinated tofu or tempeh into wraps and sandwiches 
  • Try a tofu stir fry or curry 
  • Snack on cold or roasted edamame 
  • Crispy baked tofu is great as a side or in a salad 
  • Blend tofu into soups or smoothies for extra protein and creaminess 
  • Swap eggs for a tofu scramble 
  • A tofu-based mouse or cheesecake make great plant-based dessert options
  • Tofu and miso paste can also be used to make ricotta



The bottom line 

Soy has been given a pretty unfair wrap by the media over recent years. However, most of this controversy can be dispelled when we look at the actual science behind these claims. 

In short, most soy foods such as tofu and soy milk are great additions to the diet, and are packed full of plant-based protein, fibre and other important nutrients. 

Unless you have a soy allergy, soy is both safe and healthy. Plus, research suggests it may even have some pretty positive health benefits such as decreasing your risk of heart disease and certain cancers. 

If you’re still concerned about how much soy is healthy for you or want more tips on ways to add more soy foods into your diet, we recommend booking in with a plant-based dietitian such as one at The PNW clinic. You can learn more about our dietitians and book in for a consult or a free discovery call here!


Article written by: PNW dietitian Georgia D’Andrea

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