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Should we include soy in our diet?

Soy is a plant-based food deriving from soya beans, which are a legume native to Asia. It is consumed in many different forms, including tofu, tempeh, soy milk, edamame beans, vegetarian meat substitutes, dairy-free cheeses and yoghurts.

It is nutrient dense, rich in polyunsaturated fats (omega-3 and –6), antioxidants, B vitamins and iron. Soy is also a great source of protein containing all of the essential amino acids, making it a complete protein. Soya beans and products using the whole beans (such as tempeh) are also a good source of fibre, which in itself is linked to a whole host of health benefits. 

So why does such a nutritious and versatile plant-based food have such a bad rep? 

There is a lot of misinformation around soy, as it is deemed one of the most controversial topics within nutrition. However, most of the negativity around soy seems to stem from poor studies conducted on animals, providing very weak evidence.  

So, let’s have a look at some of the common misconceptions of soya and see what the evidence has to say…. 

Eating soy can cause cancer 

The common myth that consumption of soy is linked to cancer originates from the misunderstanding of differences between oestrogen and isoflavones.  

Isoflavones are a type of plant-based compounds called phytoestrogens. Although they have a similar chemical structure, isoflavones function completely differently to oestrogen. In fact, phytoestrogens are estimated to be between 100 – 100,000 times weaker than oestrogen found in humans, and therefore any effect they have is very weak. 

However, by binding to oestrogen receptors, the isoflavones in soy act as antioxidants, as they block the oestrogen. There is plenty of research to show the protective effect this has against cancers and other diseases. In fact, human studies have shown consistently that regular, moderate soya consumption lowers the risk of not just breast cancer, but breast cancer reoccurrence in recovered patients.  

The reason for this misunderstanding is due to previous studies done on rodents. It has since been discovered that rodents metabolize isoflavones in a completely different way to humans, leading to the misleading conclusion that isoflavones promote the growth of breast cancer. 

Eating soy causes hormone imbalance in men 

Following on from the concern around phytoestrogen contained in soy, there have been multiple different myths and rumours regarding its effect on testosterone, and whether eating soy will cause ‘man boobs’. 

There is a strong evidence base that soy does not affect the production of testosterone in men whatsoever. There are several studies examining soy protein or isoflavone supplementation that suggest no significant changes on men’s testosterone, oestrogen, sex hormone binding globulin protein, or semen quality. 

In fact, there is good evidence to suggest that soy is linked to a significantly lower risk of developing prostate cancer. 

As for the ‘man boobs’, this rumour stems from a single, very scientifically weak, case study, in which a 60-year-old man developed breasts and sexual dysfunctions after consuming almost 3 litres of soy milk a day for 6 months. Not only is this an unrealistic amount of soy for anyone to consume, the man in question’s medical history is unknown, and his symptoms went away after he stopped consuming the soy. These findings have also not been reproduced in any studies since. 

Soy contains antinutrients 

This is based on the fact that soybeans contain high concentrations of phytate (phytic acid). Phytate is found in seeds, nuts, legumes, and grains. It can bind strongly to certain nutrients (such as iron and zinc), forming insoluble complexes that cannot be absorbed by the intestine. 

However, soaking, sprouting, cooking and fermenting are all ways to reduce the phytate content in soy, which it almost always is before consumed, meaning the effect of phytates is negligible. 

There are good and bad types of soy 

Soy can range from being minimally processed (such as edamame), moderately processed (tofu and soymilk), to isolated components that are used as ingredients (such as soy protein isolate, or soy fibre).  

The more processed the soy is, the less nutrients it contains as they are lost along the way. However, this does not make them ‘unhealthy’, they are still a great, low-fat source of complete protein. They just simply contain a little less nutrients. Furthermore, these more processed forms of soy are generally not consumed on their own, but as an ingredient, meaning the nutrients can be made up elsewhere. 

The bottom line 

Soy is a highly nutritious and versatile plant-based protein, and can be enjoyed by all, not just vegetarians and vegans. It comes with a whole host of health benefits when consumed moderately, and there is no need to be weary of the negative health claims that have derived from poorly conducted animal studies, that are not generalisable to humans. 



Contribution by Rebecca Horton ANutr