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Soy Protein side effects and myths


Soy protein – friend or foe?

The benefits and shortcomings of soy protein seem to have foodies, nutrition bloggers and scientists at loggerheads. Is it a super food or a food to avoid? Unsubstantiated scientific claims have led to myths about soy, clouding perceptions of this protein. Before making your own mind up about soy, consider the following debunking of 3 common myths.

What is soy?

Soy protein is a complete dietary protein, made from soybeans that are often used in tofu and various meat substitutes.

Myth Busting

Myth 1 - Men shouldn't eat soy

Some believe that the intake of soy can cause hormone imbalances and feminising changes to a man’s body, including the not so desirable ‘man boobs’. This myth has evolved due to the content of oestrogens in soy, a hormone mostly found in women that plays a major role in regulating sexual development, and the belief that soy protein can reduce testosterone levels. Although soy does contain naturally occurring plant oestrogens called isoflavones, scientific research suggests that isoflavones do not act in the same way as hormonal oestrogen, and do not affect oestrogen¹ or testosterone levels in healthy men2.

Myth 2 - Soy is bad for my health

On the contrary, soy can contribute to our overall nutritional well-being in a number of ways. Soy is a good source of protein, which is essential for our muscles to grow and rebuild, and contains minerals such as iron to help us produce red blood cells to carry oxygen around our bodies, and magnesium to help us to turn the food we eat into energy. Soy is low in total fat and in particular low in saturated fat, the ‘bad’ fat which can raise cholesterol levels in our blood, and should therefore be minimised to protect our health. There is some concern that soy has been suggested to increase the risk of breast cancer, however there are currently no known human studies to support this claim.

Myth 3 - Soy won't help me build muscle

Soy is known as a ‘complete protein’, meaning it provides all of the essential building blocks of proteins (amino acids), that we can’t make in our own bodies, including the conditionally essential amino acids Arginine and Glutamine. Conditionally essential meaning that the amount of these amino acids in our system falls during periods of heavy training, and therefore we need to top up our stores via our diet during this time. The protein content of soy will contribute towards your daily protein requirements, and help to support muscle development.

Is it all about soy?

When it comes to the question; which type of protein should we eat to optimally support our hard work in the gym? We don’t have to rely on eating one type of protein. We can combine our intake of fast, intermediate and slow release proteins to build new proteins in our body, via muscle protein synthesis. Whey protein is a rapidly digested protein that releases its amino acids relatively quickly into the bloodstream, which are then ready to use to make new proteins as little as 30 – 60 minutes post ingestion. Casein on the other hand is a slowly digested protein, releasing amino acids steadily over a longer period of time. Soy protein is digested at a slower rate than whey, but faster than casein, meaning it bridges the appearance of amino acids into the blood stream between these two proteins. Simply, the combined ingestion of these proteins can help provide the sustained release of amino acids needed to build new proteins, and thus help to support muscle development.

To sum up, many scientists and nutritional health experts strongly support soy protein as a great option to include as part of your diet. Soy protein side effects are unsubstantiated myths and, although the total amount of protein per 100 g is lower than other sources of animal proteins (such as whey or casein), when consumed in combination with other proteins, it provides the perfect accompaniment to any active individuals’ or vegetarian diet.

Learn more about how protein can support your training ‘Guide to Whey Protein’


1. Allen NE, Appleby PN, Davey GK, Key TJ. (2001). Soy milk intake in relation to serum sex hormone levels in British men. Nutr Cancer 41(1-2):41-6.

2. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ (2010). Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril 94:997–1007.