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The Myths About Soy

The Myths About Soy

If you have any interest in health food and nutrition, you’ll know that soy consumption has been a hot topic of debate for quite some time, leading to various myths and misconceptions, both for human and animal consumption.

However, despite the misinformation and misinterpretation of research, soy is in fact a versatile and nutritious food with various health benefits. Soybean contains the highest concentrations and best combination of many of the essential amino acids and is seen as the best quality vegetable protein available for horses. Not only do soybeans have a superior amino acid profile, different parts can be used to provide protein, energy, and fibre. Soy can be added to feed in different ways, including soybean meal, soy oil, and soy hulls.

With help from our inhouse scientists and veterinarians, we’ve looked at some of the common myths surrounding the use of soy in horse feed to help you make an informed choice when selecting feeds and supplements. 

Myth: Phytoestrogens cause strange behaviour  

Whilst it is true that soybeans contain phytoestrogens (chemical compounds which can mimic the actions of natural estrogens found in all mammals, including horses and humans), there is a catch. The phytoestrogens found in soybeans are called isoflavones and these isoflavones are very weak estrogen-like compounds. In other words, while there may be many soy isoflavones in a horse’s feed, their impact is very small.

Research has also indicated that not all isoflavones are the same, and one particular type called coumestrol, exerts a much more powerful effect on animals than the varieties found in soybeans. Coumestrol is commonly found in Lucerne/Alfalfa (one of the more common horse feeds) and has a good likelihood of causing estrogenic or anti-estrogenic activities in horses, more so than soybeans. 

Therefore, it is a myth that the phytoestrogen levels found in soybeans will cause strange behaviour or even infertility in horses.

Myth: Soy causes muscle wastage 

Another problem often talked about in soybeans is the presence of an anti-nutrient compound called “trypsin-inhibiting factor” (or TIF). Trypsin is a natural enzyme that helps break protein down into individual amino acids so they can be absorbed and used to build muscle. Some plants, including soybean, contain TIF which, as the name suggests, inhibits the activation of Trypsin in the small intestine. As a result, the protein in the diet isn’t completely broken down, meaning much of the amino acid content in a horse’s feed isn’t absorbed, resulting in low protein levels. This problem can easily be fixed by destroying the TIF by heat in the production process, essentially roasting the soybeans. 

All soy ingredients in EquaCare products have been heat-treated correctly to negate these TIFs and ensure your horse is getting all the nutrients they need from their feed.  

Myth: Soy causes allergies and digestive issues 

Food allergies are thought to be uncommon in horses [1], however if you do suspect your horse may have sensitivities or allergies towards soy, your veterinarian will be able to determine this through testing and elimination diets. 

Soybeans are lactose free, which reduces digestive issues, as all horses are lactose-intolerant. Roasted soybeans (like those prepared to destroy TIF, as mentioned above) are also more easily digestible. 

Research shows that soy may also have anti-inflammatory properties. In other species, soy intake has been shown to improve systemic inflammation [2] and intestinal inflammation, [3] and reduce the negative effects of inflammation on bone and cardiovascular health [4].  Extracts from soy and other plants have been shown to decrease inflammation [5] and improve cartilage health in horses with osteoarthritis. [6]

Whilst soy is not the only source of plant protein available, its high crude protein content of around 44 - 48% makes it an excellent choice for horses with elevated protein requirements. 

So, as you can tell, it’s easy to see why soy is one of our favourite protein sources for EquaCare products as a safe and beneficial source of the nutrients your horse needs to stay in peak physical condition and health. 


  1. Logas, D.Food Allergy in the Horse: A Dermatologist’s View. Advances in Equine Nutrition. 2009.
  2. Fanti, P, et al. Positive effect of dietary soy in ESRD patients with systemic inflammation—correlation between blood levels of the soy isoflavones and the acute-phase reactants. Nephrology Dialysis Transplantation. 2006.
  3. Basson, AR, et al. Regulation of Intestinal Inflammation by Soybean and Soy-Derived Compounds. Foods. 2021.
  4. Droke, EA, et al. Soy isoflavones avert chronic inflammation-induced bone loss and vascular disease. J Inflamm. 2007
  5. Ownby, SL, et al. Expression of pro-inflammatory mediators is inhibited by an avocado/soybean unsaponifiables and epigallocatechin gallate combination. J Inflamm. 2014.
  6. Kawcak, CE, et al. Evaluation of avocado and soybean unsaponifiable extracts for treatment of horses with experimentally induced osteoarthritis. Am J Vet Res. 1998.
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