Soy: The Good, Bad and Confusing!

If there’s any horse feed that can cause heated debate about whether or not it is suitable and even valuable for horses- then soy may well take the cake.

For the purpose of this article, we will focus on soy bean meal and feeds containing processed soy- not soy oil or soy bean hulls- as these components alone have significantly different nutritional value and properties.


Soy is one of the more widely fed significant sources of protein in a horse’s diet. It is a common misconception that protein is only needed to promote weight gain, muscle condition or the development of topline. Whilst it is important in these instances (in combination with appropriate exercise and training), protein is absolutely essential for a multitude of functions in all horses at any life stage and exercise level: it is a critical component of normal immune function; facilitates proper growth and repair of the musculoskeletal system; and is needed for the majority of metabolic processes in the horse. Even horse’s who aren’t in heavy work and/or actually need to lose weight need sufficient protein in their diet.

Protein is comprised of smaller units called amino acids. Horses require 21 amino acids, 12 of which a horse can synthesise (produce) themselves. The remaining nine (and in the case of illness or injury- potentially ten) amino acids must be present in a horse’s diet. These are known as ‘essential amino acids’. In addition to all essential amino acids being present in the diet, they must be present in appropriate quantities- so a horse has enough of each of them to facilitate various bodily functions. If they fall short of any essential amino acids then health, normal development, performance and recovery may be compromised.

With the exception of oils, practically all horse feeds, grains and forages contain SOME protein- Depending on the exact feed source, the actual amounts of protein contained in these feeds will vary. Different sources of protein are comprised of different sequences of amino acids. This influences how useful they are to a horse. Some high protein feed sources are actually of little value to a horse, as they lack decent amounts of essential amino acids that a horse needs, or they are unable to be properly digested and absorbed by a horse.

So how does all this relate to soy? Soy is one of the best dietary sources of lysine for our horses and ponies. Lysine is a very important essential amino acid in the equine and it is generally one which is lowest in our horse’s diets. Lysine is absolutely essential for normal muscle growth and turnover, helps to promote collagen integrity (important for healthy bones, skin, muscles, tendons and ligaments), is critical for proper immune function, and may further help bone health through improving calcium absorption and utilisation. Animal studies have also shown that diets low in lysine may be associated with anxiety. Thus, a diet low in lysine may be one of many potential contributing factors in some cases of horses with an anxious tendency.

For horses competing in endurance events, optimising hydration pre-event is an important consideration. Commercial soy feed products which require soaking before being fed, can help to increase fluid intake in horses. It’s important to keep in mind that there are other feeds which can be used for this same purpose. Soy is not the only option.

Being a low starch and protein rich feed, processed soy meal can aid in stabilising blood glucose levels- thus helping to prevent rapid spikes in insulin levels. High insulin levels are a driving factor of equine metabolic syndrome and are commonly implicated in many cases of laminitis. Horses with Pituitary Pars Intermedia Dysfunction (PPID) (also commonly referred to as ‘Cushings’) and Polysaccharide Storage Myopathy (PSSM)- a component in many cases of ‘tying up’, may also particularly benefit from a diet low in starch and containing adequate quantities of quality protein.

The phytoestrogen content of soy is an important, yet often misunderstood concept. Soy is unique in that it contains a high concentration of isoflavones, a type of plant oestrogen (phytoestrogen) that is similar in function to the hormone oestrogen but with much weaker effects. Soy isoflavones can bind to oestrogen receptors in the body and can either increase or decrease oestrogen levels in the body. Research has demonstrated that when oestrogen levels in the body are naturally low, then ingestion of phytoestrogens may help to increase the body’s levels.  When oestrogen levels in the body are naturally high, then consuming phytoestrogens may help to reduce oestrogen levels, as they bind to some of the receptor sites that naturally produced oestrogen may bind to.

Potential Cons

The phytoestrogen activity of soy may also potentially be a drawback of this feed. Excessively high intakes of phytoestrogens have the potential to decrease testosterone levels, an important consideration in stallions and colts. Soy certainly isn’t the only source of phytoestrogens commonly found in horse diets: clover grass is rich in phytoestrogens. Before attributing potential positive or negative effects of phytoestrogens in a horse’s diet, the total quantity of all phytoestrogen rich food sources needs to be taken into account.

Soy is commonly attributed to causing allergic symptoms in horses- everything from itchiness and skin irritation, colic and digestive disturbances, to overly-reactive behaviour and respiratory issues. Whilst soy is a common allergen in humans, the same has not yet been demonstrated in horses. Diagnostic tests for equine food allergies are difficult to carry out, time-consuming and often unreliable. In clinical practice, I find that the majority of horses with allergy related symptoms (whether these be environmental or food related) improve significantly when overall nutritional status and specific components of both immune and gut health have been therapeutically addressed. Generally something which may appear to cause allergic symptoms in a horse is not the real culprit.

A significant valid concern with soy is the fact that it is one of the most commonly genetically modified (GM) crops. Whilst this is only an issue in some parts of the world, it is certainly worth taking into consideration. Animal studies with GM foods indicate that they may cause some common toxic effects such as hepatic, pancreatic, renal, or reproductive effects and may alter blood parameters and immune function.

Soy is high in omega six fatty acids. Whilst these types of fatty acids are an essential component in our horse’s diets, they are generally present in higher than desirable amounts. Of further importance is the percentage of omega six fatty acids in proportion to omega three fatty acids. High levels of omega six fatty acids with accompanying low levels of omega three fatty acids are known to promote inflammation. Adding large amounts of soy products to a diet already high in omega six fatty acids further accentuates the shift towards a pro-inflammatory tendency. This can  have wide-ranging effects from impairing recovery from training, competition and exercise; compromising immune function; and exacerbating already existing inflammatory conditions such as osteoarthritis, desmitis, placentitis and laminitis to name a few.

Uncooked or even inadequately cooked soybean meal has some anti-nutritional factors. Trypsin inhibitor is the most significant one. Trypsin is an essential enzyme which helps horses to properly digest protein, allowing for absorption and metabolism of the amino acids which make up the protein. Trypsin inhibitor found in uncooked soybean prevents trypsin from being able to properly break down proteins. This means that in the presence of trypsin inhibitors a large percentage of all protein from all sources a horse is consuming is unable to be properly digested, absorbed and metabolised.

Some concerns exist that soy can have a negative effect on thyroid function and can alter the levels of thyroid hormones. Whilst the majority of data is from human studies, it is unlikely that regular consumption of small amounts of soy containing foods have significant impact on thyroid function in healthy subjects. In cases where hypothyroidism already exists (keeping in mind that diagnosis and true cases of hypothyroidism in horses is rare), then reviewing the appropriate feeding of soybean containing products is warranted.


Processed soybean meal and feeds containing this product can be a valuable inclusion in the diets of many horses. Factors which may caution against the feeding of soy to some horses include specific health concerns and where in the world the soy is grown and produced. Soybean meal MUST be properly heat treated to prevent trypsin inhibitor from interfering with protein digestion. The quantity of soy products fed and the overall diet of the horse are crucial factors to take into consideration.

Rarely is anything inherently good or bad: the same may be said of soybean products. The most important consideration is that one size doesn’t fit all: feed the individual horse in front of you!

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