The Truth About Soy Allergies

We recently introduced an article that debunks some of the myths about feeding soy to horses. It can be found HERE. The article addresses some concerns owners may have when feeding soy to their horses. One common worry with owners is that their horse may be allergic to soy.

Further, if they discover their horse is, in fact, allergic to soy, coupled with so few soy-free choices out on the market to feed horses, it can be troubling.  

Food allergies are a real concern in horses. A food allergy is described as a condition in which the immune system has a reaction, sometimes severe, to food. A horse’s symptoms of an allergic reaction to food can be:

  • Hives, which are small, swollen raised areas of skin
  • Itchy skin (pruritus), eczema, or general irritation of skin
  • Bloating or colic
  • Respiratory distress, wheezing, swollen tongue
  • Behavioral abnormalities due to feeling unwell
  • Severe reaction leading to anaphylactic shock, organ failure, and potential death

Thankfully, food allergies are rare in horses. Severe allergic reactions even more so.  

While data is lacking in horses, some foods have been shown to be more allergenic in humans. The foods shown to cause the most allergies in people include milk, eggs, wheat, peanuts, tree nuts and soy.

No population-wide studies have been done to determine the percentage of horses impacted by soy allergies. In fact, as discussed in our Food Allergies in Horses article, it is difficult to verify a true allergy. Rough estimates (emphasis on rough because there is no way to verify) of the prevalence of soy allergies in horses range from less than 1% and up to 5%. This would mean that out of the over 9 million horses estimated to be in the United States, less than 90,000 or up to 450,000 may be allergic to soy protein. That is a noteworthy population of horses.

During allergic reactions to food, the immune system "reads" a foodstuff as harmful and reacts the way it does because it is trying to minimize further exposure. Allergic reactions are often described as an overreaction of the immune system and, thus, why the symptoms often are rapid and, at times, intense.

Any allergy is often very difficult to diagnose by an equine practitioner. It is said allergies are multifactorial and additive. This means that there can be many causes of a specific allergy for a horse. Additionally, a horse may be allergic to more than just once substance.

Our Food Allergies in Horses article details how feed allergies are detected. It also details how you can test or determine if your animal is allergic to specific feed ingredients and how to provide nutritional support to horses suffering from allergies. It is worth expanding and reviewing some of that information here. First, when determining potential causes to an allergy, a complete history should be compiled about the horse. Information should include:

  • Any commercial feeds fed, past and present
  • Hay (pasture) types, past and present
  • Any treats given, past and present
  • Age of onset of symptoms
  • Dates (season) when onset of symptoms began
  • Any insect or parasite control measures
  • Any changes in environment or management

The purpose is to get as complete a picture as possible of all potential allergens. Once the feed becomes the primary suspect of the offending substance, then the only reliable way of determining if a horse has a soy (or any food) allergy is by an elimination food trial. This is because both intradermal allergen testing and serum testing are unreliable for determining food allergies in horses. 

Briefly, during an elimination trial, the suspected allergen (feed ingredient) is removed from the diet. If soy were a suspected allergen, it would be advised to eliminate any soy in the diet completely. If this is, in fact, an allergy you suspect your horse has, you may consider our Wholesome Blends line, which is 100% soy-free. Then, observe your horse to see if any allergic reactions subside. It takes anywhere from 4 to 6 weeks to see clinical improvement of allergies. If you see improvement, you should then reintroduce the offending substance (in this case, soy) to the horse’s diet. If the horse’s allergies return, then you can likely assume that feed ingredient was the offending substance.   

The above example simplifies an elimination feed trial when, in fact, it is much more complex. For example, just switching from a feed containing soy to a soy-free feed, we may also be eliminating other potential food allergens, such as wheat. It is always best to consult with an equine practitioner or, if you suspect your horse is suffering from food allergies, Tribute® Equine Nutrition is always available for consultations. We can help guide you on how to design a proper elimination feed trial for your horse.

Article By: Nicole Rambo, Ph.D., Chris Mortensen, Ph.D.
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