Many owners place great faith in nutritional supplements to ward off disease and inflammation in their performance horses. And, indeed, cautious optimism is warranted. During the 2020 American Association of Equine Practitioners’ Convention, held virtually, Lori Warren, PhD, PAS, an equine nutritionist at the University of Florida, reviewed what scientists know about the horse’s immune system and the potential to enhance disease resistance through dietary means.

Horses in training and competition invariably face stressors that tax their immune systems, including strenuous exercise, long-distance hauling, and concentrated stabling at showgrounds and racetracks where they’re at increased risk of exposure to pathogens (disease-causing organisms). In addition to a good vaccination program, a balanced diet is one of the best defenses a horse has against disease, said Warren. Every nutrient is important, she emphasized. Any deficiencies or imbalances in essential amino acids, fatty acids, vitamins, minerals, and fiber can compromise immune function.

But what if the horse’s recommended daily requirements are being met? Can specific nutrients confer a protective or therapeutic benefit when fed in higher amounts?

Antioxidants

Antioxidants are among the most popular supplements, and vitamin E is one of the most studied. Deficiencies have been associated with neurologic and inflammatory diseases, and veterinarians typically recommend supplementation for affected horses. However, for healthy athletes whose daily requirements are being met, research suggests feeding extra vitamin E does not provide additional immune-boosting benefits, said Warren. Ditto for selenium and vitamin C.

In fact, it’s unclear what role vitamin C plays in immune function, she observed. The National Research Council has not established a daily requirement for vitamin C in the equine diet, perhaps because horses can synthesize it in the liver, and excess C gets flushed from the body. That said, Warren noted that some horses with active inflammation might benefit from receiving an antioxidant “cocktail” that includes vitamin C. She also cautioned that if owners have been feeding vitamin C and then decide to discontinue doing so, it should be tapered off rather than stopped cold turkey so the body’s natural synthesis of vitamin C can resume.

Omega Fatty Acids

Omega-6 and omega-3 fatty acids are recognized for their immune-boosting properties. They can’t be synthesized so they must be present in the horse’s diet. A forage-rich ration that includes pasture and hay is an often-discounted source of omegas, Warren said, so supplemental fat might not be needed.

Flaxseed, linseed, sea algae, and cold-water fish oils are good sources of omega-3s. Fish oil and algae have an advantage in that the fatty acids’ molecular structure provides a biological shortcut to being used by the cells in the body. Corn, soy, and other plant oils are good sources of omega-6s. Warren emphasized that omega-6 is not the “bad guy.” In fact, it’s just as important as omega-3—the two work together within cells. Study results have shown that horses with respiratory disease and osteoarthritis receiving both omega-3s and -6s experience a reduction in inflammatory markers. Although no hard conclusions have been drawn, the American College of Veterinary Internal Medicine recommends adding the omega-3 docosahexaenoic acid to the diets of horses with inflammatory airway disease.

Functional Fiber

Warren also addressed the immune-supporting properties of dietary fiber, specifically what she deems “functional fibers.” These are indigestible carbohydrates that pass through the digestive tract until reaching the hindgut, where they are fermented by microbes. Also marketed as “prebiotics,” these fibers are more than just a simple energy source, she said. They exert a positive effect on immunity by:

1. Establishing beneficial bacteria in the hindgut;
2. Binding to pathogens;
3. Supporting epithelial cells; and
4. Interacting with immune cells.

Because horses are herbivores, the largest part of their diet should be fiber-rich hay and pasture, said Warren. Other sources of soluble fiber—such as beet pulp, soy hulls, and whole oats—can be added to the ration. Fiber components such as oligosaccharides and beta-glucans present in certain cereal grains have been shown to have positive effects in other species, including humans, she noted. For example, who hasn’t heard about the cholesterol-reducing benefit of oatmeal?

However, nutritional advantages do not always transfer from one species to the next, which is why more research needs to be done in horses, she said. During one University of Florida trial, researchers supplemented the diets of 2-year-olds in training with oat beta-glucans. The youngsters were then stressed for 12 hours by elevating their heads, similar to what they might experience during long-distance transport. The idea was to provoke an inflammatory response in the airways. The group that received the oat beta-glucans, however, fared no better than the control group.

Yeast

Yeast is yet another promising feed additive. In one study, researchers fed pregnant mares yeast in the final weeks before foaling. After birth, they had higher-quality colostrum (antibody-rich first milk), greater passive transfer (of antibodies to foals), and stronger neutrophil function in their foals. In a separate study, researchers fed dams and foals yeast supplements and noted measurable increases in fecal IgA— protective factors that are “first lines of defense” against pathogens in the gut, Warren explained. In yet another study, senior horses fed a yeast culture supplement had an improved immune response, as well as a reduction in inflammatory cytokines, when vaccinated against equine influenza.

Of course, determining dietary effects on the horse’s immune response is a science that’s still in its infancy, Warren said. More equine-specific research is needed before she feels comfortable recommending that owners feed certain nutrients above NRC requirements—unless a special condition warrants it. Until then, as simple as it sounds, she emphasizes the key to maintaining a healthy immune system is a balanced diet.