Nutrition

Using Nutrition to Boost the Immune System

By Paul Basilio

Exercise is widely recognized as a stressor that can cause neuroendocrine and hormonal changes that weaken a horse’s immune system. For heavily exercised performance horses that are often transported long distances and kept near other horses, this can be a perfect recipe for compromised immunity.

In addition to vaccination and infectious disease standard care, what more can be done to strengthen a horse’s natural defenses?

“Research on the impact of diet on the immune system in the horse is in its infancy,” said Lori K. Warren, PhD, associate professor of equine nutrition at the University of Florida. “Unfortunately, we don't have a lot of hard facts and prescriptive recommendations.”

She does, however, have some valuable insights that she shared in a virtual session at the 66th Annual AAEP Convention.

Vitamins and Antioxidants

Vitamins and antioxidants support the immune system either through their antioxidant role or by contributing to the body’s natural defenses by strengthening epithelial barriers, cell-mediated immunity, or antibody production.

Vitamin E is often studied for its role in mitigating exercise-induced oxidative stress, but there is a limited amount of literature on immune-related outcomes, she explained. Although studies do confirm the role of vitamin E in immune function and immune support, they are usually comparing the role of vitamin E supplementation vs. a diet deficient in vitamin E.

“Evidence is lacking that feeding vitamin E above established requirements will confer any additional advantages in an otherwise healthy horse,” Dr. Warren said. “For horses in work, the vitamin E requirement ranges from 1.6 to 2 IU/ kg of body weight, which is about 1,000 IU per day for a 500-kg horse. In horses that have neurological disease or in those with myopathies, feeding vitamin E above these requirements is warranted.”

Selenium is another key oxidant that is largely evaluated vs. a deficient diet. Horses typically require about 1 to 1.25 mg/ day for a 500-kg horse. While studies have shown declines in innate and acquired immunity in horses fed selenium-deficient diets, there has not been much work done on feeding above the requirements.

Dr. Warren’s team performed a study that compared a diet containing the National Research Council (NRC) requirement for selenium vs. one that contained 3 times the requirement. The horses were unfit at the beginning of the study, but they underwent 2 hours of strenuous exercise daily during the trial.

“Muscle sampling revealed a greater antioxidant capacity and a reduction in lipid peroxidation products in the early post-exercise period when horses received the higher level of selenium,” she explained. “From an immune standpoint, higher selenium intake resulted in a more rapid remediation of the leukocytosis that we typically see following exercise.”

She added that other immune measurements, such as lymphocyte proliferation and neutrophil function, were unaffected.

“Collectively, these results suggest that selenium could be important in stressful situations or big challenges but feeding selenium above requirements is not likely to confer addition benefits under normal training conditions,” she said.

And too much selenium can be toxic. In 2009, 21 polo ponies died after an overdose of selenium used to help muscle recovery.

For vitamin C, there is plenty of research available, but the results have not demonstrated a clear benefit in reducing oxidative stress. There is no dietary requirement established for vitamin C, and it is assumed that synthesis of the vitamin in a horse’s body is enough to meet their needs.

Immunomodulation with Fatty Acids

Fatty acids have long been known to influence the immune response through several mechanisms, including gene expression, cell signals, cell membrane composition and by serving as a substrate for more bioactive molecules. Both omega-3 and omega-6 fatty acids are important to include in a horse’s diet in the proper balance.

“Ultimately, it’s the balance of omega-6 and omega-3 fatty acids in the body—or really within the cell— that determines the overall response,” she said.

Prior research has shown that omega-3 fatty acids can reduce markers of inflammation. However, prior research has also shown the opposite.

“It generally takes a high supplementation rate— anywhere from 50 to 75 mg/kg of body weight—to elicit changes in markers of inflammation,” Dr. Warren explained. “That is much higher than is typically recommended by supplement companies, and probably too high to be economically sustainable for most horse owners.”

She also explained that although omega-6 fatty acids are often maligned, they are by no means the villain in the fatty acid saga. “There have been claims that omega-6 fatty acids can cause inflammation, but there is little support for this in the literature.”

In one of her studies, Dr. Warren found that yearlings supplemented with a high-fat source of either fish oil or corn oil—which is rich in omega-6—had similar reductions in the inflammatory marker PGE2 when compared with a diet where no fat was supplemented.

“From a clinical standpoint, most people are interested in supplementing omega-3 fatty acids to horses with inflammatory conditions such as osteoarthritis,” she added. “A relatively high amount of EPA [eicosapentaenoic acid] and DHA [docosahexaenoic acid] from fish oil was shown to alter the fatty acid composition of synovial fluid in several studies, but it’s been a mixed bag in terms of having an impact on inflammatory markers or clinical outcomes.”

She added that omega-3 supplementation in healthy horses typically has no benefit.

The American College of Veterinary Internal Medicine has recommended 1.5 g of DHA for the treatment of inflammatory airway disease, but those recommendations are quite lower than the dosages that affect inflammatory markers in the literature, according to Dr. Warren.

Omega-3 fatty acids appear to have the greatest impact in horses that have a heavy inflammatory load, which echoes some of the effects that are found in antioxidant nutrients.

“It does appear that the longer-chain omega-3 fatty acids, such as EPA and DHA and their sources, are more effective than supplying omega-3 as the parent fatty acid alpha-linolenic acid. However, there’s not enough research for me to able to systematically arrive at an effective dose that I can pass along,” she said.

PREBIOTIC AND FUNCTIONAL FIBERS

More than 70% of the immune system is associated with the gastrointestinal tract, and it serves as an important first line of defense against pathogens. Because horses are herbivores, the fibers are critical.

Some dietary fiber, called prebiotics, colonizes the gut with beneficial bacteria, binding to pathogens to block attachment to the intestine and interacting directly and indirectly with immune cells associated with the gut mucosa. Others have probiotic effects include mannan-oligosaccharides (MOS), beta-glucans and pectin. These fibers are abundant in mare colostrum and milk.

“As fibers are fermented and cleaved down to shorter oligosaccharides in the hind gut, they certainly can exert positive effects in that part of the GI tract,” Lori K. Warren , PhD said. “I think our interest is to try to supply these shorter fibers through the diet so that they can also exert their effect in the small intestine.”

Recommending to clients that they feed their horse a diet high in fiber that supplies the functional fiber precursors is a solid first step toward enhancing the immune system.

“Encourage the use of a variety of fiber sources, especially those that are high in water-soluble fiber, which are often referred to as highly digestible fiber,” she said. “We’re looking for things that contain beta-glucans— such as whole oats—and things that contain pectin, like beet pulp and soybean hulls. All of those are a good approach until we can learn more about the functional fibers in the horse.”