Oxidation is a normal metabolic process that allows horses to transform the carbohydrates, fats, and proteins they devour in meals into energy. An unavoidable side effect of oxidation is the creation of free radicals—compounds that have the potential to irreparably damage cells. Free radicals are particularly harmful to the cell membranes (structures responsible for keeping destructive entities away from delicate inner organelles).

Under normal circumstances, substances called antioxidants thwart much of the wreckage caused by free radicals. However, oxidation speeds up during athletic effort due to increased oxygen consumption and accelerated aerobic metabolism. The end result of unchecked oxidation in the bodies of equine athletes could be muscular fatigue severe enough to compromise performance.

In instances of strenuous exercise, natural stores of antioxidants have difficulty providing sufficient protection against the cascade of free radicals generated from aerobic metabolism. Thus, supplementation of antioxidants is particularly helpful in warding off the ill effects of mass-produced free radicals associated with intense exercise. Horses with an inadequate reserve of antioxidants might experience muscle soreness or stiffness during an exercise bout and prolonged recovery following hard work.

Vitamin E

Vitamin E contributes most generously to the natural antioxidant defenses of the horse. Feeds typically fed to horses have variable vitamin E concentrations. Cereal grains such as corn, oats, and barley contain minimal vitamin E, and processing might further decrease vitamin activity. While vegetable and soybean oils possess substantially more vitamin E than grains, refining can diminish content. Even if they undergo only minimal refining, these oils have such low inclusion rates in diets that their contribution to total vitamin E intake is miniscule.

Horses can derive sufficient amounts of vitamin E from fresh forage or hay, however, the vitamin content abates as forages mature and are harvested. Up to 90% of the vitamin content might be lost between the pre-bloom or boot stages and complete heading out in grasses. Losses also occur in legumes, although to a lesser extent. Storage negatively impacts vitamin composition as well. In one month, for instance, a 50% loss in vitamin E can occur in stored hay.

Because of the irregularity in vitamin E content of forages and other feedstuffs, the nutrient is often added to fortified feeds. Synthetic forms of vitamin E are absorbed well by horses, however, natural forms are far more digestible.

Vitamin E deficiencies are often thought to precipitate nervous disorders such as equine degenerative myeloencephalopathy (EDM), a disease characterized by deterioration of the brain stem and spinal cord. Ataxia (incoordination) is the foremost sign of EDM, usually beginning in the hind limbs and progressing to the forelimbs. Equine motor neuron disease, a debilitating neurologic affection that can cause profound paralysis and death, is often partially attributed to vitamin E insufficiency. Treatment for both diseases centers on the provision of megadoses of vitamin E, often 10 to 20 times the normal daily requirement. In some cases of EDM, supplemental vitamin E has completely arrested signs, although few horses return completely to normal.

Vitamin E is often linked with selenium, a micromineral that possesses potent antioxidant properties. Because it is an essential component of glutathione peroxidase, an intercellular enzyme that helps prevent the formation of free radicals, selenium is integral in the diets of performance horses. In addition to inadequate antioxidant defenses, a selenium deficiency can be detrimental to the muscular, reproductive, and immune systems.

Vitamin C

Vitamin C, often referred to as ascorbic acid, also plays a pivotal role in neutralizing free radicals. Because of its water-soluble nature, vitamin C can work both inside and outside the cell to combat free radical damage. In the exercising horse, perhaps the foremost contribution of vitamin C is its synergistic relationship with vitamin E. Once a molecule of vitamin E inactivates a free radical, its ability to short-circuit others is forsaken. In the presence of vitamin C, however, vitamin E can be regenerated to continue its raid on free radicals. The rejuvenating properties of vitamin C, therefore, make it an essential ingredient in an effective antioxidant supplement.

Vitamin C is not included in most horses' diets because the liver synthesizes sufficient quantities under normal circumstances. In periods of stress, such as during sustained exercise, vitamin C levels might drop and reduce the efficiency of antioxidant mechanisms in the body. In one study completed by Virginia Polytechnic Institute and State University, 35 endurance horses competing in 80- and 160-km races incurred vitamin C depletion, suggesting supplementation might be necessary to maximize antioxidant defenses.

An antioxidant cocktail has been advocated by human physicians for several years, and its positive effects have proven effective in nourishing the equine athlete as well. A triad of antioxidants including vitamin E, selenium, and vitamin C ensures a degree of coverage not afforded by vitamin E alone.

Article reprinted with the permission of copyright holder Kentucky Equine Research (KER). Visit www.ker.com for more horse health and nutrition information.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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