Vitamin E and Selenium Status in Horses

Vitamin E and selenium both function as antioxidants in the cell. Selenium is part of the enzyme glutathione peroxidase, which breaks down peroxides. Vitamin E is a fat-soluble vitamin that inserts itself into the cell membrane and neutralizes free radicals. Both selenium and vitamin E can offset a deficiency of the other, as long as both are not deficient.

Selenium deficiency or selenium-responsive conditions have been associated with several problems in horses, including white muscle disease, sporadic tying-up, retained placenta,
reduced fertility, masseter (jaw) muscle myopathy, and heart failure or difficulty swallowing in foals.

The best sample to assess selenium adequacy is whole blood. Because erythrocytes cannot produce more glutathione peroxidase, whole blood selenium concentration reflects long-
term selenium status over the lifespan of the sampled red blood cells (120 to 150 days). It is recommended to use royal blue top (trace mineral) tubes when measuring blood selenium concentration. Horses require 0.1 ppm of selenium per kg of dry matter intake (mg/kg); this equates to approximately 1 mg of selenium per day for a 1,000-lb (454-kg) horse.

Serum is the most commonly used sample for measuring vitamin E. The vitamin E requirement is approximately 1 IU/kg body weight/day, or 500 IU/day for an 1,100-lb (500-kg) horse. The source of vitamin E is very important, especially when treating horses for equine motor neuron disease, equine degenerative myelopathy, or other vitamin E responsive conditions. Natural (d-α-tocopherol), water-soluble sources of vitamin E are better absorbed and preferentially used by the horse. Synthetic vitamin E sources (d,l-α-tocopherol) contain a mixture of eight chemically different sources of vitamin E, only one of which is d-α-tocopherol. Studies have shown that natural vitamin E is more bioavailable than synthetic vitamin E and significantly raises both serum and cerebrospinal fluid concentrations of α-tocopherol compared to synthetic vitamin E.

Vitamin E deficient myopathy is a newly recognized cause of muscle loss and weakness. It is unknown if this is a unique problem or an early, reversible stage of equine motor neuron
disease. Affected horses have low serum (<2 µg/ml) and muscle concentrations of vitamin E. Supplementation with 5,000 IU natural vitamin E/day for six weeks has been shown to restore serum and muscle vitamin E concentrations to normal and most horses make a gradual return to normal exercise.

Providing Dietary Protein to Horses

Owners of dogs and cats know that their pets are carnivores—meat eaters—and commercially available food for these animals contains high protein levels similar to what would be found in the diets of wild felines and canines. Horses, on the other hand, are herbivores—plant eaters—and at first glance, it would not seem that grass, the most natural equine diet, would contain a great deal of protein.

However, grasses and legumes, either fresh or dried, usually do contain enough protein for maintenance of idle or lightly worked horses. Grass has a low to medium level of protein and does not contain a large amount of lysine, an essential amino acid. Alfalfa or lucerne, a legume, is an excellent source of protein, with levels that can approach 18 to 20 percent in the best-quality hay. Since horses need only about 10 or 11 percent crude protein as adults, and about 12 to 14 percent during growth, alfalfa could easily supply most or all protein requirements, though a straight alfalfa diet is not balanced for other nutrients.

In general, a mature horse doing little work needs only enough protein to maintain body tissues. Requirements for protein go up as horses are put into an exercise program, and the need increases with the workload because these active horses must build lean muscle tissue. Breeding stallions and late-pregnant mares need protein levels similar to those of horses in moderate to heavy exercise. Lactating mares have the highest protein requirement, up to twice as much as a mature horse in
moderate exercise. Finally, growing horses need high-quality protein as they increase in size and weight.

The National Research Council’s Nutrient Requirements of Horses contains guidelines for the amount of crude protein needed by horses of every size, age, stage of growth, reproductive status, and exercise level. Horse owners don’t need to pick up a copy of this publication, however, if they purchase horse feed that has been formulated and produced by a reputable manufacturer. Feed bags and labels indicate the type and amount of feed necessary to meet the needs of many classes of horses, from mature/idle to growing, lightly worked, heavily exercised, breeding, or retired equines.

Supplementing a forage-based diet by feeding a concentrate according to the bag’s directions will usually provide sufficient protein. Knowing that their equines need more protein than idle animals, owners of hard-working sport horses or lactating mares may wonder whether they should boost dietary protein levels through the use of special protein or amino acid supplements. Studies in both horses and human athletes have shown that this type of supplementation has little or no positive effect.

However, the intensity of the actual training program in that study is somewhat difficult to interpret. Several research experiments conducted at Kentucky Equine Research (KER) and other locations have examined the effects of dietary protein on metabolic responses to exercise. In two of these studies, lactate accumulation during exercise was lower when horses received a diet containing a high level of protein, but in a third study, lactate accumulation was unaffected by dietary protein level. In the KER study, it was found that muscle glycogen was somewhat lower in horses receiving a high-protein diet, while another study showed no effect of protein level on muscle glycogen concentration. At this point, there are no solid research data to support the theory that protein supplementation may be beneficial to equine athletes. Even in human athletes, benefits have only been sporadically reported. Because the branched-chain amino acids are the primary amino acids catabolized during exercise, researchers have also evaluated supplementation of these amino acids (especially leucine). Horses receiving a branched-chain amino acid supplement had lower lactate levels than unsupplemented horses in one study, but this study involved only mild exercise, so results might not apply to heavily worked horses. Further research in this area may turn up some positive benefits of protein supplementation, but at present, there is no recommendation for this practice.

Effect of Adding Soybean Oil to a Horse’s Ration

When fat is substituted for carbohydrate isocalorically (calorie for calorie) in a horse’s ration, blood glucose and insulin responses to feeding are reduced. It was unclear, however, whether this response was simply due to reduced glucose in the diet or if fat affects glycemic response in some other manner. An experiment carried out at Kentucky Equine Research (KER) was designed to evaluate whether adding fat to a grain meal would affect glucose and insulin response to feeding when the level of grain intake remained the same.

Nine Thoroughbred horses were used in this two-period switchback design experiment. Five of the horses were in training and were physically fit, and four were untrained. During period one, each horse was fed 2.27 kg (5 lb) of a grain mix, which consisted of 72% oats, 20% corn, and 8% molasses at 7:00 a.m. Five of the horses were also fed 200 ml (170 g) of soybean oil mixed into the grain. At 8:00 a.m. each horse was given 2.72 kg (6 lb) of mature bluegrass hay. Blood samples were taken from each horse by jugular catheter before feeding and at 1, 2, 3, 4, 6, 8, 10, and 12 hours post feeding. Water was available to the horses at all times. The same procedure was followed two weeks later with the soybean oil added to the grain of the four horses that served as controls during the first period.

Blood samples were collected and analyzed for lactate, glucose, and insulin at the conclusion of the study. Blood glucose was significantly lower one hour after feeding when soybean oil was added to the diet. Glucose remained lower for 3 hours post feeding. After 6 and 10 hours, blood glucose was higher in the fat-supplemented group. Insulin was lower in the fat-supplemented group 1 hour after feeding. After 8 and 10 hours, insulin was higher in the fat-supplemented group. Plasma L-lactate tended to be higher in the control group 4 hours after feeding and higher in the fat-supplemented group 6 hours after feeding.

These data suggest that the addition of fat (soybean oil) to a grain meal will affect glucose and insulin response to feeding. These effects are independent of the amount of carbohydrate in the diet and may be due to differences in the rate of gastric emptying when fat is included in the diet.

This article was based on information in a paper titled “Responses of Blood Glucose, Lactate and Insulin in Horses Fed Equal Amounts of Grain With or Without Added Soybean Meal” by J. Pagan, T. Rotmensen, and S. Jackson.