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Q: I would like to know more about supplementing horses’ diets with vitamin E. Is it something that benefits all horses, or will green grass and sunshine be enough? Would a high-level performance horse benefit from additional vitamin E? Along that line, I often see vitamin E supplements that contain selenium. I live in an area where selenium is already added to complete feeds. Would it be safe to feed two selenium sources? —KS, via email

A: Vitamin E is a term used to describe a group of compounds known as tocopherols and tocotrienols. In equine nutrition we pay most attention to alpha-tocopherol. Alpha-tocopherol plays an important role in something called the glutathione pathway, which protects cells from oxidative damage, as well as reacting with free radicals produced by processes such as the lipid peroxidation chain reaction. However, other forms of tocopherol, such as gamma tocopherol, have unique and potentially equally important functions. At this time, though, the data on their specific roles and importance are limited.

Free radicals cause damage in cells because they have an odd number of electrons, which gives them an unstable electrical charge. In an attempt to become stable they “steal” electrons from other molecules. This causes a new molecule to be unstable, and it might not be able to perform its function within the body. A chain of oxidative destruction can result within the cell, which can negatively impact cell function and potentially cause cell death. Free radical formation is a natural consequence of various molecular processes within the cell, and we often hear it referred to as “oxidative stress.” It’s easy to label free radicals as “bad,” but they are, in fact, a perfectly natural consequence of the body using fats and carbohydrates for energy.

By deploying antioxidants, the body can control free radical damage. Antioxidants bind to or inhibit the free radicals, thus reducing the likelihood of an uncontrolled chain of oxidative damage. Antioxidants can be vitamins (such as E and C), minerals (such as selenium), or enzymes (such as glutathione peroxidase). As long as the free radicals don’t outnumber the available antioxidants in the tissues, oxidative stress can be avoided.

As exercise increases, so does the working muscle’s energy demands and the number of free radicals produced. To avoid damage to muscle cells during exercise, adequate levels of antioxidants must be available to counter all the extra free radicals being generated. This is why, as a horse’s workload increases, so do his vitamin E requirements. Signs of oxidative damage in working horses include muscle soreness and stiffness and slower-than-expected recovery from intense exercise.

The National Research Council’s (NRC) requirement for vitamin E in the form of alpha-tocopherol for a mature 1,100-pound horse at rest is 500 IU (international units) per day. Once a horse is in light work this requirement increases to 800 IU. Heavy works requires 1,000 IU. Some researchers feel the NRC underestimates vitamin E requirements, especially in working horses, and that those horses consuming high-fat diets might also require higher intakes of vitamin E. High-fat diets can lead to greater peroxidation and, therefore, a greater need for antioxidants.

The form of vitamin E in the diet is important, with natural d-alpha tocopherol being absorbed from the digestive tract more readily than synthetic dl-alpha tocopherols. Commercial feeds are not obligated to state the form of vitamin E included in their products, which are often labeled as containing a vitamin E supplement. Synthetic forms have the lowest relative bioavailability, followed by natural acetate forms, natural alcohol, and micellized (chemical process that changes some compounds to easily absorbable structures) vitamin E, which is essentially water-soluble and highly bioavailable. Natural d-alpha tocopherol found in many supplements is inherently unstable and, as a result, is often found in its acetate form. By binding the acetate to the alpha-tocopherol chemically, the acetate protects it from damage when exposed to oxidative forces that may exist, especially in the feed. Once ingested, the digestive tract enzymes release the d-alpha tocopherol for absorption with its oxidative properties intact.

An added complication to this picture is that each horse utilizes vitamin E in its liver differently. This was brought home to me when a client who was feeding her three horses the exact same diet did bloodwork to assess their vitamin E status. Despite feeding more than the NRC requirement and feeding each horse the same, one came back with high, but normal, values; one mid-range; and the third low, but normal. For this reason I no longer make blanket recommendations on vitamin E provision for my clients. We ensure that the diet is providing the minimum NRC requirement, and then we test whole blood vitamin E to see whether further supplementation is required. Vitamin E supplements containing natural sources of vitamin E are not cheap, so we are not supplementing unless it’s necessary.

Good-quality grass pasture is an excellent source of vitamin E in all natural forms. A horse that is sustaining itself on good-quality grass pasture will be consuming significantly more vitamin E than the NRC requirement. However, because vitamin E is not heat-stable, its levels in hay can decrease over time. For this reason, horses on poor pasture or that cannot maintain themselves on pasture alone and must be supplemented with hay or that only receive hay should also receive an additional source of vitamin E. The amount of vitamin E provided in good-quality commercial feeds should meet NRC requirements as long as they’re fed per the manufacturer’s guidelines. However, the form in the feed will have an impact. The horse’s veterinarian should test to see whether an additional supplemental source is required. This is especially true if your horse is showing signs of low vitamin E status or has a neurologic condition worsened by low vitamin E levels.

Many supplemental sources of vitamin E on the market have added selenium. Whether this will result in a diet too high in selenium depends on a number of things, including the level of selenium in forage (often unknown), whether any commercial feeds and supplements already being fed contain selenium, and the total volume of feed fed. A mature 1,100-pound horse at rest or in light work has a requirement for 1 milligram of selenium per day increasing to 1.25 milligram if in heavy work. However, he’s not likely to suffer any ill effects if his selenium is even twice that intake. In fact, horses’ true selenium requirements per the NRC is unknown.

Research has suggested an intake of 0.1 milligram of selenium per kilogram of dry matter consumed (1 milligram for a 1,100-pound horse eating 2% of its body weight per day) will prevent classical deficiency; however, some studies show greater equine influenza antibodies in foals from mares that received 3 milligrams of selenium per day compared to 1. This suggests that the intake needed for optimal immune function might be higher than that needed to avoid classical deficiency symptoms. Based on studies in other species, a maximum tolerable concentration of selenium in horses is estimated at 0.5 milligram/kilogram dry matter consumed, while the U.S. Food and Drug Administration typically recommends a concentration of no more than 0.3 milligram/kilogram dry matter consumed. As the guidelines can vary depending on total feed intake, the more total feed your horse consumes each day the greater the total number of milligrams of selenium he can consume, with the estimate of 1 to 2 milligram total per day being the basic recommendation. Therefore, to determine whether your horse’s diet can handle additional sources of selenium, you need to calculate the amount coming from all sources in the diet.

Again, because individual assimilation can vary, I recommend you have your horse’s selenium levels tested so you know how the selenium levels in his diet correspond to actual levels in his body. From there, you can work with your veterinarian and/or equine nutritionist to make any necessary adjustments.

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