When Do Horses Need Vitamin E? - Equine Health and Welfare

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When Do Horses Need Vitamin E?

HCBC Staff
Created: November 18, 2015

as a horse’s workload increases, his requirement for vitamin E goes up, too. Signs of oxidative damage in working horses include muscle soreness and stiffness and slower-than-expected recovery from intense exercise.

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 most attention is paid 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, there are other forms of tocopherol, such as gamma tocopherol, that have unique and potentially equally important functions. At this time, however, the data on their specific roles and importance is 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 the free radicals or in some other way inhibit them, 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 the working muscle’s demand for energy is increased and the number of free radicals produced goes up. To avoid damage to muscle cells during exercise, adequate levels of antioxidants must be available to counter all the extra free radicals that are being generated. This is why, as a horse’s workload increases, his requirement for vitamin E goes up, too. 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 anti-oxidants.
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 lastly micellized (chemical process which 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, it is often found in its acetate form. By chemically binding the acetate to the alpha-tocopherol the acetate protects it from damage when exposed to oxidative forces that may exist especially in the feed. Once ingested the enzymes of the digestive tract release the d-alpha tocopherol for absorption with its oxidative properties intact.
An added complication to this picture is that how each individual horse utilizes vitamin E once it reaches the liver varies widely. This was brought home to me when a client who had three horses and was feeding them the exact same diet did blood work to assess 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’s values were low, but normal. For this reason I no longer make blanket recommendations on vitamin E provision for my clients. We insure that the diet is providing a minimum of the NRC requirement, and then we test whole blood vitamin E to see whether further supplementation is required. This has the added benefit because vitamin E supplements containing natural sources of vitamin E are not cheap and 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 due to the fact that vitamin E is not heat stable, it is not as unabundant in hay and levels in hay can decrease over time. For this reason horses on poor pasture or that cannot maintain themselves on pasture alone and have to be supplemented with hay or that only receive hay should be receiving an additional source of vitamin E. The amount of vitamin E provided in good quality commercial feeds should insure that the NRC requirements are met as long as those are fed per the manufacturers 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 you horse is showing signs linked to low vitamin E status or has any kind of neurologic condition worsened by low vitamin E levels.
Many supplemental sources of vitamin E on the market do 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 there is selenium in any commercial feeds and supplements already being fed, and the total volume of feed fed. While a mature 1,100-pound horse at rest or in light work has a requirement for 1 mg of selenium per day increasing to 1.25 mg if in heavy work, no ill effect is likely even at twice that intake. In fact the true requirement for selenium in horses per the NRC is unknown.
Research has suggested an intake of 0.1 mg/kg dry matter consumed (1 mg 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 mg of selenium per day compared to 1 mg per day. 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 mg/kg dry matter consumed, while the U.S. Food and Drug Administration typically recommends a concentration of no more than 0.3 mg/kg dry matter consumed. As the guidelines can vary depending on total feed intake it means that the more total feed your horse consumes each day the greater the total number of milligrams of selenium can be consumed with the estimate of 1 to 2 mg total per day being the basic recommendation. Therefore, to determine whether your 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 that you have your horse’s selenium levels tested and then you will know how the level of selenium in the diet corresponds to actual levels in your horse. From there you can work with your veterinarian and/or equine nutritionist to make any necessary adjustments.
Do you have an equine nutrition question? Thunes and The Horse’s editors want to hear from you! Send your questions to [email protected]
About the Author

Clair Thunes, PhD
Clair Thunes, PhD, is an independent equine nutrition consultant who owns Summit Equine Nutrition based in Sacramento, California. She works with owners/trainers and veterinarians across the United States and globally to take the guesswork out of feeding horses. Born in England, she earned her undergraduate degree at Edinburgh University in Scotland and her master’s and doctorate in nutrition at the University of California, Davis. Growing up, she competed in a wide array of disciplines and was an active member of the United Kingdom Pony Club. Today, she serves as the regional supervisor for the Sierra Pacific region of the United States Pony Club. As a nutritionist she works with all horses from WEG competitors to Miniature Donkeys and everything in between.

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