Vitamin E seems to be at the forefront of many discussions about horse nutrition these days. Contrary to appearances, vitamin E is not a vitamin in and of itself. Rather, it’s a group of eight compounds—four tocopherols and four tocotrienols—that are fat soluble. This vitamin is important for myriad functions in the horse’s body, including but not limited to, and in no particular order, the eyes, the reproductive tract, and the neuromuscular system. The most important function of vitamin E seems to be as a biological antioxidant. In this way, and probably by other ways that haven’t been determined yet, vitamin E serves to maintain many normal body functions of the horse.
Vitamin E-related problems generally manifest in the horse’s neuromuscular system. In young horses, these include nutritional myodegeneration, neuroaxonal dystrophy, and equine degenerative myeloencephalopathy; in older horses, a lack of vitamin E is associated with vitamin E-deficient muscle problems or equine motor neuron disease.
The grass is greener and packed with vitamin E
Happily, for most horses, there is ample vitamin E provided in the diet. Green grass is a great source of vitamin E—most things that are green have a good bit of it. Those horses lucky enough to have access to green grass also get lots of vitamin E. That said, horses do not require vitamin E on a daily basis. Horses store vitamin E quite well in the liver; they can last many months in the face of inadequate dietary intake. This, then, explains why horses that don’t have access to green grass in the winter don’t immediately start showing problems related to a lack of vitamin E. An adult horse with previous adequate dietary intake can go approximately 18 months without vitamin E before problems become noticeable.
Of course, some horses are not so lucky as to always have access to green grass, such as those kept in arid, desert climates. However, even in these areas, not all horses show signs of vitamin E deficiency. Even when a whole herd is likely to be deficient in vitamin E, only certain horses show clinical signs. Whether or not a horse ever shows problems related to a vitamin E deficiency relates to things such as its age when deficiency develops, how long the horse was deficient, genetics, other dietary deficiencies or excesses, and probably a number of other as-yet-undetermined factors.
Further complicating the picture, some horses show no apparent bad effects of a vitamin E deficiency (as determined by serum levels). Unfortunately, one cannot simply draw blood, check vitamin E levels, and be sure a horse has a problem. A proper diagnosis of a vitamin E-related problem is made with a combination of vitamin E status, clinical signs, and muscle biopsy results, as well as eliminating other diseases with similar signs. A normal vitamin E level for a horse is considered to be greater than 2 μg/ml.
Supplementing vitamin E
It is important to be aware of the type of vitamin E that is being supplemented. Vitamin E comes in two main types: natural or synthetic. Here’s a case where “natural” really is better. The natural form of vitamin E is in an “RRR” form. This can either come in a powder (RRR-αTocopherol acetate) or as a water-dispersible liquid (RRR-αTocopherol). The powder is more stable and has a longer shelf life, but isn’t absorbed quite as well as is the liquid.
The “synthetic” form of vitamin E can be noted by its spelling. Synthetic vitamin E is identified as “All-rac-α-Tocopheryl acetate” or “dl-α-Tocopheryl-acetate” on the supplement label. Tests have shown the synthetic form of vitamin E is not absorbed as well by the horse as is the natural form. If it has been determined a horse should be supplemented with vitamin E, it makes sense to use the formulation the horse can use most efficiently.
Some caveats (in no particular order):
1) Some horses don’t respond to vitamin E supplementation, even when it’s given at appropriate levels and in a bioavailable form.
2) A good number of horses that develop diseases related to a deficiency of vitamin E do not respond to vitamin E supplementation (estimates are that could amount to as many as 40 percent of vitamin E-deficient horses).
3) There’s an injectable form of vitamin E available for horses that also contains selenium. However, the injectable form does not have enough bioavailable vitamin E to affect a horse’s serum levels of vitamin E.
4) Vitamin E supplementation should be done for a specific reason, and should be followed with serum tests to see if supplementation is effective.
5) Vitamin E levels greater than 4 μg/ml (micrograms/ml) are too high.
6) Giving extra vitamin E to any horse with a suspected neurological problem or muscle problem doesn’t make much sense if the horse has normal serum vitamin E levels.
7) In human medicine, there is little in the way of good research to show vitamin E supplements are beneficial. Most good human trials from the past few years have reported inconclusive, or even negative, results. Further, vitamin E supplements might actually be harmful in some circumstances.
The dietary requirement for vitamin E was last examined back in 2007, and it was set at 1 to 2 IU/kg of body weight. In cases where supplementation is recommended for horses, doses are usually along the lines of 5000 to 10,000 IU per day. Dosing with the liquid is the most effective way to supplement; however, it also is the more expensive form of supplementation. While dosing with the natural powder has been shown to increase serum vitamin E levels, achieving normal serum levels in horses in which deficiencies can be shown requires a higher dose and longer periods of time than if the liquid is chosen as a supplement. Still, under any circumstances, the best and least expensive way to supplement a horse with vitamin E is to get him to eat grass for at least six months a year.
David W. Ramey, DVM, is a Southern California equine practitioner who specializes in the care and treatment of pleasure horses. Visit his website at doctorramey.com. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.