Vitamin E is a fat-soluble vitamin with important anti-oxidative functions for all animals. There are eight forms of vitamin E, with alpha-tocopherol being the most common and the most important regarding function.

Vitamin E’s structure contributes to its function, by having a fatty side chain that allows it to be embedded in cell membranes, and a segment that allows it to neutralize damaging free radicals (an atom or molecule with an unpaired electron, which makes it highly reactive).

Intense exercise and/or disease states that increase the body’s metabolic rate may increase free radical and ROS production above the natural defenses, and can cause cellular damage leading to tissue injury and potentially further disease processes.

Most metabolic processes in the body produce free radicals or reactive oxygen species (ROS – highly reactive forms of oxygen) without consequence. Vitamin E functions as a free radical scavenger, and donates a hydrogen atom to free radicals to neutralize them.

Oxidative stress is the term used to describe an imbalance between the production of free radicals and reactive oxygen species, with the body’s natural antioxidant defenses. Intense exercise and/or disease states that increase the body’s metabolic rate may increase free radical and ROS production above the natural defenses, and can cause cellular damage leading to tissue injury and potentially further disease processes – for example, in humans an increased risk of cancer.

The National Research Council’s Nutrient Requirements of Horses (2007) recommends that a 500 kg horse receives 500 IU (international units) of vitamin E per day, although most nutritionists recommend closer to 750-1000 IU per day, particularly if a horse is in work. When looking at supplemental vitamin E, it may be expressed as either IU or mg, and may be from natural or synthetic sources. One IU of natural vitamin E (d-alpha-tocopherol) is equal to 0.67 mg of alpha-tocopherol, while synthetic forms of vitamin E are less potent (1 IU synthetic vitamin E, dl-alpha-tocopherol = 0.45 mg alpha-tocopherol). However, natural vitamin E is relatively unstable and is often supplemented in feeds as alpha-tocopherol-acetate.

Vitamin E works closely with selenium, which is a component of glutathione peroxidase, another free radical and ROS detoxifyer. Vitamin C also scavenges free radicals, and therefore the entire antioxidative nutrient status of the diet is important.

Sources of Vitamin E

The best source of natural vitamin E is pasture. Depending on the plant species, pasture may have between 50-400 IU of vitamin E per kg dry matter of plant material. Therefore, a horse eating only pasture, could be consuming between 500–4,000 IU of vitamin E daily from pasture alone. When plants are cut for hay production, the drying and storage process causes significant breakdown of vitamin E, decreasing to values from 10-100 IU near harvest down to little to no vitamin E after a few months of hay storage.

Other feed ingredients such as grains or oils may contain some vitamin E; however, the form may not be alpha-tocopherol, so most commercial feeds have added vitamin E to ensure requirements are met, particularly if a horse is not on pasture. Most commercial feeds have between 100-350 IU/kg, so the amount provided in the diet depends on how much is fed (eg. 2 kg of a 300 IU/kg vitamin E supplemented feed might provide 600 IU of vitamin E). Because many athletic horses have increased anti-oxidant needs and beyond what feeds they might consume, further supplementation is often warranted.

Too little?

Vitamin E deficiencies have not been reported for horses at pasture, so horses that consume primarily hay are those that might need supplementation. It is difficult (and expensive) to test hay or pasture for vitamin E, so nutritionists tend to presume the hay does not contribute significantly to the vitamin E in the diet. Therefore, the decision to supplement with additional vitamin E will require an evaluation of vitamin E in any commercial feeds or supplements provided.

In addition, vitamin E is one of the few nutrients where blood concentration can also be helpful to determine if supplementation is warranted. This is because some horses may metabolize, or require, different amounts of vitamin E and it may be difficult to properly assess status though dietary analysis alone. Normal alpha-tocopherol concentrations in equine blood is 2–4 micrograms/ml.

Because vitamin E is fat soluble, excess vitamin E consumed is stored in the fat, and so deficiency symptoms don’t tend to appear until the stores are depleted, which may take several weeks of low intake. Deficiency symptoms may range from muscle trauma due to oxidative stress, to equine motor neuron disease (EMND) that may occur with prolonged vitamin E deficiency.

Too much?

Vitamin E is relatively non-toxic, and it is not uncommon to see some horses supplemented with more than 5,000 IU/day. However, research has shown that excessive supplementation may negatively affect the status of other vitamins, in particular beta-carotene (the precursor to vitamin A). High levels of supplementation have also not been shown to provide added protective benefits for intense exercise.

Therefore, while some horses may warrant supplementation above NRC recommendations, too much may actually be detrimental. Be sure to work with an equine nutritionist to help your determine if you horse needs vitamin E supplementation.