Vitamin E is a fat-soluble vitamin that acts as an essential antioxidant for horses. Horses need vitamin E in their diet because they cannot synthesize it on their own. Vitamin E supports normal nerve and muscle function, and helps maintain a healthy immune system. Vitamin E protects the fatty membrane of every cell.

Fresh, green grasses and forages contain vitamin E, and horses maintained mostly on lush pasture should get enough by grazing fresh grass. If the diet contains more hay than grass, however, it’s important to note that harvesting grasses and forages for hay causes rapid degradation of vitamin E that continues during storage. Horses fed mostly hay will most likely have a low vitamin E intake.

Horses with vitamin E deficiency are subject to increased oxidative stress and muscle pain after exercise. They may have weakened immune systems, and will often recover slowly. These horses are also at increased risk of frequent coughs and colds. Insufficient Vitamin E can make some equine neurological disorders worse and can cause neurological disease in foals, and horses of all ages. So let’s talk about it.

What Is Vitamin E?

Vitamin E isn’t actually a single compound. It is shared term used to encompass eight fat-soluble compounds called tocopherols (saturated) and tocotrienols (unsaturated). Tocopherols and tocotrienols have four structural variants called alpha, beta, gamma, and delta, with alpha-tocopherol being the form most common and bioactive to horses.

Of the different forms of alpha-tocopherol, RRR-alpha-tocopherol (more commonly knows as d-alpha-tocopherol) is the naturally occurring form found in plants and is more biologically available to horses than synthetic all-rac-alpha-tocopherol (better known as dl-alpha-tocopherol). This distinction is important when comparing supplements because two products containing the same number of International Units (IU) may not provide the same biological activity.

Fresh green forage is naturally rich in vitamin E, which is one reason horses with regular access to good-quality pasture generally receive more vitamin E than horses relying primarily on stored hay. Unfortunately, vitamin E is relatively unstable and its concentration declines during harvesting, drying, and storage. The longer hay is stored, the greater the potential loss. 

What Does Vitamin E Do in the Body?

Vitamin E’s best-known role is as a fat-soluble antioxidant. It helps protect cell membranes from oxidative damage by interrupting chain reactions involving free radicals and preventing the oxidation of polyunsaturated fatty acids within cell membranes.

Think of it as part of the body’s antioxidant defense system. Normal metabolism, exercise, immune activity, and other physiological processes generate reactive oxygen species. These molecules aren’t inherently bad — they are a normal part of biology — but excessive oxidative damage can harm lipids, proteins, and cellular structures.

Vitamin E helps keep that damage under control, particularly within the lipid-rich membranes of cells.

This is especially important in muscle and nervous tissue. Vitamin E works alongside other components of the antioxidant system, including selenium, to protect cells from oxidative damage. In horses, inadequate vitamin E has been associated with neuromuscular disorders including equine motor neuron disease (EMND) and equine degenerative myeloencephalopathy (EDM), as well as nutritional muscular dystrophy. 

Vitamin E also has functions beyond antioxidant activity. It is involved in cell signaling and gene regulation, supports normal immune function, and helps maintain normal cellular and neurological function.

How Is Vitamin E Absorbed and Metabolized?

Because vitamin E is fat-soluble, its absorption is closely connected to normal fat digestion.

After a horse consumes vitamin E, it enters the small intestine, where bile salts and dietary fat help incorporate it into micelles. These tiny structures allow vitamin E to come into contact with and cross the intestinal lining. Vitamin E is then packaged into lipoproteins, transported through the lymphatic system and bloodstream, and ultimately delivered to the liver and other tissues.

The liver plays a particularly important role in determining which form of vitamin E remains in circulation. A protein called alpha-tocopherol transfer protein (α-TTP) preferentially binds alpha-tocopherol and helps incorporate it into circulating lipoproteins, allowing it to be distributed throughout the body.

Vitamin E that isn’t retained or used is metabolized, primarily in the liver, into water-soluble metabolites that can be eliminated from the body. The details of vitamin E metabolism in horses are not as well characterized as they are in humans and laboratory species, and some of what we know about equine vitamin E metabolism is extrapolated from other animals. 

This fat-soluble nature is also why vitamin E absorption can be affected by problems with fat digestion or intestinal absorption. Horses with certain gastrointestinal or malabsorptive conditions may therefore have difficulty maintaining adequate vitamin E status even when dietary intake appears sufficient.

How Much Vitamin E Does a Horse Need?

This is where things get a little more complicated.

The 2007 National Research Council (NRC) recommends a minimum daily intake of 1-2 IU per kg body weight. For a 500-kg (1,100-lb) horse, this works out to approximately 500 IU/day for a sedentary adult, increasing to around 1,000 IU/day for a heavily exercised horse under the NRC framework.) However, these numbers are best thought of as baseline dietary recommendations, not necessarily the amount that every horse should receive as a supplement. In the two decades since the last Nutrient Requirements of Horses was researched and written, many nutritionists argue that these levels are too low. Thus, this is our “baseline for sustaining life.”

A horse consuming abundant fresh green pasture may obtain substantial vitamin E naturally. Conversely, a horse living primarily on stored hay will have considerably less dietary vitamin E available in forage. Research examining real-world management has found that suboptimal vitamin E status is relatively common, particularly in horses with limited pasture access or poor pasture quality.

Exercise can also influence vitamin E requirements. Prolonged aerobic exercise increases oxidative stress, and horses performing substantial amounts of endurance or other aerobic work may have greater needs. Horses consuming higher-fat diets may also have increased antioxidant requirements because lipid metabolism can increase oxidative load. 

Natural vs. Synthetic Vitamin E

When shopping for a vitamin E supplement, you’ll commonly encounter natural d-alpha-tocopherol and synthetic dl-alpha-tocopherol. Natural vitamin E contains the RRR-alpha-tocopherol form that is preferentially retained and utilized by the body. Synthetic vitamin E contains a mixture of stereoisomers, only one of which is the naturally occurring RRR form. As a result, natural vitamin E has greater biological activity per unit than synthetic vitamin E. This means you shouldn’t automatically compare products based solely on the number of IU on the label. The form of vitamin E matters.

Does Your Horse Need a Vitamin E Supplement?

Not necessarily.

A horse receiving abundant, high-quality fresh pasture may obtain plenty of vitamin E without additional supplementation. On the other hand, horses consuming primarily stored hay, particularly horses with no pasture access, are more likely to benefit from supplementation. That being said, horses can vary considerably in how they absorb, transport, metabolize, and utilize vitamin E. Even two horses of the same breed and build, living under the same conditions and consuming the same diet, can have very different circulating vitamin E levels and requirements.

Other factors that may warrant closer consideration include:

• Heavy or prolonged aerobic exercise
• High-fat diets
• Limited access to fresh pasture
• Growing horses
• Pregnant or lactating mares
• Certain gastrointestinal or malabsorptive conditions
• Allergies
• IR/EMS/PPID
• Existing vitamin E deficiency or deficiency-associated disease

For horses with suspected deficiency or neuromuscular disease, supplementation should be guided by a veterinarian. Blood alpha-tocopherol testing can also provide useful information about vitamin E status, although interpretation needs to take the individual horse and clinical picture into account. 

Can You Give Too Much?

Yes.

Vitamin E has traditionally been considered relatively safe, but more is not automatically better. The NRC established an upper safe dietary concentration of approximately 20 IU/kg body weight/day, equivalent to around 10,000 IU/day for a 500-kg horse under the assumptions used in its calculations. High-dose supplementation has also produced potentially undesirable effects in research horses, including interference with absorption of other nutrients. 

Therapeutic doses may be considerably higher in specific diseases, but those doses belong under veterinary supervision and shouldn’t be confused with routine supplementation for a healthy horse.

Summary

Vitamin E is essential, but it is worthwhile to perform bloodwork for horses if vitamin E deficiencies are suspected. And honestly, it’s not a bad idea to include a vitamin E test as part of your annual vet check-ups and bloodwork — especially if your horse’s diet is primarily hay, if they are IR/ID or have EMS, or have myopathies or other pathologies that increase oxidative stress..

If your horse’s blood come back as low, or you think your horse might benefit from additional vitamin E, you should first be asking: “How much vitamin E are they already getting from their diet?”

And remember: a balanced diet is about more than individual nutrients. Vitamin E works as part of a larger antioxidant and nutritional system, including selenium and other nutrients. The goal isn’t to maximize one nutrient, it is to provide the horse with what they actually need, in the right amounts, as part of an appropriately formulated diet.