Cell membrane integrity must be pristine to control sodium and calcium ion flow across muscle cell membranes to initiate muscle contraction. Other antioxidants, such as selenium, also help protect membranes from free radical damage.

“Several important medical conditions can result due to inadequate antioxidant status,” says Carrie Finno, PhD, DVM, Dipl. ACVIM, associate professor and Gregory L. Ferraro Endowed Director of the Center for Equine Health at the University of California, Davis, School of Veterinary Medicine. “Although similar in some ways, these conditions have very different clinical presentation and age of onset.”

Examples include:

• Vitamin E deficient myopathy is a reversible vitamin E deficiency that leads to muscle weakness and atrophy (wasting). Supplementing with water-dispersible formulations of natural vitamin E can reverse the condition.
• Equine motor neuron disease is a lower motor neuron disease (similar to Lou Gehrig’s disease, aka ALS, in people) that affects the spinal cord, peripheral nerves, and muscle. Affected horses often tremble and are reluctant to stand still. Vitamin E supplementation leads to improvement in up to 40% of cases, but this might not happen in some horses if the disease has progressed too far.
• Equine neuroaxonal dystrophy/degenerative myeloencephalopathy (eNAD/EDM) is an inherited neurologic disease that requires both a genetic predisposition and low vitamin E intake early in life. Affected foals show signs of incoordination, usually by 6 to 12 months of age. Treatment with vitamin E will not reverse clinical signs, but supplementing genetically susceptible dams can help prevent the disease in their foals. Although inherited, there is no genetic test currently available for eNAD/EDM.

“Green grass serves as the most important natural dietary source of vitamin E for horses,” says Finno. “As soon as grass is dried, the potency of vitamin E in the forage declines dramatically. Because there is no effective treatment for eNAD/EDM and limited success with treating EMND, it is imperative that we as caretakers prevent these conditions from developing in the first place.”

If horses are not able to access fresh pastures (or pastures with adequate levels of vitamin E), they’ll need supplemental vitamin E in their diets.

The National Research Council (NRC), the “nutrient bible” for horses, recommends horses receive 1-2 international units (IU) of vitamin E per kilogram of body weight. This means an average 1,000-pound horse needs 450-900 IU of vitamin E every day.

“Blood vitamin E levels in healthy, grazing horses fall between 3 and 5 µg/mL, and this is the level that we should try to achieve and maintain in horses we are supplementing with vitamin E,” Finno said.

Because some horses require more or less vitamin E than others, the only way to provide individualized supplementation is to measure blood vitamin E levels. Further, blood vitamin E levels can vary depending on the type of supplement owners offer.

Finno explains that the first generation of vitamin E supplements were termed “synthetic” because they contained alpha-tocopherol (as alpha-tocopherol acetate), the main antioxidant component of vitamin E, in all of its different chemical configurations. Later, research demonstrated that only the “natural” form of vitamin E effectively increased vitamin E levels in horses’ bloodstreams. Most recently, alcohol-based vitamin E supplements, termed “water-dispersible,” were developed for use in horses. These are the most effective supplements because the water-dispersible formulation is readily absorbed and the natural vitamin E quickly increases blood and tissue levels in most horses.

Considering vitamin E’s long list of benefits in multiple species, it’s not surprising that the equine industry enthusiastically embraced offering vitamin E supplements.

“Historically, there was little concern associated with giving too much vitamin E,” says Finno. “This was mainly due to the fact that most vitamin E supplements were not very well absorbed in the horse and the NRC levels were based on these synthetic vitamin E supplements. With newer formulations, however, excessive supplementation can easily occur.

“Luckily, studies show that detrimental effects may not occur until supplementation reaches 10-20 times the recommended amount,” she adds. “However, that study used synthetic vitamin E products. With the water-dispersible natural formulation, excess levels can occur quite easily. This reiterates the importance of frequently checking blood levels in supplemented horses.”

Given the complexity of vitamin E supplementation, Finno encourages horse owners to talk with their veterinarians or equine nutritionists before adding supplements to their horses’ diets. She also stresses the importance of checking supplemented horses’ blood vitamin E levels regularly to ensure they remain within the normal range.