Veterinarians frequently suggest supplementing the diets of horses with certain neurologic disorders with vitamin E. But why? What disorders does it help? And which of the eight types of vitamin E supplement is most beneficial? Fortunately, it's not quite as complicated as it might seem.
At the 2013 American College of Veterinary Internal Medicine Forum, held June 12-15 in Seattle, Wash., Carrie Finno, DVM, PhD, Dipl. ACVIM, presented a review of vitamin E supplementation in horses with neurologic disorders.
Vitamin E Basics
Vitamin E is a fat-soluble biologic antioxidant believed to help horses maintain normal neuromuscular function, explained Finno, a researcher in the University of Minnesota Equine Neuromuscular Diagnostic Laboratory.
"Vitamin E … is a complex nutrient consisting of eight closely related, fat-souble, naturally occurring compounds that form two groups: tocopherols (saturated) and tocotrienols (unsaturated)," she said. "There are four individual isoforms—α, β, γ, and δ—within each group. The majority of the commercially available vitamin E supplements for horses contain natural or synthetic α-tocopherol, the most biologically active (or available) and researched isoform.”
Measuring blood serum or plasma α-tocopherol concentrations is the easiest way to determine if a horse has adequate vitamin E levels, Finno said. Measuring α-tocopherol concentrations in adipose (fat), muscle, and liver tissue or cerebrospinal fluid (CSF) can also help determine a horse's vitamin E status, although samples are decidedly more complicated to obtain.
Finno said a horse has adequate vitamin E levels if the plasma or serum α-tocopherol concentration measures more than 2 µg/mL (microgram per milliliter), has marginal concentrations if it ranges from 1.5 to 2 µg/mL, and is considered deficient if the concentration falls below 1.5 µg/mL.
Many horses with vitamin E deficiencies show no outward ill effects, Finno said. A select few, however, develop signs of neuromuscular disease.
"Development of neuromuscular disease appears to depend upon the age of the animal when the deficiency develops, the duration of the deficiency, the genetic make-up of the individual, and other concurrent dietary deficiencies or excesses," she explained.
Some of the disorders linked to vitamin E deficiencies include neuroaxonal dystrophy and equine degenerative myeloencephalopathy, vitamin E deficient myopathy, and equine motor neuron disease. While vitamin E deficiencies play a role in these issues, veterinarians must consider other factors during diagnosis, Finno said: "The diagnosis of (these) neuromuscular disorders … requires not only determination of vitamin E status, but also the appropriate clinical signs, supporting clinical pathology and/or muscle biopsy results, and elimination of other possible diseases."
Neuroaxonal Dystrophy and Equine Degenerative Myeloencephalopathy (NAD/EDM)—Finno said NAD and EDM are two closely related disorders differentiated by the location of the axonal degeneration in the horse's nervous system. "Although the pathophysiology is not completely defined, there is strong evidence of a genetic component whose expression is highly influenced by vitamin E deficiency during the first year of life," she said.
Most affected horses begin showing signs by the time they are six to 12 months old, Finno said. Common clinical signs include symmetric ataxia (incoordination), a base-wide stance, and proprioceptive deficits (lack of physical awareness of limbs and their placement). Additionally, most affected horses have low serum α-tocopherol levels, she said.
The gold standard diagnostic test for NAD/EDM is a post-mortem histopathologic examination. Therefore, veterinarians typically look at clinical signs and serum α-tocopherol levels and rule out other neurologic disorders to diagnose the disorder in the living horse, Finno said.
Unfortunately, NAD/EDM isn't treatable, and there haven't been any cases of "spontaneous resolution," Finno said. And while many NAD/EDM horses are treated with vitamin E in hopes of resolving the neurologic signs, "there is strong evidence that vitamin E supplementation of affected horses does not lead to neurologic improvement," she said.
The good news is that breeders might be able to help reduce the likelihood of NAD/EDM in future generations: "Supplementation of dams with vitamin E during pregnancy may lead to a decreased incidence of NAD/EDM during the next foaling season," Finno said.
Vitamin E Deficient Myopathy (VEM)—Finno said some vitamin E deficient horses demonstrate muscle atrophy and weight loss without changes in the motor nerves, known as VEM. She explained that it's still not clear if this disorder is its own entity or whether it's a precursor to equine motor neuron disease.
VEM horses present with a loss of muscle mass, toe dragging, poor performance, and muscle weakness and fasciculations (twitching), Finno said.
Veterinarians can diagnose VEM using several different tools. The most specific indicator, she said, is a certain histopathologic finding in a sacrocaudalis dorsalis muscle (located above the tailhead on either side of the spine) biopsy. While some affected horses have low serum α-tocopherol levels, all cases to date have had low muscle α-tocopherol concentrations, Finno said.
Therefore, if the serum α-tocopherol concentration is normal, it does not exclude VEM and it is still worthwhile to have a muscle biopsy performed in suspect cases.
"Horses with VEM are remarkably responsive to appropriate vitamin E supplementation and show a complete recovery," Finno said.
Equine Motor Neuron Disease (EMND)—The final disorder Finno described was EMND, an acquired condition that affects horses' motor nerves. Researchers are still working to understand exactly what causes this disorder but know EMND is associated with a diet devoid of vitamin E.
Horses with EMND typically have low serum α-tocopherol levels and have consumed a vitamin E deficient diet for at least 18 months, Finno said. Other clinical signs include muscle wasting and fasciculations, frequent weight shifting in the hind limbs, low head carriage, and, in some cases, recumbency (inability to rise).
Choosing a Supplement
Choosing a supplement—regardless if it’s a joint health product, a digestive aid, or one of the many other types of feed additives—can be challenging. With the myriad of products to choose from, owners have a lot to consider before making a decision, and vitamin E supplements are no exception. With natural and synthetic types to choose from, what should an owner look for?
Carrie Finno, DVM, PhD, Dipl. ACVIM, a researcher in the University of Minnesota Equine Neuromuscular Diagnostic Laboratory, explained, "Natural vitamin E is composed of one isomer (RRR-α-tocopherol), and it is the most bioactive form in animal tissues. Synthetic vitamin E, also called dl-α-tocopherol, is a mixture of eight isomers, of which only one is identical to the natural isomer. These eight isomers vary greatly in relative biopotency."
Finno referenced one recent study in healthy horses that found natural vitamin E formulas had up to 2.52 times the bioavailability of the synthetic vitamin E formula. Additionally, she said, two commercially available water-dispersible natural vitamin E products (Elevate WS and Nano-E) had 5.59 and 6.13 times the bioavailability of a synthetic vitamin E product, respectively. Both products produced a significant increase in horses' serum α-tocopherol levels, while the synthetic product did not, she added.
"It is important to realize that the absorption and metabolism of vitamin E in healthy horses may differ from horses with a vitamin E deficiency," she cautioned. "Thus, the choice of the type of vitamin E supplement should differ depending upon whether a maintenance supplement is being selected for a horse with normal serum α-tocopherol or whether there is a need to rapidly increase serum α-tocopherol in a horse with low serum α-tocopherol and clinical signs of vitamin E deficiency."
If questions arise, consider consulting a veterinarian or equine nutritionist for assistance.
EMND diagnosis is based on clinical signs along with histology and serologic tests. She said pathologists often examine a tailhead muscle biopsy for histopathologic abnormalities typically associated with EMND, or they identify myelinated axon degeneration on a biopsy of the spinal accessory nerve's ventral branch.
Veterinarians typically treat EMND with high doses of α-tocopherol, Finno said. About 40% of cases show improvement within six weeks, she said, and affected horses could appear "normal" within three months. She cautioned that putting affected horses back into work could cause their condition to deteriorate again.
To Supplement or Not To Supplement
So should all horses receive supplemental vitamin E? Not necessarily. Although vitamin E doesn't accumulate in horses' bodies to the point of toxicity, some disorders—including coagulopathy (impaired blood clotting) and impaired bone mineralization and beta-carotene absorption—have been reported in horses with very high dietary vitamin E concentrations, Finno said.
The current vitamin E recommendation for mature horses is 1 to 2 IU/kg (international units per kilogram) body weight per day, she said.
"Before implementing supplementation it is important to measure serum α-tocopherol concentrations in order to determine if there is an underlying deficiency and in order to monitor the efficacy of supplementation," Finno said.
If supplementation is required, "natural-source, water-dispersible forms of vitamin E … seem like the obvious choice for optimal treatment as they are five to six times more bioavailable (a greater amount is absorbed by and made available in the horse's body) than synthetic vitamin E."
Vitamin E is an important part of a horse's diet, especially one at risk of or affected by neuromuscular disorders. It is important to work with a veterinarian to treat neurologic problems, if present, or to determine if a particular horse is vitamin E deficient and would benefit from supplementation.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.