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In the complement of nutrients required by horses, vitamin E ranks high in importance by horse owners, likely because of its reputation as an antioxidant and because adequate intake staves off neuromuscular disease. As one recent study shows, though, even supplemented horses that appear to consume more than enough vitamin E may not have adequate levels in their bodies.*

“Horses obtain vitamin E primarily from green pasture,” said Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research. “However, horses in certain modern management situations are not allowed to graze. In these instances, horses should be supplemented with vitamin E.”

Horses require 1-2 international units (IU) of vitamin E per kilogram body weight in their daily diet. An average 500-kg (1,100-lb) horse would need 500-1,000 IU of vitamin E daily.

In a recent study, researchers set out to determine the effect of supplementation on vitamin E and selenium levels of 330 horses living in Oregon, a geographical region with selenium-deficient soil. Estimated vitamin E intake was determined by the amount of pasture turnout and owner-reported supplementation. Access to fresh, green grass for more than six hours per day was assumed to provide 500 IU of vitamin E. In addition, information regarding the amount of supplemental vitamin E offered horses was recorded. To determine vitamin E status, plasma samples from the horses were tested.

“The amount of vitamin E in hay was considered minimal because the vitamin E content in forage declines rapidly once harvested,” Crandell explained.

The researchers drew three key conclusions from the study:

  • Suboptimal blood concentrations of vitamin E were identified in about one-third of the horses included in the study;
  • Horses with less than six hours of fresh grass were more likely to have deficient or marginally low levels of vitamin E in their blood than horses grazing more than six hours a day; and
  • Despite the fact that over 85% of owners offered their horses supplemental vitamin E, nearly 40% of horses in the study were undersupplemented, with less than 500 IU vitamin E offered on a daily basis.

“The study revealed 20% of the horses that appeared to have adequate vitamin E intake actually had low levels of vitamin E when plasma was tested,” said Crandell.

According to the researchers, low vitamin E levels despite seemingly appropriate intake could be rationalized in several ways, including the type of vitamin E (natural versus synthetic), quality of the supplement, failure of the horse to absorb the vitamin E from the gastrointestinal system, amount of fat in the horse’s overall diet, and age of the horse, which could affect absorption rate.

“Kentucky Equine Research offers Nano-E, a water-soluble natural vitamin E supplement. When used correctly, this helps avoid preventable diseases associated with vitamin E deficiency,” Crandell explained. “When supplementing with vitamin E, consider having your horse’s vitamin E levels tested routinely and alter your supplementation approach based on those results,” Crandell advised.”

*Pitel, M.O., E.C. McKenzie, J.L. Johns, and R.L. Stuart. 2020. Influence of specific management practices on blood selenium, vitamin E, and beta-carotene concentrations in horses and risk of nutritional deficiency. Journal of Veterinary Internal Medicine 34:2132-2141.

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