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Renowned for its roles in bone health and maintenance of phosphorus and calcium balance, vitamin D might have different physiological functions in horses than once thought, according to Danish researchers*. Because of this, current recommendations for vitamin D requirements—6.6 IU/kg body weight based on recommendations by the National Research Council**—might not be optimal for horse health.

“Much of our understanding of the role of vitamin D in horses comes from human research or information extrapolated from other species,” said Kathleen Crandell, Ph.D., a nutritionist at Kentucky Equine Research. “This means that what we thought we knew about horse’s dietary requirements for vitamin D might not, in fact, be correct or complete.”

Key points in the review on vitamin D included the following:

  • The physiology of vitamin D (i.e., the way it “works”) in horses may be very different than other species;
  • It is widely accepted that horses can obtain vitamin D via exposure to sunlight  and through dietary sources (i.e., vitamin D supplements or roughages), but this “fact” has not been proven;
  • Circulating vitamin D levels in the bloodstream appear to be very low in horses compared with other species; and
  • Vitamin D appears less involved in maintaining a normal balance between calcium and phosphorus in horses than other animals.

Based on their comprehensive review of the literature, these researchers concluded that overall, our knowledge of vitamin D requirements for horses is based on outdated literature.

Interestingly, the researchers also noted, “…the use and management of horses has changed dramatically during the last 25 to 50 years. Hence, research in the vitamin D physiology and nutrition of modern riding horses is highly necessary, before a much-needed update on the recommended vitamin D supplementation of horses can be carried out.”

*Hymøller, L., and S.K. Jensen. 2015. We know next to nothing about vitamin D in horses! Journal of Equine Veterinary Science. 35:785–792.

**NRC. 2007. Nutrient Requirements of Horses, 6th Rev. Ed. National Academies Press. Washington, D.C.

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