We Know Next to Nothing About Vitamin D in Horses!

•We know next to nothing about vitamin D in horses and requirements need an update.•The vitamin D physiology and nutrition of horses are very different from other species.•Circulating levels of vitamin D metabolites in horses are very low.•It is unknown if horses can synthesize endogenous vitamin D in the skin.•The natural supply from roughage is unknown.

Very few references on vitamin D in horses exist, but the limited research available suggests that the vitamin D physiology of horses may be very different from other species. Horses can obtain vitamin D both through endogenous synthesis in the skin during sunlight exposure and through dietary sources either from synthetic vitamin D supplements or the natural vitamin D content of roughages. However, regardless of the source of vitamin D, circulating levels of vitamin D metabolites in plasma are generally reported to be very low in horses and vitamin D appears less involved in maintaining normal calcium and phosphorus homeostasis in horses than in other species. Current recommendations on the vitamin D supplementation of horses are based on a scarce amount of more or less outdated literature. Very little research has been carried out regarding the vitamin D physiology of horses and even less regarding the efficiency of different sources of vitamin D in the nutrition of horses. Furthermore, 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.