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Is Your Horse Getting Enough Vitamin D?By Kentucky Equine Research Staff · January 19, 2017

Vitamin D plays many important roles in the physiology of the horse, such as enabling calcium absorption and maintaining bone health. Several studies also show that vitamin D supports the immune system, especially when confronted with respiratory illnesses such as inflammatory airway disease (IAD) and influenza.

One recently published study in human subjects*, however, questioned this theory, finding no association between low vitamin D levels and risk of influenza infection.

Despite the fact that the recent human study did not find an association between low circulating vitamin D levels and risk of infection with the influenza virus, the study authors did note that several other studies have shown that lower vitamin D levels in the bloodstream are associated with a higher risk of respiratory tract infections.

While it is important to appreciate that results from human studies may not be applicable to horses because of the different ways humans and horses metabolize vitamin D, that study highlights the potential role of vitamin D in preventing illness, not just supporting bone health, and the need for more research. For example, a recent equine study noted that sick neonatal foals frequently have low vitamin D levels, which appear to be associated with increased disease severity and mortality**.

Horses obtain vitamin D from both their diet and the sun, using ultraviolet rays to produce vitamin D in the skin. Interestingly, the vitamin itself isn’t active in the body; it must first be metabolized into a biologically active form.

At present, optimal dietary and circulating levels of vitamin D in horses remain unclear. Current recommendations for vitamin D by the National Research Council+ indicate that horses not exposed to sunlight should receive 6.6 IU vitamin D/kg body weight; however, horses exposed to sunlight are able to meet their vitamin D requirements without supplementation.

If you’re concerned about your horse’s vitamin D status, consult with a Kentucky Equine Research (KER) nutrition advisor. Horses normally have lower circulating levels of vitamin D than other mammals, and blanketing in the winter can potentially inhibit a horse’s ability to synthesize vitamin D.

KER offers a low-calorie supplement, Micro-Max, appropriate for horses and ponies that require supplementation but do not need the extra energy provided by concentrates. Micro-Max contains vitamin D as well as a number of other nutrients important to equine health. Horse owners in Australia should look for Gold Pellet.

*Xu, C, V.J. Fang, R.A. Perera, et al. 2016. Serum 25-hydroxyvitamin D was not associated with influenza virus infection in children and adults in Hong Kong, 2009-2010. Journal of Nutrition. 146(12):2506-2512.

**Kamr, A.M., K.A. Dembek, S.M. Reed, et al. 2015. Vitamin D metabolites and their association with calcium, phosphorus, and PTH concentrations, severity of illness, and mortality in hospitalized equine neonates. PLoS One. 10(6):e0127684.

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