Vitamin D in horses
What is Vitamin D?
Vitamin D exists in several forms. The most common vitamin D forms are ergocalciferol (D2) and cholecalciferol (D3). Contrary to popular belief, vitamin D does not come directly from sunlight, however it is activated from a precursor form into its active form by sunlight. The D2 version is produced by plants and fungi when they are exposed to sunlight, and then are absorbed by the horse when it eats the plants. D3 is made in the horse’s (and human’s) body from cholesterol during a chemical reaction in the skin when exposed to sunlight(1). Therefore, horses make their own vitamin D just by being in the sunlight.
Vitamin D is one of the fat-soluble vitamins, meaning it requires ingestion with a fat or oil to be absorbed by the body(2). Plants do have a small percentage of fats which aid in absorption of the vitamin D contained in those plants.
What role does vitamin D have in the body?
Vitamin D regulates skeletal health by controlling the absorption of calcium, magnesium and phosphate from the intestines. These three minerals are the major building blocks of your horse’s bones and helps keep them strong(1). Vitamin D also regulates the reabsorption of calcium from the kidneys and controls the flow of calcium into and out of the bones(2).
Vitamin D is thought to have a role in the immune system and help resist infection, susceptibility to cancer, and survival under compromised circumstances(1). Vitamin D has also been shown to influence cell growth(2).
Vitamin D deficiency
Just like humans who don’t go out in the sunlight much become vitamin D deficient, there is a concern that horses stabled indoors for large portions of the day or full time may become vitamin D deficient. This is of particular concern as vitamin D deficiency might be a factor that could increase the risk of skeletal injury(1). If there is not enough vitamin d produce or the diet is low in then your horse can develop poor muscle contractions, bone deformities and even bone fractures(3).
A vitamin D deficiency may also predispose a horse to inflammatory airway diseases such as equine asthma (heaves) and other ailments often found in stabled horses. Vitamin D deficiency is not a concern for horses who are outdoors for at least part of the day. There are no known cases to date of vitamin D deficiency in horses maintained in a practical setting with light exposure(2).
Vitamin D toxicity
Normal management of horses with full-time turnout in sunlight will not result in a toxic level of vitamin D3. However, over supplementation of vitamin D can lead to calcification of soft tissue (including the heart, kidneys and blood vessels(3)) and dealth(2). Vitamin D should only be supplemented if a horse is housed inside full time with little to no sun exposure and it is recommended by a veterinarian.
Dietary Sources
Vitamin D is found in both plants and animals. Grasses will have sufficient vitamin D for your horse. Sun-cured hay, particularly alfalfa, has some vitamin D2. Most commercial feeds are low in vitamin D content, so should not result in toxicity.
Vitamin D supplements
A majority of vitamin D supplements are in the form of vitamin D3. There is no current research that details whether vitamin D2 or D3 are more biologically available, however it is thought that D2 might be more active(1).
References
1. Riggs, C. M. (2019) The significance of Vitamin D in horses, in WEVA – International Congress – Verona, 2019.
2. NRC (2007) Nutrient Requirements of Horses. 6th revised edition. The National Academies Press: Washington, DC.
3. Getty, J. M. (2010) Feed your horse like a horse. Dog Ear Publishing: USA.
