Aspect Key Points
Importance Vitamin D is essential for bone health, muscle function, and immune response.
Bone Health Facilitates calcium absorption, preventing weakened bones and rickets.
Muscle Function Ensures adequate calcium for muscle contraction, crucial for athletic horses.
Immune Response Modulates immune and inflammatory responses, aiding in infection defence and injury recovery.
Sources Sunlight and dietary intake (sun-dried forage like hay).
Sunlight Horses synthesize vitamin D3 through UV exposure, but in negligible amounts.
Dietary Sources Reliable source of vitamin D2 from plants and high-quality hay.
Supplementation Necessary for horses with limited sunlight exposure or restricted diets.
D2 vs. D3 D2 is a safer, more natural option, while excessive D3 can disrupt vitamin D metabolite balance.

Vitamin D for Horses: Importance and Sources

Vitamin D, often referred to as the "sunshine vitamin," plays a crucial role in the health and well-being of horses. This essential nutrient is involved in various bodily functions, including bone health, muscle function, and immune response. Understanding the importance of vitamin D and its sources can help horse owners ensure their animals receive adequate nutrition for optimal health.

The Importance of Vitamin D for Horses

Vitamin D is vital for maintaining calcium and phosphorus balance in the body, which is essential for healthy bone formation and maintenance. It works in conjunction with calcium and parathyroid hormone to regulate the absorption of these minerals, supporting the development and integrity of bones and teeth. Additionally, vitamin D is involved in muscle function and immune system modulation, making it a critical nutrient for overall health.

Bone Health and Calcium Homeostasis

Vitamin D's primary role in bone health is to facilitate the absorption of calcium from the diet. Without sufficient vitamin D, horses may struggle to absorb enough calcium, leading to weakened bones and conditions such as rickets. Although rickets is rare in horses, it underscores the importance of maintaining adequate vitamin D levels to prevent skeletal issues.

Horse eating hay for Vitamin D

Muscle Function and Immune Response

Vitamin D also plays a role in muscle function by ensuring adequate calcium availability for muscle contraction. This is particularly important for athletic horses, such as racehorses, which require optimal muscle performance. Moreover, vitamin D has been shown to modulate immune and inflammatory responses, helping horses fend off infections and recover from injuries more effectively.

Sources of Vitamin D for Horses

Horses can obtain vitamin D from two primary sources: sunlight and diet. However, the effectiveness of these sources can vary based on several factors, including geographical location, management practices, and individual horse characteristics.

Sunlight Exposure

When horses are exposed to ultraviolet (UV) light from the sun, a chemical reaction in their skin produces vitamin D3 (cholecalciferol). This form of vitamin D is then stored in fat cells and later processed by the liver and kidneys into its active form. However, studies have shown that horses synthesize negligible amounts of vitamin D3 from sunlight, making dietary sources more critical for maintaining adequate levels.

Horse basking in sunlight

Dietary Sources

Dietary intake is the most reliable source of vitamin D for horses. Vitamin D2 (ergocalciferol) is found in plants, particularly in sun-dried forage such as hay. Horses that graze on pasture or consume high-quality hay typically receive sufficient vitamin D2 to meet their needs. For stabled horses or those with limited access to grazing, vitamin D supplementation may be necessary to prevent deficiencies.

Vitamin D Supplementation

Supplementation can be particularly important for horses with limited sunlight exposure or those on restricted diets. Thoroughbred racehorses in Hong Kong, for example, have been shown to require vitamin D supplementation due to their stabled conditions and lack of grazing opportunities. These horses often receive vitamin D3 supplements in their feed to maintain adequate serum levels of the nutrient.

Horse supplement product

Balancing Vitamin D2 and D3

While both vitamin D2 and D3 can be used to supplement horses, there is evidence suggesting that D2 may be a more natural and safer option. Naturally managed horses primarily rely on dietary vitamin D2, and studies have indicated an inverse relationship between serum concentrations of 25-hydroxyvitamin D2 (25OHD2) and 25-hydroxyvitamin D3 (25OHD3). This suggests that excessive D3 supplementation could potentially disrupt the balance of vitamin D metabolites in the body.


Ensuring that horses receive adequate vitamin D is essential for their health and performance. While sunlight exposure can contribute to vitamin D synthesis, dietary intake remains the most reliable source. High-quality forage and, when necessary, appropriate supplementation can help maintain optimal vitamin D levels, supporting bone health, muscle function, and immune response. By understanding the importance and sources of vitamin D, horse owners can make informed decisions to promote the well-being of their animals.

Horse in pasture

For more detailed information, you can visit the Your Horse magazine and the Kentucky Equine Research websites.

Horse health supplements

For more detailed information on how to ensure your horses are getting enough Vitamin D, check out the latest studies on the National Center for Biotechnology Information and Thoroughbred studies in Hong Kong.

Horse in sunlight

Additionally, for a deeper dive into vitamin D and its effects on horses, you can refer to these scientific articles and research studies.


What are the sources of vitamin D in animal feed?

Vitamin D in animal feed primarily comes from sun-dried forages such as hay, which contain vitamin D2 (ergocalciferol). Additionally, commercial feeds and supplements can provide vitamin D3 (cholecalciferol) to ensure horses receive adequate amounts, especially if they have limited access to sunlight.

What is the best source of vitamins for horses?

The best sources of vitamins for horses are high-quality forages like hay and pasture grass, which provide a range of essential vitamins including vitamin D2. Supplementation may be necessary for horses with specific dietary needs or limited grazing opportunities. Always ensure supplements are tailored to the horse's specific health requirements.

What is vitamin D give its sources and importance?

Vitamin D is an essential nutrient involved in calcium and phosphorus balance, crucial for bone health, muscle function, and immune response. Horses obtain vitamin D from sunlight exposure and dietary intake, primarily from sun-dried forage such as hay. Supplementation can be necessary for horses with limited sunlight exposure or specific dietary restrictions.

What causes vitamin D deficiency in horses?

Vitamin D deficiency in horses can be caused by inadequate exposure to sunlight, as UV light is needed for the skin to synthesize vitamin D3. Additionally, poor dietary intake of vitamin D-rich foods, such as sun-dried forage, or the lack of appropriate supplementation in stabled horses can lead to deficiencies. Other factors include geographical location, management practices, and individual horse characteristics.