English
Vitamin D is a fat-soluble vitamin that is essential for horses, influencing bone health, muscle function, and the immune system. Adequate vitamin D intake is considered important for growing horses to support normal bone development.
In horses, vitamin D is believed to play a role in regulating levels of calcium and phosphorus in the blood. This vitamin may influence the absorption of these minerals from the diet, their storage and release in bone, as well as their excretion in urine.
Vitamin D is abundant in fresh forages and sun-cured hay, but levels decrease with prolonged storage. While mature, idle horses can get enough vitamin D from forage, supplementation may be warranted for growing horses, lactating and pregnant mares, or horses fed old hay.
Like other mammals, horses can also synthesize some vitamin D when exposed to sunlight. However, endogenous production of this vitamin is limited, suggesting that dietary sources are more important to consider.
Vitamin D for Horses
Vitamin D is different from other vitamins in the equine diet because it functions more like a hormone than a traditional nutrient. It circulates in the bloodstream and attaches to vitamin D receptors in various tissues throughout the body.
This binding mechanism is characteristic of hormones, which act as messengers to regulate bodily functions. Through this mechanism, vitamin D can influence a wide range of physiological processes. [4][5]
Physiological Function
In human health, vitamin D is well-known for its roles in supporting bone health and maintaining normal blood levels of the minerals calcium and phosphorus. This vitamin also plays key roles in the immune system, regulating inflammation, and inhibiting cancer cell proliferation. [3]
In horses, the metabolism and function of vitamin D remains an active area of research. Current studies indicate that horses have significantly lower blood levels of vitamin D compared to other species. Additionally, these levels in horses appear to be less influenced by dietary modifications and sunlight exposure.
Vitamin D also appears to be less important for maintaining normal calcium and phosphorus levels in horses compared to other animals. [3] However, it is clear that horses do require vitamin D for normal skeletal development and can be susceptible to vitamin D deficiency, particularly early in life.
Calcium and Phosphorus Regulation
The most well-described role of vitamin D in other species is maintaining calcium and phosphorus homeostasis. Homeostasis is the process by which a biological system maintains a stable internal environment despite changes in external conditions.
Maintaining normal calcium and phosphorus levels in the blood is critical for bone strength, muscle function, energy production and more. Vitamin D exerts an influence on several tissues to regulate levels of these minerals in the blood: [1][2][6]
- Bone tissue: Vitamin D is involved in balancing the processes of bone breakdown and synthesis, which affects calcium concentrations in the blood.
- Intestinal tract: Vitamin D stimulates the absorption of calcium and phosphorus from the gut.
- Kidneys: Vitamin D helps reduce urinary losses of these minerals by promoting their reabsorption from the kidneys.
This understanding is based on research in other species and may not reflect the physiological role of vitamin D in horses. Studies indicate that horses do not rely on vitamin D for calcium absorption in the gut and they tend to excrete more calcium in their urine. [3]
Other Roles
Other physiological roles of vitamin D have recently been described:
- Tissue repair: Vitamin D may activate satellite cells in the skeletal muscle of horses. These are stem cells that aid in tissue repair after exercise. [7]
- Inflammation and immune function: In animal models, vitamin D deficiency increases susceptibility to infection and autoimmune disorders. [8][9]
- Cell growth and differentiation: In animal models, vitamin D has been observed to regulate cell growth, division, and function. [8][9]
Further research is necessary to establish whether these additional roles of vitamin D, observed in other animal models, are also significant for the health and well-being of horses.
Vitamin D Requirements
Vitamin D requirements for horses are established by the NRC’s Nutrient Requirements of Horses (2007) and are largely estimated from research in other species. [6]
It is thought that most horses can meet their vitamin D requirements primarily through consuming forages, and to a lesser extent, from synthesis in the skin. [2][6]
However, there are instances where horses may require supplementation to prevent a deficiency in vitamin D.
Mature Horses
For mature, non-exercising horses, the estimated daily vitamin D requirement is 6.6 IU per kg of body weight. [6] This is approximately 3,300 IU of vitamin D per day for a horse weighing 500 kg (1100 lb).
Mature horses that have access to fresh grass hay likely do not need vitamin D added to the diet. However, horses that are consuming artificially dried hay or hay that has been stored for a long time may need supplemental vitamin D. [2][6]
Growing Horses
To support healthy bone development, growing horses have a higher requirement for vitamin D. [6] This requirement is greatest during the first six months of life when growth is most rapid.
By five years of age, the horse is assumed to have reached its mature weight and the vitamin D requirement stabilizes at 6.6 IU per kilogram of body weight, aligning with the needs of adult horses.
For growing horses, vitamin D requirements are as follows: [6]
- 0 – 6 months: 22.2 IU per kg of body weight
- 7 – 12 months: 17.4 IU per kg of body weight
- 13 – 18 months: 15.9 IU per kg of body weight
- 19 – 24 months: 13.7 IU per kg of body weight
Broodmares
Broodmares may require more vitamin D to support the needs of their developing foal and milk production. Research shows that mare’s milk contains higher concentrations of vitamin D than human milk. [22] However, the 2007 NRC guidelines do not indicate an increased requirement for vitamin D in pregnant or lactating mares.
More recently, experts recommend providing supplemental vitamin D to these mares starting in late pregnancy and continuing through lactation. [1][2]
Exercising Horses
Similar to broodmares, the NRC currently does not indicate an increased vitamin D requirement for exercising horses. However, young horses in training programs should be supplemented to ensure they obtain enough of this vitamin for skeletal development and muscle health. [1]
Assessing Vitamin D status
A blood sample can be taken to evaluate the vitamin D status of your horse. Normal concentrations in the blood range from 1.9 – 18 ng / mL of vitamin D. [2]
Like other species, a horse’s circulating vitamin D concentration is affected by location and time of year. Your veterinarian will need to consider these factors when assessing your horse’s vitamin D status. [2][16]
Vitamin D Deficiency
Overt vitamin D deficiency has not been reported in horses under normal dietary and management conditions. However, other issues may arise due to low vitamin D status.
For example, in hospitalized foals low vitamin D may contribute to inflammation along with irregular calcium and phosphorus levels. [17]
Rickets
Rickets is the most well-studied disease caused by vitamin D deficiency in growing animals. It is characterized by bone deformities due to decreased calcium and phosphorus content of bone and cartilage. [6]
Despite experimental attempts to induce rickets in horses through controlled deprivation of sunlight and dietary vitamin D, only milder forms of impaired bone growth have been observed. This outcome suggests that horse