Vitamin D for Horses: Benefits, Dietary Sources & Signs of Deficiency

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.

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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 horses may have a different physiological response to vitamin D deficiency compared to other species. ^[3][6]

It is worth noting that rickets is not the only issue caused by vitamin D insufficiency; it is simply one of the most well-studied.

Sources of Vitamin D

There are two main forms of vitamin D in the horse’s body: ^[3]

• Vitamin D₂ (ergocalciferol), which is primarily obtained from plant sources, such as forage.
• Vitamin D₃ (cholecalciferol), which is synthesized in the skin through exposure to sunlight or supplied in feeds and supplements.

Vitamin D₂ is the main form of this vitamin found in the horse’s bloodstream. Unless supplemented, horses have almost undetectable levels of vitamin D₃ in their blood. ^[3][11][20]

This suggests that natural dietary intake from forages is a more significant contributor to vitamin D levels than synthesis in the skin due to sun exposure. ^[11][12]

Vitamin D in Forages

The vitamin D₂ found in forages is not produced by the plants themselves, but rather by fungi growing on them. During the haymaking process, these fungi begin to synthesize vitamin D₂ in response to sun exposure.

These organisms have a cholesterol-like molecule in their cell membrane known as ergosterol, which is converted to vitamin D₂ when exposed to sunlight. ^[19]

The fungi continue to produce vitamin D after grass is cut to make hay, so long as they receive sunlight. For this reason, sun-cured hay has higher vitamin D content than artificially-dried forages that are not exposed to sunlight. ^[1]

Forage Concentrations

The amount of vitamin D in forage is variable and depends on the level of sun exposure during the wilting process. Factors influencing this include: ^[19]

• Geographical region
• Time of year
• Weather conditions
• Wilting time in field

Vitamin D₂ is also most abundant on the leaves of the plant. As a result, the forage species and maturity influences vitamin D levels, with leafy alfalfa hay generally having higher concentrations than grass hay. ^[6][13]

Vitamin D content of different types of forages (dry matter basis) ^[1]

Storage Conditions

The vitamin D content of hay is not stable and will degrade over time when the hay is stored. ^[2] This gradual reduction in vitamin levels is also true for other nutrients, such as vitamin E, vitamin C, and beta-carotene.

Consequently, horses that are fed hay which has been stored for an extended period may need supplementation to avoid nutrient deficiencies.

Vitamin D Synthesis in the Skin

Many animals can produce vitamin D₃ in the skin when exposed to ultraviolet radiation from the sun. ^[14] In this process, the fat compound 7-dehydrocholesterol is cleaved by UV light, creating a precursor to vitamin D₃.

This precursor molecule either spontaneously converts to vitamin D₃ within the skin or undergoes active conversion by enzymes in the liver and kidneys​​. ^[3]

Seasonal Variation

The efficiency of converting 7-dehydrocholesterol into vitamin D₃ in horses is largely dependent on the intensity of sunlight. ^[3] Notably, the angle of the sun changes during the winter, causing light in the UV-B spectrum to be deflected.

Horses that live north of 51°N latitude, which includes most of Canada, do not synthesize vitamin D in their skin between September and April. ^[3] These horses may be more susceptible to vitamin D deficiency, although this seasonal effect is less pronounced than in other species. ^[15]

Exposure to Sunlight

Other factors can influence sun exposure, such as the time of day that horses are turned out and blanketing practices.

However, seasonal variations and management practices that affect exposure to sunlight do not consistently impact vitamin D₃ levels in horses. ^[11]

Horses also generally have very low or undetectable levels of vitamin D₃ in their blood. This suggests that exposure to sunlight is less important than dietary intake of this vitamin for horses. ^[11][20]

Vitamin D Supplements

Horses that require vitamin D supplementation can obtain it through supplements containing vitamin D₃ (cholecalciferol) or commercial feeds that are fortified with this nutrient.

• For young horses in early growth stages, the suggested concentration is between 800 and 1,000 IU of vitamin D₃ per kg of dry matter in the diet. For a 325 kg (715 lb) yearling consuming 6.5 kg dry matter per day, this is between 5,200 – 6,500 IU per day. ^[23]
• For mature horses and those in later growth stages, the recommended concentration of vitamin D₃ in the diet is 500 IU per kg of dry matter. For a 500 kg (1100 lb) horse consuming 10 kg dry matter per day, this is equivalent to 5,000 IU per day. ^[23]

The upper tolerable limit of vitamin D intake is 44 IU per kg body weight, equivalent to 22,000 IU per day for a 500 kg (1100 lb) horse. This is the maximum amount of vitamin D intake that is considered safe for a horse, beyond which there could be a risk of adverse health effects.

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Vitamin D Absorption and Activation

Vitamin D is a fat-soluble vitamin, which means it is absorbed in the small intestine along with dietary fats. After absorption, it and other large fat molecules are transported via the lymphatic system before entering the bloodstream. ^[1][2]

Once vitamin D enters blood circulation, it can follow one of three paths: ^[1][2]

• It can be activated and function as a hormone, playing a role in various physiological processes.
• It may be stored in the liver, kidneys, and adipose (fat) tissues for future use.
• Alternatively, it can be metabolized and eventually excreted from the body.

Vitamin D Activation

Both forms of vitamin D, D₂ and vitamin D₃, are biologically inactive until they undergo a two-step conversion process to become active: ^[2]

• Step 1: Vitamin D is first transported to the liver, where it’s converted into 25-hydroxyvitamin D, also known as calcidiol.
• Step 2: Calcidiol then undergoes further conversion, typically in the kidneys, to become 1,25-dihydroxyvitamin D, also known as calcitriol, which is the active form of vitamin D.

Calcitriol plays an active role in the body by regulating mineral balance, supporting bone health and influencing various other bodily functions, including immune responses and cell growth regulation.

Vitamin D Metabolism in Horses

Equine researchers investigating the function of vitamin D have noted several features that are unique to horses: ^[3][21]

Circulating Vitamin D

The circulating levels of vitamin D are quite low compared to other species. For example, normal levels in cattle are up to three times higher than in horses. ^[3]

This could be because horses naturally have higher levels of calcium and lower levels of phosphorus in their blood than other species. They may be less reliant on vitamin D to increase calcium absorption from the gut or decrease the loss of these minerals in the urine. ^[3]

Calcitriol Conversion

The second step of vitamin D activation, converting calcidiol to calcitriol, does not appear to occur in the kidney of horses unlike in other species where the bulk of conversion occurs in the kidneys.

It is yet to be determined where this process occurs in horses. ^[1][3]

Vitamin D Toxicity

While vitamin D supplementation is beneficial to help horses meet their dietary requirements, excessive intake can be detrimental to the health and wellbeing of horses. Vitamin D toxicity can result from: ^[2][6]

• Improper feed formulation
• Excessive supplementation with vitamin D₃
• Ingestion of calcinogenic plants

Calcinogenic Plants

Calcinogenic plants are those that contain steroidal glycosides, which are metabolized by microbes in the hindgut to produce calcitriol, the active form of vitamin D. These plants include: ^[2]

• Cestrum Diurnum (Day blooming Jasmine)
• Solanum Torvum (Turkey Berry)
• Trisetum flavescens (Yellow Oat-grass)

Tissue Calcification

The main consequence of extremely high vitamin D intake is calcification of soft tissues such as the heart, arteries, intestine, lung, and kidney. This affects their function and can ultimately be fatal.

Calcification from vitamin D toxicity can lead to symptoms such as: ^[1][6][18]

• Limb stiffness
• Tachycardia
• Recumbency (lying down)
• Loss of appetite
• Weight loss
• Excessive drinking and urination

When horses are over-supplemented with vitamin D at toxic levels, it can take several days or even weeks for symptoms of toxicity to become evident. ^[2]

If your horse is diagnosed with vitamin D toxicosis, it is recommended to immediately remove all supplemental sources of vitamin D, calcium, and phosphorus from the horse’s diet. ^[2]

Additionally, limiting the horse’s exposure to sunlight may help manage the condition. Feeding artificially dried hays, which typically have lower vitamin D content compared to sun-cured hays, may also be beneficial in such cases ^[2]

Summary

• Vitamin D is a fat-soluble vitamin that is naturally found in the equine diet in forages. It is also commonly added to feeds as well as vitamin and mineral supplements.
• Vitamin D is well-known for its role in bone health and regulating calcium and phosphorus levels in other animals, but its role in these processes might not be as significant for horses.
• Overt vitamin D deficiency has not been reported in horses, but specific groups such as growing horses, lactating and pregnant mares, and young exercising horses could benefit from supplementation.
• Vitamin D toxicosis can occur in horses due to excessive supplementation or exposure to toxic plants.