Fat Soluble Vitamins

Vitamin A, a fat soluble vitamin, has a number of roles in the horse's body. The main one, and most well-known one, is the role it plays in night vision.

One form of A combines with opsin to produce rhodopsin. Rhodopsin is the visual pigment that helps recognize the presence of light energy and transform it into a signal that travels the nervous system. This nervous system signal is then what allows the horse to see.

However, A also has other functions in the horse. It regulates gene expression during cell differentiation. Due to this regulation, it is very important in reproduction and the creation of the embryo.

Lastly, A is important to help maintain the innate and adaptive immune response to infection.

Traditional feedstuffs usually fed to horses, such as forages, cereal grains, and plant protein supplements, are relatively low in retinol. However, retinol is present in pro-vitamin A compounds, also known as carotenoids.

One of the most well-known carotenoids is beta-carotene, and it is found in most feed ingredients that are used for horses. It is highest in forages and lower in cereal grains.

Pasture is the forage that contains the highest level while mature grass hays contain the least of the forages.

Corn is the cereal grain that has the most beta-carotene, but it has significantly less than the forages.

Beta-carotene is broken down in the small intestine and liver of the horse to be converted into A.

According to the latest edition of the NRC's Nutrient Requirements for Horses, the maintenance level of A for horses is 30 IU/kg BW. For growth the requirement is 45 IU/kg BW, while for breeding, gestation, and lactation it is 60 IU/kg BW.

Vitamin A Deficiency

Vitamin A deficiency is characterized by night blindness, which has been reported in horses.

However, clinical signs of deficiency (mainly night blindness) are hard to induce in horses, and require very low levels of carotene intake over a long period of time (at least a year or more).

However, impaired growth has been reported in growing ponies deprived of carotene. As a result, it is thought that growth parameters are a more sensitive indicator of carotene deficiency than clinical signs.

It also appears, based on the results of a number of studies, that horses may have an ability to adapt to very low levels of carotene intake, making deficiency of A even less of a worry for horse owners.

Vitamin A Toxicity

Vitamin A toxicity is more of a worry in horses than deficiency. However, it should be noted that toxicity due to beta-carotene intake has never been reported. Toxicity results in fragile bones, hyperostosis (simply the overgrowth of bone), teratogenesis (production of a malformed fetus in a pregnant mare), and other disorders.

It has also been implicated in developmental orthopedic disease in growing horses.

The presumed upper safe limit for intakes of A is 16,000 IU per kg of dry matter (16,000 IU/kg DM).

Vitamin D, as one of the fat soluble vitamins, can be stored in the body.

Its main role in the body is calcium homeostasis, or keeping the calcium levels at a constant level. It also influences the growth and differentiation of cells to some degree.

When managing the calcium levels, D most often works in the digestive tract. It helps calcium to be absorbed from the intestine and helps it be reabsorbed from the kidney.

However, it also helps calcium be released from bones, as well as be taken back up into bones. This is why most calcium supplements on the market also contain Vitamin D.

In the horse diet, D is relatively low. The only place it is really found naturally is sun-cured alfalfa.

However, it is not an issue that D is not found in the diet, because all normal, healthy horses (and humans for that matter) synthesize D in their body through exposure to sunlight.

The only time low dietary inclusion would be a problem is if the body cannot synthesize it, or the horse is deprived of sunlight.

Vitamin D Toxicity

The only symptom of Vitamin D toxicity is hypercalcemia, or calcification of soft tissue.

However, since horse diets are low in D, and the body only makes as much D as it needs, D toxicity is pretty rare in horses.

The only time it would be a concern is if your horse is being fed multiple supplements or fortified feeds containing D. In that instance, it would be prudent to figure out how much D he is getting, and ensure it is below the upper safe limit of 3,300 IU/day.

Vitamin D Deficiency

Vitamin D deficiency is also not a huge concern in horses, due to the fact that they need very little compared to most animals.

Rickets is the disease that develops if a horse does not get enough D. Rickets is also called soft-bone disease, and the main symptom is bone deformities, especially in the limbs.

However, in studies done on horses, visible outward signs of rickets were never reported, though bone growth and development was affected in ponies that were deprived of sunlight and had no D supplementation.

In most practical horse-keeping situations, assuming the horse gets at least some exposure to sunlight, or some supplemental D in the diet, D deficiency is not going to be a problem.

Vitamin E is another one of the fat-soluble vitamins. Its primary function in your horse's body is as an anti-oxidant. It is fat-soluble, so it can easily penetrate cell membranes (which are made up of fat, or lipids) and serves as one of the main antioxidants that protects these membranes.

Amounts of E in the horse's diet varies considerably. The main sources of it are forage. However, when talking about forages (and therefore naturally occuring sources of E) content declines the longer the forage is stored.

The content also varies based on how mature the forage was at harvest -- older forages have less E activity than younger plants.

Grains have a lower content of E than forages. And again, the amount of E found in any given grain varies depending on harvest conditions.

To make up for all this variation, most feed companies fortify their feeds with extra E.

Vitamin E Deficiency

White muscle disease is the disease that is commonly thought of when vitamin E deficiency is being discussed. This disease is also known as nutritional muscular dystrophy, and is a degenerative disease that affects the heart and skeletal muscles of foals under a year of age.

A deficiency of E has been implicated in white muscle disease, however, the available studies point to selenium deficiencies being the primary cause of the disease. E supplementation is used, along with selenium supplementation, to treat the disease.

Another disease in horses that may be related to E deficiency in horses is Equine Motor Neuron Disease (EMND). EMND is a degenerative neuron disease that affects lower motor neurons of horses 2 years and older. This disease is characterized by sudden and severe onset of trembling, a constant shifting of the back legs when the horse is standing, more than usual lying down, and muscle wasting. It can also affect the eyes. Several studies have shown that this disease occurs after a prolonged E defeciency.

Vitamin E Toxicity

And more good news for horse owners...

...vitamin E toxicity is not a concern in horses, even with large amounts of supplementation. The upper safe dietary limit, which has not been determined specifically in horses, is set at 1,000 IU per kg of dry matter (1000 IU/kg DM) based on observations in other species.

Impaired bone calcification is the major symptom that is shown for E toxicity in other species.

Vitamin K has one major, and very important, role in the horse's body. Through a complex process it helps with blood clotting. The Gla-proteins (which are part of that complex process) that are made from K are also important in bone metabolism and heart health.

So, K is a pretty important vitamin to sustain life.

Similar to other vitamins, the best source of K in your horse's diet is his forage...




...and cereal grains in his diet contain very little K.

Dietary requirements for K have not been determined in the horse, but most horses in normal housing situations easily obtain all of the K they need from their diet.

Vitamin K Deficiency

Problems with blood clotting is the major symptom of vitamin K deficiency. It is caused by those Gla-proteins being formed the wrong way and not being able to do their job correctly.

In humans, K deficiency has also been implicated in diseases affecting bone and heart health.

However, thankfully for us horse owners, K deficiency due to the horse not consuming enough has never been reported. However, if the horse consumes K antagonists (substance that works against K, making it unable to do its job), deficiency symptoms can appear.

One of these antagonists, dicoumarol, is produced by moldy sweet clover hay. Problems with blood clotting from consuming moldy sweet clover hay (and thus dicoumarol) HAVE been reported in horses. Yet another reason to make sure you never feed moldy hay to your horse.

Additionally, the therapeutic use of warfarin in horses can interfere with the metabolism of K, causing blood clotting problems.

Vitamin K Toxicity

Vitamin K toxicity from overconsumption of the vitamin has not been reported in horses.

It is estimated that the toxic level of K by ingestion in the horse is at least 1,000 times the daily recommended intake. That's the good news.

The bad news is that in one study, a researcher managed to cause acute renal failure in every study horse when he administered a single dose of menadione (the synthetic form of K) by injection according to the manufacturer's recommendations.

This, paired with the fact that phylloquinone (one of the two naturally occurring forms of K) injections appear safer in newborn human babies, has led to the suggestion that phylloquinone injections be used if a horse needs to have K administered by injection.