Their Role in the Diet

Many processes in the body are supported by vitamins. They are essential for normal metabolism, as well as supporting growth and gestation. Vitamins also play a role during exercise, exercise recovery and supporting the immune system. With all of the jobs that vitamins have in the body, it is easy to understand how deficiencies will negatively impact your horse’s health.

Types of Vitamins

Vitamins are a diverse group of compounds. Broadly, they are categorized as fat-soluble and water-soluble. Vitamins A, D, E and K are fat-soluble vitamins. Fat-soluble vitamins are absorbed along with dietary fat and they are stored in body fat. When excessive levels of fat-soluble vitamins are fed, they accumulate in fat stores, which can pose risk of Vitamin A and D toxicity.

Water-soluble vitamins dissolve in water upon entering the body and because of this, they are not stored for later use. Excess water-soluble vitamins are excreted in urine, which decreases the likelihood of creating a toxicity. The water-soluble vitamin category encompasses the B vitamins (thiamine, riboflavin, folic acid, niacin, pantothenic acid, biotin, vitamin B6 and vitamin B12) and Vitamin C.

Vitamin A

Vitamin A plays crucial roles in reproduction and gestation, as well as supporting the immune system. It is also key for the maintenance of bone and soft tissue.

Vitamin A is not found in the natural diet of the horse. Instead, Vitamin A is synthesized by the horse from beta-carotene in the small intestine. Beta-carotene is a Vitamin A precursor that is found in high quality pasture grasses in abundant quantity, but when forage is cut to make hay, the curing process destroys much of the carotene content. Even high-quality pasture as the sole source of forage may not provide enough Vitamin A for a horse in heavy work and many horses have limited access to pasture. For this reason, Vitamin A needs to be provided by the feed portion of the diet for many horses.

Commercial horse feeds are fortified with Vitamin A, instead of beta-carotene. This is due to the poor stability of beta-carotene in feed. Synthetic beta-carotene has been explored as a supplement for horses, but the conversion of synthetic beta-carotene to Vitamin A in the small intestine of the horse was found to be low.

Vitamin D

Vitamin D plays a significant role in calcium homeostasis – making it very important for bone health. Some Vitamin D is found in basic feed ingredients, most predominately sun-cured hay, but in general, the amount of Vitamin D in the natural diet is low. Instead, Vitamin D is synthesized by the horse in the skin, but in order for this process to occur, the horse has to have regular access to sunlight.

Because of the importance of Vitamin D and the variability in which horses get consistent access to sunlight, Vitamin D is included in commercial horse feeds to prevent deficiencies.   

Vitamin E

Vitamin E is an antioxidant that supports neuromuscular function. Similar to Vitamin A, a diet of fresh grass is a good source of Vitamin E, but the stability of Vitamin E in harvested forage is low. Of all of the Vitamins, Vitamin E gets the most attention and you can read more in depth on Vitamin E here.

Vitamin E is one of the few nutrients that can be measured in the blood with a high level of accuracy, and before embarking on Vitamin E supplementation, it is recommended to check a horse’s Vitamin E status. The good news is that Vitamin E has a wide toxicity range, which means you are unlikely to cause health issues with too much Vitamin E, but the added cost of unnecessary supplementation could be spent in many other ways to support your horse’s wellness.  

There are different forms of Vitamin E that can be used in commercial feeds. The form of Vitamin E is important, but often misunderstood. Synthetic Vitamin E (dl-alpha-Tocopheryl Acetate) tends to get the bad rap, as it is less bioavailable than natural (d-alpha-Tocopheryl Acetate) Vitamin E. However, it is important to remember that less bioavailable doesn’t mean it is not bioavailable!

Natural Vitamin E is our first choice when addressing an acute Vitamin E deficiency because it raises serum Vitamin E levels more quickly, but synthetic Vitamin E is appropriate and effective for maintaining a healthy Vitamin E status.

Synthetic Vitamin E does hold the advantage in maintaining stability over time in stored feed, which makes it a useful addition to horse feeds, even though it has a slightly lower bioavailability than natural Vitamin E.

Vitamin K

Vitamin K is involved in blood clotting, bone metabolism and vascular health. Vitamin K is found in forages and nutritional deficiencies in horses have not been identified. Certain drugs and toxins can impair vitamin K metabolism and result in symptoms of deficiencies, but these situations are very rare. 
There are several different types of Vitamin K. The Vitamin K in feedstuffs is the phylloquinone form, but Vitamin K can also be produced by intestinal bacteria and this is in the form of menaquinone. While nutritional deficiencies have not been identified, commercial horse feeds are often fortified with menadione, a synthetic form of Vitamin K, to reduce risk of deficiencies in situations where horses do not have adequate access to forage.

Water-Soluble Vitamins

Of the nine water-soluble vitamins, requirements for only two of them have been defined for the horse – thiamin and riboflavin. Thiamin and riboflavin both play a role in the production of ATP, which is a form of energy used by the horse to support normal function and athletic performance.

Many of the B vitamins, including thiamin and riboflavin, are synthesized by the microbes that live in the horse’s digestive tract. Horses with healthy digestive tracts that are fed good quality forage are unlikely to be deficient in B vitamins; however, commercial horse feeds are often fortified with B vitamins to support horses who do not have adequate access to forage or are doing a poor job of synthesizing B vitamins due to imbalances in the intestinal microbiome.

Vitamin C is a water-soluble antioxidant that is synthesized by the liver of the horse. It is assumed that a horse’s Vitamin C needs are met by liver production under most circumstances, although in senior horses and horses in heavy work, this may not be the case. Vitamin C fortification in horse feeds is done to maximize your horse’s health, rather than prevent Vitamin C deficiency. A heat stable form of Vitamin C is required to maintain stability through the pelleting process.

Vitamin Toxicity

As with many nutrients, it is important to ensure that vitamin requirements are met, but it is equally as important to prevent over supplementation. Feeding a commercial horse feed at label recommended feeding rates will ensure that requirements are met without being grossly exceeded.
To determine if your horse’s vitamin needs are being met, reach out to our team for a personalized feeding plan at any time!
 

Nicole Rambo, Ph.D.