Provision of Vitamins Important in Convalescing Horses

A well-rounded diet for horses includes all of the vitamins necessary for top-notch health. These vitamins are especially important when a breakdown in tissue occurs due to injury or illness. A closer look at the most prominent vitamins in the equine diet underscores their necessity.

Vitamin A

Vitamin A is perhaps the most important of the vitamins and is indispensable for support of growth and health of a horse.  Deficiency causes defects in bone growth, defects in growth and differentiation of epithelial tissues, and lowered resistance to disease and infection.  Vitamin A is important for the proper function of osteoclasts, which are responsible for resorption and remodeling of bone. Without vitamin A, excessive deposition of periosteal bone occurs from the unchecked action of osteoblasts. Vitamin A is necessary for collagen synthesis and cross-linking so it is important for tendon and ligament strength. Excessive doses of vitamin A may increase inflammation through its action on lysosomes. Retinoic acid, one of the active forms of vitamin A in the body, can synergize with thyroid hormone or glucocorticoids to enhance growth hormone secretion. Growth hormone has been shown experimentally to enhance the healing process in soft tissue injuries.

Carotenes are the natural source of vitamin A for the horse, because they are found in abundance in green forages.  Unfortunately, much of the carotene content is destroyed by oxidation in the process of curing hay.  Horses are not able to absorb sufficient quantities of beta-carotene from hay to meet their requirement, except possibly well-made, early-bloom alfalfa (lucerne) hay consumed soon after it is made. In Nutrient Requirement of Horses, the NRC recommendations for vitamins are usually close to the minimal amount necessary to prevent deficiency signs but not necessarily the optimal amount needed by the animal. Feeding at the NRC recommended level may lead to suboptimal performance even though the animal appears normal. Optimal supplementation of vitamin A for the horse has been hypothesized to be more than twice the NRC recommendation. Very little research on vitamin A supplementation of the performance horse has been done.  Further work is needed to define vitamin A requirements for exercise in the horse, but look for a supplement containing at least 40,000 IU of vitamin A or a feed supplying more than 10,000 IU/kg.

Vitamin D

Vitamin D is actually a hormone and adequate sunlight results in the production of sufficient vitamin D from 7-dehydrocholesterol in the skin.  Hence vitamin D is not required in the diet if sufficient amounts of sunlight are received.  Sufficient vitamin D must be present for calcium and phosphorus to be absorbed; a vitamin D deficiency markedly reduces absorption of both minerals.  Lack of adequate photoproduction of vitamin D₃ or inadequate dietary supplementation of vitamin D, leads to the failure of bones to calcify normally. This metabolic disease is known as rickets in the young and osteomalacia in adults. Many performance horses spend the vast majority of the day in a box and after injury, a horse may be confined in a box all day, so vitamin D supplementation may be warranted, especially after a bone injury.

Apart from exposure of the horse to sunlight, vitamin D₂ results from ultraviolet irradiation of hay during drying and curing. Vitamin D is also added to supplements and feeds. No absolute requirement has yet been established for the horse. Optimal supplementation for horses with limited access to sunlight is not yet known. Extensive bone remodeling was found in young horses undergoing training resulting in changes in calcium, phosphorus, and vitamin D levels in serum and bone.  Logically, anything that stresses the bone structure of the animal will increase the amount of bone remodeling, and therefore may increase the need for vitamin D.  An increase in vitamin D intake has been found to increase efficiency of calcium and phosphorus absorption in the intestinal tract, which would increase the supply of these nutrients as needed in times of stress due to exercise or injury.

Vitamin E

Vitamin E has numerous functions in the body, many of which are still not completely understood.  Vitamin E is essential for the integrity and optimum function of muscular, circulatory, nervous, and immune systems.  Its action as a natural antioxidant is seen as the underlying factor of most vitamin E functions.  Vitamin E is considered to be the most effective natural lipid-soluble chain-breaking antioxidant. The action of vitamin E is very important in cell membranes, protecting them from peroxidative damage. This is especially important in the mitochondrial membrane, where enzymes involved in respiratory chain energy production are located.  Selenium, like vitamin E, also acts to prevent lipid peroxidation, however with differing mechanisms.  The two complement each other, one appearing to be able to compensate for the absence of the other.

Vitamin E is abundant in green growing pastures. The content diminishes with plant maturation, and harvesting the forage diminishes the quantity of vitamin E present. Storage of the hay further decreases the amount of vitamin E as much as 50% in the first month of storage. Vegetable oils are relatively high in vitamin E content, but as amounts fed are in cups rather than kilograms, they make a minor contribution to vitamin E intake.  In practice, the vitamin E content of other feedstuffs is variable and not readily predictable because of handling and storage time. Therefore, it is common practice to supplement animal feeds with vitamin E, and feeds or supplements should contain sufficient vitamin E to supply at least 1000 IU. Significant confusion of actual vitamin E requirement exists because of the difficulty in determining the amount needed to prevent deficiency symptoms and the amount needed to produce optimal antioxidant and immune function. Recent research has shown that natural source vitamin E is more bioavailable than synthetic forms, and is increasingly being used in feeds and supplements. Some new-generation natural vitamin E liquids have been shown to rapidly increase serum levels, and high doses (5000 IU) have value as a supplement after injury to aid the healing process.

Vitamin K

The major function of vitamin K is in blood coagulation. The vitamin is required for the activation of the four plasma clotting factors. Recently, vitamin K has also been found to have a role in the activation of a number of other proteins throughout the body, some specifically identified in the skin and bone. This includes osteocalcin, which is responsible for facilitating bone mineralisation, so vitamin K could have a positive effect on net bone formation.

Vitamin K₂ is the compound synthesized by intestinal microbes, while vitamin K₁ is the compound found in green plants and vitamin K₃ is a synthetic product. It is generally assumed that vitamin K is synthesized by microorganisms of the cecum and colon in sufficient quantities to meet requirements. Green leaves are the richest natural source of vitamin K, and the vitamin remains present even after the green has diminished. The nature of the diet of the horse, pasture and/or hay, should provide more than adequate quantities of vitamin K, but research is needed into any benefits of supplementation after injury to bone. Conditions that interfere with vitamin K function include gastric ulcers, mycotoxins in the feed, and long-term antibiotic treatment. These conditions may predispose to vitamin K deficiencies and benefits from supplementation.

B vitamins

Whilst the B-group vitamins do not have any specific documented role in the healing process, horses recovering from injury may benefit from B vitamin supplementation. Pain may increase metabolic rates, which increases the need for B vitamins as they have significant roles in energy metabolism. Stress can decrease the synthesis of B vitamins in the large intestine and if the horse is not eating, there can be a short term benefit from oral or injectable B vitamin supplementation.

Most of the emphasis on nutrition and hoof wall integrity is on the vitamin biotin.  It is thought that the normal horse has a biotin requirement of 1-2 mg per day. Studies have shown that supplemental biotin at levels of 15-20 mg per day have had positive effects on histological characteristics of the hoof wall tissue.  However, patience is important as the horse takes six to nine months to grow a new hoof. Recent research has shown that it takes six months for appreciable differences between treated and control horses and 18 months for statistically significant differences. Biotin treatment reduces the incidence and severity of hoof horn defects, increases tensile strength, and improves condition of the white line. However, in this experiment it did not increase hoof growth rate, although field reports usually indicate that hoof growth is increased. Unfortunately, biotin does not work for every horse with bad feet.

Combination supplements that incorporate biotin, zinc, and methionine work better than pure biotin products.

Vitamin C

Vitamin C is necessary for the synthesis of collagen, blood vessel formation, and the healing of skin wounds.  It is an essential cofactor in the hydroxylation of protein and lysine during the formation of collagen, and when deficiencies occur collagen formation is retarded.

Vitamin C is an important antioxidant, and it removes excess oxygen thus protecting fats and proteins in cell membranes.  These actions would suggest that vitamin C is an important supplement to consider in this discussion, yet the horse can synthesise vitamin C in tissues from glucose, and oral sources are poorly absorbed so supplementation of vitamin C is unlikely to be necessary or beneficial.  Stress may reduce normal synthesis and these horses have lower blood levels, but oral doses of 20 g daily are probably required if vitamin C is to be given.