Vitamins are organic compounds that are required in the diet in very small amounts, and that perform a variety of functions in the body including serving as antioxidants and cofactors of enzymes. They are also involved in cell differentiation and growth. Despite only being required in minute quantities, deficiencies can result in serious disorders, while excess amounts (toxicity) can also cause problems. Vitamins are classified as either fat-soluble or water-soluble, based on their chemical properties associated with their absorption, transport, storage and excretion.
Fat-soluble vitamins (vitamins A, D, E and K) are absorbed and transported in association with fats. They require bile salts, for example, to facilitate absorption from the digestive tract. Furthermore, they can be stored along with body lipids, and are poorly excreted. Thus, it may take time for stores to be depleted before any signs of deficiency are noticed. Excessive intake of fat soluble vitamins may, however, result in toxicity because excretion is limited.
Water-soluble vitamins (the B-complex vitamins and vitamin C) are easily absorbed, but are not retained in the body, unless they are bound to enzymes as cofactors. When consumed in excess, the water-soluble vitamins are excreted in the urine. Thus, toxicity from water-soluble vitamins is rare.
Many of the vitamins are not actually required in the diet of horses. Vitamin C (ascorbic acid), for example, is produced from glucose in the liver of the horse and is, therefore, not required in the diet (unlike humans, non-human primates, guinea pigs and fruit bats, who cannot produce their own vitamin C and do require it in their diets). Vitamin D can be synthesized in the skin upon exposure to sunlight and, therefore, may not be needed in the diet. The B-complex vitamins and vitamin K are produced by the microbial organisms in the large intestine of the horse in sufficient quantities, such that deficiencies are rare and are usually caused by something affecting their production or use.
Vitamin A (aka retinol) is not found naturally in the horse’s diet. Instead, the horse converts beta-carotene (or other carotenoids) to vitamin A within the small intestine and liver. Carotenoids are found in the greatest quantities in fresh forages such as pasture (grasses and legumes), and in smaller amounts in stored forages (such as hay, though the amounts vary significantly with storage time) and cereal grains.
Vitamin A (in the form of retinyl-palmitate or retinyl-acetate) is usually added to commercial feeds for horses because access to fresh pasture may be limited. Vitamin A functions primarily as rhodopsin, which is involved in vision. Vitamin A also plays a role in reproduction and immunity. Deficiency of vitamin A is noted by night blindness, though upwards of 200 days are required to induce these symptoms because vitamin A is stored in the liver. Toxicity of vitamin A has been associated with bone fragility and developmental orthopedic diseases. Toxicity due to beta-carotene, however, has not been reported (because it is watersoluble before being converted into vitamin A), thus toxicity only results in horses when vitamin A has been added to the horse’s diet in excessive quantities (most likely due to over-supplementation or manufacturing errors of commercial feeds).
Vitamin D (ergocalciferol, cholecalciferol) is produced by the skin upon exposure to sunlight. It is also found in sun-cured forages such as hay. Because horses may not have sufficient sunlight exposure (particularly in the winter months in Canada where sunlight may be limited, or when horses are on stall rest or are heavily blanketed), commercial feeds are often supplemented with cholecalciferol. The function of vitamin D is to help facilitate the absorption of calcium from the digestive tract, reabsorption of calcium from the kidney and regulate bone calcium mobilization. Thus, deficiency in vitamin D status results in decreased calcium within bone, resulting in rickets or other bone disorders, though this is rare in horses. Vitamin D toxicity results in calcification of non-bone tissues and death. Toxicity is not due to excessive synthesis in the skin from sunlight, but rather from over-supplementation.
Vitamin E (tocopherol) is found in high concentrations in fresh forages (pasture), while hay and cereal grains have significantly lower amounts. Because of this variation, commercial feeds are usually fortified with vitamin E (tocopheryl acetate). Vitamin E functions as an antioxidant to protect tissues against oxidative damage. Vitamin E deficiency results in white muscle disease in foals, or equine motor neuron disease in older horses, though these disorders often occur in conjunction with a selenium deficiency. Toxicity of vitamin E is rare, even at high intakes, though it may affect the absorption or status of other vitamins. High intakes of vitamin E are common among athletic horses in effort to prevent exercise induced muscle damage, though research does not unequivocally support this claim.
Vitamin K (phylloquinone, menaquinone, menadione) is produced by the microbes found within the large intestine of the horse, but is also found in plants such as forages and cereal grains. Deficiency only arises with exposure to compounds that counteract vitamin K activity, such as dicumerol, which is found in some mouldy clovers.
Vitamin C (ascorbic acid) is synthesized in the liver of horses from glucose, and is found in small quantities in several fruits and vegetables that may be consumed by horses. Because vitamin C is synthesized by the horse, deficiency symptoms such as scurvy have not been reported in horses. Reduced vitamin C concentrations in the blood, however, have been reported in older (>20 years) horses, though it is unknown if this is a result of decreased synthesis or increased use (or both). Vitamin C functions as an antioxidant and is involved in the synthesis of collagen. Exercise, disease or stress may also affect vitamin C status, and, therefore, it is commonly supplemented. Because vitamin C is water-soluble, toxicity has not been reported in horses.
The B vitamins include thiamin (B1), riboflavin (B2), niacin, pantothenic acid, pyridoxine (B6), biotin, folate (folic acid) and cobalamin (B12). In general, the B vitamins function as cofactors of enzymes involved in energy metabolism. As indicated above, the B vitamins can be synthesized by the microbes within the horse’s digestive tract, and they are also found in variable amounts in many equine feeds. Because of this, B vitamin deficiency is rare. Supplementation may, however, be warranted in cases where microbial activity may be impaired, such as with oral antibiotic therapy. There are also some substances that interfere with B vitamin metabolism such as bracken fern (thiamin) or pyrimethamine, which is used to treat EPM (folate). Biotin supplementation has also been shown to improve hoof quality in some horses. There have been no reported cases of toxicity of any of the B vitamins in horses. Recommended Intake
The vitamin requirements for horses are only well-established for vitamin A and E, with some estimated requirements established for vitamin D, thiamin and riboflavin (see table above). If a horse is fed a diet of good quality hay (only stored for a short period of time) or pasture, as well as a commercially fortified grain mix, he is likely getting the vitamins he needs. A few vitamins (vitamin E and biotin) may be further supplemented in hopes to improve antioxidant status or hoof quality respectively.