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Vitamin C in Horse DietsBy Kentucky Equine Research Staff · December 24, 2013

Vitamin C is perhaps one of the most misunderstood vitamins in horse nutrition. Known also as ascorbic acid, vitamin C has various roles in the body, many of which revolve around its antioxidant properties.

Vitamin C plays a pivotal role in neutralizing harmful free radicals. Because of its water-soluble nature, vitamin C can work both inside and outside the cell to combat free-radical damage. Vitamin C also helps by regenerating vitamin E. Besides its antioxidant functions, vitamin C is needed for collagen synthesis, hormone synthesis, conversion of vitamin D3 to calcitriol, bone calcification, and antihistamine control. Vitamin C deficiency could produce poor hair coat, depressed immune system, hemorrhage, delayed wound healing, degenerated or enlarged adrenal glands, scoliosis, and lordosis.  

Humans are very cognizant of the need for supplemental vitamin C because without it a person will develop scurvy, a common affliction among seafaring individuals who lacked fresh vegetables and fruits in their diets. However, this may lead to confusion about what horses need with regard to vitamin C; while humans need to have vitamin C in their diet, horses do not. Humans lack the enzyme to convert glucose to vitamin C, but horses produce that enzyme, L-gulonolactone oxidase, in the liver. For horses to have adequate vitamin C in the diet, they need a properly functioning liver and adequate glucose, which serves as substrate for the conversion. For a normal healthy horse, vitamin C supplementation is pointless.

Just getting horses to absorb supplemental vitamin C is a challenge. Research has revealed that large amounts of supplemental vitamin C must be consumed before a change in blood ascorbate levels is registered, and these amounts are many times what might be considered the requirement (if the horse had a requirement). Because supplemental vitamin C is not absorbed well by the intestine of the horse, over three grams of vitamin C per day has to be fed in order to make an impact on the blood vitamin C levels.

Because horses are efficient at producing the enzyme needed for the conversion of glucose to vitamin C, when horses are fed supplemental vitamin C over long periods of time, there is concern that it will down-regulate the natural production of that enzyme. For this reason, long-term supplementation of large amounts of vitamin C is not recommended if it is not necessary. Even short-term supplementation can have a negative effect. If a horse has been on high levels of vitamin C supplementation for 10 or more days, it is recommended to avoid abrupt withdrawal of the supplement and to wean off the horse gradually. It was observed that abrupt cessation of supplementation of ascorbic acid after 10 days in weanling horses caused below normal blood ascorbate levels that lasted for three weeks. This was believed to be caused by the influence of supplementation on the enzyme production in the liver, which is slow to respond once the feedback mechanism has initiated a pause in enzyme production.

The addition of vitamin C in horse feeds has offered no benefit, mainly because it is added in very small amounts. Further, vitamin C is very susceptible to oxidation, so its potency decreases rapidly when it is part of a premix used in commercial feeds. Other forms of vitamin C used in supplements may be more stable. Vitamin C is normally added to joint supplements because of its roles in the formation and maturation of collagen, but there are no data on horses on the value of vitamin C in a joint supplement.

There are circumstances when supplemental vitamin C would be helpful, namely any time the horse’s immune function is depressed when the additional antioxidant may give a needed boost to the system. As the demands increase on the body, producing stress, the body may not be able to synthesize sufficient vitamin C to keep up with the demand. Horses that work really hard, like endurance horses, eventers, and racehorses; horses in prolonged stressful situations (such as a long transportation or multiday competitions); postoperative and post-trauma horses; horses suffering from wounds, infections or diseases; arthritic horses; horses with heaves or allergies; and aged horses with chronic infections or decreased immunity may benefit from supplemental vitamin C.

Researchers have looked for ways vitamin C supplementation could benefit horses in different stressful situations. For example, researchers found foals exhibiting high levels of stress during weaning by stall confinement had lower than normal plasma ascorbate levels, which is indicative of high vitamin C need and inadequate production. In another research study, vitamin C supplementation (20 g per day) increased antibody response to vaccines in aged horses, especially those with pituitary dysfunction or Cushing’s syndrome. Blood ascorbic acid was found to be low in the plasma and in the fluid in the lungs of horses with recurrent airway obstruction, and it is recommended these horses get 30 mg/kg body weight per day.  

Recent collaborative research has shown that horses with surgical colic had higher isoprostane metabolite levels (produced by free-radical-induced peroxidation of arachadonic acid and are sensitive markers of oxidative stress and ischaemia) than groups of horses with medical colic or unaffected control horses. As isoprostanes may exacerbate the oxidative stress in a compromised gastrointestinal tract, antioxidant and cell protectant therapy should be used as adjunct therapy in surgical colic cases. Vitamin C, DMSO, and lignocaine are recommended in addition to high doses of natural vitamin E.

Vitamin C is recommended for horses suffering in the acute stage of equine motor neuron disease (EMND) while undergoing treatment with high levels of vitamin E. Researchers investigated the effectiveness of a nutrient supplement that included vitamin C and niacin (as well as tyrosine) in alleviating the clinical signs of anhidrosis (inability to sweat). The supplement appeared to improve heat dissipation in nonexercised anhidrotic horses by increasing the amount of body sweat area.

Plants are a natural source of vitamin C and green growing grass has plenty; however, hay is virtually devoid because of the oxidative instability of vitamin C. Grains are not particularly high in vitamin C but have a huge upsurge as they begin to sprout. It is suggested that the vitamin C content of oats can increase by up to 600% on sprouting. Sprouts may be a good way to provide organic vitamin C in times of stress and for horses in heavy training, although the efficacy of absorption and the actual requirement requires more research.

In short, it is not necessary to supplement vitamin C to a horse, but its powerful antioxidant action makes it an attractive supplement if problems arise.