Written By Dr. Kris Hiney

Previously, we have discussed two important fat soluble vitamins which serve an important anti-oxidant function in the horse, vitamin A and E.  We will continue to discuss anti-oxidants as we transition to the water soluble vitamins essential to the health and well-being of the horse.  As humans, we are probably very familiar with vitamin C or ascorbic acid/ascorbate, as it is a commonly supplemented vitamin.  After all, who hasn’t reached for an orange in order to get their share of this important vitamin (Despite the fact there are many more nutrionally dense sources of vitamin C!)?  People often turn to vitamin C during times of stress or illness, especially the common cold, to try and fight off pathogens.  But what does vitamin C do in the horse, and should you be supplementing it?

Typically, most individuals are familiar with vitamin C’s role as an anti-oxidant, but it also serves as a co-factor for a host of enzymes.  Specifically, vitamin C is necessary for the formation of collagen, which appears throughout the body in connective tissue of tendons, ligaments, blood vessels etc.  Vitamin C also is necessary for the synthesis of carnitine (the molecule which allows fatty acids to be transferred into the mitochondria for oxidation) as well as tyrosine and other neurotransmitters.  Vitamin C supplementation, along with other anti-oxidants, has actually been shown to improve cognitive disfunction in aging dogs.

Vitamin C is synthesized in horses, but not in man, guinea pigs or a variety of other species.  Therefore in humans, vitamin C is a dietary necessity, but it is not required  in the diet of the average horse.  The horse is capable of converting glucose through a variety of enzymatic reactions into ascorbic acid. This synthesis is adequate in most scenarios.  So when might vitamin C be beneficial to the horse?   Presumably when there is a need for greater amounts of anti-oxidants in the body.
We have discussed the role of anti-oxidants before.  The body uses oxygen as the final electron acceptor in the electron transport chain during the capture of energy in the form of ATP.  Normally this process produces a harmless, and even useful byproduct – water.  However, a small proportion of these reactions does not go according to plan, but instead creates a harmful molecules known as reactive oxygen species or ROS.  In actuality, the formation of free radicals is a normal part of metabolism and serves as cell signaling systems.  In fact, the creation of free radicals stimulates the adaptive response seen with athletic training.  Therefore, we should not aim to eliminate their presence entirely.  However, in excess, these free radicals can do immense damage to the body as they damage DNA, cell membranes etc.  Reactive oxygen species have been implicated in carcinogenesis, aging, cognitive function etc.  Ascorbate aids in the anti-oxidant cascade by regenerating the reduced form of vitamin E and other anti-oxidants.

Horses which are intensely exercised will naturally produce a greater number of reactive oxygen species due to the increased rate of metabolism.  It is not uncommon for those individuals involved in more strenuous equine sports (endurance rides, three day eventing etc.) to routinely supplement their horses with anti-oxidants.  In studies which have examined the use of vitamin C in horses, there appears to be a difference in response relative to the intensity of the work being performed.  In polo ponies, plasma ascorbic acid was higher in ponies which were considered to be more intensely working than the lighter worked ponies, despite both groups receiving supplemental vitamin C.  Similarly, endurance horses supplemented with vitamin C had a higher plasma ascorbic acid level at the beginning of the race compared to the control horses, but the difference between plasma vitamin C levels  between the two groups grew smaller throughout the race.   The unsupplemented horses actually increased their plasma ascorbic acid levels throughout the race, presumably through the mobilization of body stores.  This differed in previous studies which showed a decrease in plasma ascorbic acid in more intensely worked horses.  This drop in ascorbic acid has also been reported in heavily raced sled dogs. Thus it may be the level of exercise which is important. Certainly this makes sense as the level of effort increases, the metabolic rate must increase and the greater percentage of ROS will be produced.   Although lacking in concrete data, it appears that additional vitamin C may be beneficial for heavily exercised horses.

Exercise is not the only form of stress which horses may experience.  Plasma vitamin C levels have been seen to be lower in horses following surgeries, traumatic wounds, strangles and episodes of exercised induced pulmonary hemorrhage.  Horses with recurrent airway obstruction also have had lower plasma ascorbic acid levels, and supplementation appears to be helpful in creating better exercise tolerance and reduced airway inflammation.  Supplementation of vitamin C also appears to help aged horses enhance their immune system and improves their response to vaccinations.  Horses do appear to tolerate large doses of vitamin C quite well, horses received 20 g /d of ascorbic acid for 8 months with no measureable negative response.  However, it has been shown that horses decrease their own natural synthesis of vitamin C when supplemented.  Therefore, when the supplement is removed, horses will have a lower plasma concentration of vitamin C compared to normal.  Therefore, prolonged supplementation may be ill advised.  Overall, like all vitamins previously discussed, supplementation of vitamin C should not be done without careful consideration of whether or not the horse would truly benefit from supplementation.