Vitamin C (ascorbic acid) comes in and out of supplement headlines. As happens with many nutrients, exaggerated claims end up being made and when these eventually prove to be unwarranted the pendulum swings too far in the other direction but there is no question vitamin C plays a pivotal role in health.
The horse’s body can manufacture vitamin C but fresh grass is a rich source and blood levels drop off in winter when on hay.
Just about everyone thinks of vitamin C in terms of the immune system and resistance to infections/”colds”. What it actually does is protect the immune system and other tissues from free radicals generated when the immune system cells are activated. It therefore ends up moderating friendly fire from immune system activity rather than actually preventing infection. Another antioxidant function is the regeneration of “used” vitamin E back to an active form.
Vitamin C is also an important antioxidant in muscle cells and all body tissues. High levels of free radicals are generated by such things as exercise and exposure to pollutants. Horses with chronic lung disease have been documented to have low levels of vitamin C in lung fluid and supplementation shown to support normal lung function.
Vitamin C also maintains minerals forming the reactive site of enzymes in their active form, is essential to production of carnitine and norepinephrine as well as creation of normal connective tissues throughout the body including integrity of blood vessels, structural framework of bone, joint cartilage, ligament and tendons and is critical for the restitution of wounds and injuries.
Most mammals, including horses, can manufacture vitamin C from glucose. Unlike other water soluble vitamins, there can also be a limited storage of vitamin C in the body, at least in humans. We don’t know much about vitamin C in horses. They make enough to never suffer from full blown deficiency (scurvy) but we don’t know how much they can ramp up production in times of increased need, like injury or infection, whether they can store the vitamin or whether production decreases with age. This uncertainty coupled with the observation that blood levels drop in stabled horses and over winter suggests some supplementation may be optimal.
Vitamin C has low toxicity, the major issue being gastrointestinal irritation and diarrhea at high doses (typically 20 grams/day or more in an average sized horse). Caution should be used in insulin resistant horses or other horses known or suspected to be iron overloaded. Vitamin C increases bioavailability of inorganic iron by changing its electrical charge and directly stimulating absorption. A daily dosage of 4.5 grams or less is best and has been shown to increase blood levels over time.
Supplementation is reasonable in horses with chronic lung irritation, musculoskeletal issues, infections or wounds, to support the body’s inherent antioxidant defenses and maintain vitamin C supplies for normal functions under conditions of high demand.
Vitamin C may be supplemented alone or in combination with plant and/or nutritional antioxidant ingredients such as bioflavonoids, grape seed and skin, vitamin E, berries, glutamine and N-acetyl-cysteine.
Eleanor Kellon, VMD