Vitamin C: Does it Help Horses with Inflammation?
Q: Over the past year my 18-year-old gelding has had several bouts of hind limb swelling that the vet has treated with antibiotics. I’m wondering whether I can feed him vitamin C to improve his immune system and, if so, how much?

A: Vitamin C is an antioxidant that plays a role in helping prevent damage to body tissues under threat from free radicals, which are generated as the immune system works to protect the body from bacteria, viruses, and other organisms. Some clinical signs of inflammation and infection might be due to free radicals. Vitamin C plays a crucial role in helping fight off infection. In an ideal situation, antioxidants such as vitamin C neutralize free radicals before damage to sensitive tissues occurs.

Vitamin C levels occurring in typical equine feed stuffs aren’t readily available; but many scientists believe that the levels are fairly low, with a horse’s most abundant source coming from fresh pasture. However, like many other species, horses synthesize their own vitamin C from glucose in the liver, making about 72 grams each day.

Classic vitamin C deficiency results in scurvy, which veterinarians have never documented in horses. Vitamin C status is assessed by measuring ascorbic acid levels in the blood, and several studies have shown lowered blood ascorbic acid levels in horses suffering from conditions such as poor wound healing, strangles, recurrent airway obstruction, poor performance, and acute rhinopneumonia.

Age Matters, Maybe

Nutritionists haven’t identified equine vitamin C requirements, but a horse’s need for vitamin C might be affected by factors such as age and disease.

Studies looking at vitamin C in horses are limited and somewhat contradictory. In one study horses over 20 years old had lower serum concentration of ascorbic acid than younger horses, suggesting that these older horses depleted vitamin C produced at a faster rate. However, another study found no serum differences between older aged horses and their younger counterparts.

While we don’t know what level of serum ascorbic acid indicates deficiency, several studies have shown a range of levels in healthy adult horses. One study looked at levels in 488 horses and reported a mean concentration of 5.9 ± 1.4 µm/ml. However across studies the range of plasma concentrations appears to be between about 0.8-6.5 mg/l and might be lower in the winter than summer.

Getting an obvious response to supplementation might be tricky, because different forms of vitamin C have different absorptions, and nutritionists believe utilization differences between individual horses is large. Most equine research has looked at supplementing with between 4.5 g and 20 g per day. Supplementation at the higher end of this range might lead to reduction in the horse’s own vitamin C production, which could create problems if supplemental sources are withdrawn suddenly without time given for the horse’s own production to restart. Additionally, while there have been no reports of toxicity in horses, it might cause gastrointestinal disturbance and work done in other species suggests that large vitamin C doses might result in allergic reactions.

Available Types of Vitamin C

The form of vitamin C you give your have might impact success. Researchers found crystalline L-ascorbate given orally has low bioavailability, while ascorbyl palmitate (sometimes sold as vitamin C ester) has far better absorption. Ascorbyl palmitate as vitamin C ester is not to be confused with Ester-C, a form often found in equine supplements because it’s thought to be gentler on the stomach. Ester-C is mostly calcium ascorbate with some other vitamin C metabolites, which supplement manufacturers believe aids in bioavailability.

Supplemental sources of vitamin C aren’t stable in premixes unless in a protected form. Manufacturers claim some   forms are more stable for use in feeds and vitamin and mineral supplements; however, if you’re unsure look for products that are solely vitamin C and that have a “use by” date on the label.

Take-Home Message

Not enough data is available for us to say for certain whether supplementing vitamin C will help horses with various disease conditions. However, should you decide to try it, select a form that has good bioavailability, feed carefully and if stopping supplementation do so gradually. Start with 5 grams a day building cautiously if no positive effect is seen and stay below 20 grams a day. Take care if stopping supplementation; gradually weaning a horse from supplements is likely a good idea.