The main function of Vitamin C is as an anti-oxidant. This means that it works to prevent oxidation, or free-radicals destroying cells.
Research is lacking when it comes to determining how much C is found in common feedstuffs. In fact, C concentrations in typical horse feeds is unknown.
What is known is that the horse is able to synthesize C from glucose. Based on a number of studies, it is currently assumed that the horse meets his entire C need by manufacturing it in his own body, meaning he does not need an outside vitamin C source.
It also appears that C deficiency and toxicity are of very little concern in horses...which is good news for us owners!
Scurvy, which is characterized by tiredness, rash on the legs, and bleeding gums, is the classic sign of vitamin C deficiency. However, scurvy has never been reported in horses.
Even though scurvy has never been reported in horses, a few studies have linked low ascorbic acid blood levels with other diseases.
It is important to realize that these studies have simply linked the two...as of yet, there has been no determination as to whether or not it is a cause and effect relationship.
For example, it could be something completely different that is causing both the low ascorbic acid blood level AND the disease -- in which case supplementing to increase the ascorbic acid blood level would not get rid of or prevent the disease.
These diseases include strangles, acute rhinopneumonia, increased wound infections after operations, and decreased performance levels.
Even more good news about C...vitamin C toxicity has never been reported in horses!
In fact, a toxic level hasn't even been established in most domestic animals. Since C is one of the water-soluble vitamins, this is not really surprising, as excess is simply flushed out of the body.