Q: I thought horses made their own vitamin C so why would I need to supplement it?
A: Healthy horses, like many other mammals, but unlike humans and hamsters, can synthesize their own vitamin C within their liver. Therefore healthy horses in light work fed a balanced diet probably don’t need supplementary vitamin C.
There is evidence however, that in certain circumstances, horses may not make enough vitamin C, or their internal levels may be depleted and supplementation can be useful. During illness or injury, if a horse has had a high parasite burden, a bad infection, and during periods of stress and if the horse is undergoing intensive exercise, extra vitamin C is recommended.
Horses with recurrent airway obstruction (RAO), which used to be called COPD, have depleted levels of vitamin C in their blood and lungs, and they should always be supplemented. Since researchers have shown that stabling challenges horses’ lungs, all stabled horses could benefit from supplementary vitamin C to help support their airway function. Horses with Cushing’s syndrome, also known as PPID, may benefit from extra antioxidants including vitamin C, since the dysfunction they have in their pituitary gland may be caused by oxidative processes.
The most commonly used type of vitamin C, ascorbic acid, is not very bioavailable to the horse, and ascorbyl monophosphate or palmitate are more useful for supplementing horses. Use 10-20 g per day for a typical 500 kg horse.
It’s important that supplementation of vitamin C is never abruptly stopped, because this could be harmful. The amount fed should be gradually decreased over at least a week and preferably two, if the supplementation is going to be stopped.