Horses and Vitamin C
Horses (unlike humans) have the ability to manufacture vitamin C from glucose, because they produce an enzyme in their liver called L-gulonolactone oxidase. Under normal conditions, horses have enough glucose in their diets to produce all the vitamin C they need. When facing certain challenges, horses may use up vitamin C faster than they can manufacture it, such as during rapid growth in young horses, or when athletic horses are performing at high levels. At these times supplemental vitamin C can be beneficial.