Optimal levels of vitamin C are beneficial, especially for older horses. However, you may read elsewhere that horses don’t need supplemental vitamin C or hear on the news that vitamin C can be toxic or damaging. Not true. The fact is it’s extremely high levels of vitamin C supplementation that can cause the vitamin to react abnormally in the body.
Here’s what you need to know:
Minimum isn’t optimal. Horses manufacture enough vitamin C on their own to prevent a true deficiency. However, that level is inadequate for optimal immune function and connective tissue/bone formation.
Older horses and those not on pasture need vitamin C. Blood vitamin C levels drop to low in horses without access to fresh grasses, and older horses in general need vitamin C in their diets. Supplement at a rate of 5 to 7.5 grams/day.
At the right level, vitamin C is an antioxidant. Too much, and it’s a free radical. Vitamin C functions as a helpful antioxidant, soaking up damaging free radicals. However, it can also turn tail and generate free radicals itself when the body tries to rid itself of any excess. How this occurs isn’t clear, but the solution is: Don’t use ridiculously high amounts that cause this reaction. Anything over 36 grams daily is high for a horse.
High acidity can cause gut reactions in the horse. High levels of vitamin C can produce gut distress and diarrhea, likely a direct result of the acidity, and probably alterations in gut organisms caused by that acidity.
Iron absorption increases with vitamin C. Vitamin C enhances iron absorption. Iron is a potential toxin and generates damaging free radicals. It can aggravate arthritis, feed bacterial infections and deplete the horse’s body of vitamins and minerals. The key is to avoid iron supplements unless the horse had a major blood loss, not to decrease the horse’s vitamin C supplementation.
Vitamin C effects on the muscles aren’t yet clear. A study (Childs, et al) that looked at the effect of vitamin C and N-acetyl cysteine supplementation in people after heavy exercise indicated that more muscle damage occurred in people taking the supplement than those who didn’t. However, the vitamin C dosage in this study was fairly high. Studies looking at the effects of vitamin C supplementation at appropriate doses noted either no change in the amount of muscle damage or some evidence of muscle protection by vitamin C.
Ester-C is a buffered vitamin C. Ester-C has a neutral pH rather than the acid one of regular C. It’s a more bioavailable form of vitamin C, meaning that it’s better absorbed. Ester-C isn’t as likely to cause the problems with iron absorption that regular C can cause.