Exercising horses generate tremendous amounts of free radicals as a consequence of energy generation and repair processes. Plant polyphenols are natural compounds with strong antioxidant capacity. The skin and seeds of grapes are rich in polyphenol, with the most well known of these being Resveratrol. Another well studied class is the bioflavonoids, particularly bioflavonoids from citrus fruits which includes Hesperidin, Quercetin and Rutin.

- Vitamin C is definitely an important antioxidant in the immune system and protects the interior of the cells Horses can synthesize their own vitamin C. However, it has been clearly shown that Vitamin C levels in lung fluid from horses with chronic lung disease are low.
- Vitamins A and E are fat soluble antioxidant vitamins which work protecting the cell membrane from attack. Vitamin A, from Beta-Carotene, is particularly important for skin
- The B vitamins are required to support active division of immune system cells and some have specific immune system functions. Riboflavin is involved with both the destruction of organisms by primitive immune cells and in cellular immunity. Thiamine is also essential to normal functioning of T cells in the cellular immune system. Folic acid deficiency has been clearly linked to increased infection risk.