Stress, be it emotional or physical, can negatively affect a horse’s overall health and quality of life. Another type of stress, oxidative stress, also wreaks havoc in horses. Oxidative stress occurs when free radical production and accumulation exceed an animal’s ability to detoxify these highly reactive, unstable molecules. In everyday life, normal metabolic reactions, such as the oxidation of sugar to produce energy, generate free radicals as byproducts.
Excess free radicals can also be produced by inhaling or ingesting various pollutants, such as those in smoky air and smog—conditions many horses in the Western United States are experiencing right now.
Free radicals have unpaired electrons, which are what make them so reactive. Once generated, they bounce around the cell damaging fatty cellular membranes, proteins, and enzymes, as well as DNA.
Oxidative stress leads to tissue damage and potentially illness due to reduced immune function, said Carey Williams, PhD, an equine extension specialist at Rutgers University.
Luckily, the body has several naturally occurring antioxidants in place to squelch the damaging molecules and stop the chain reactions of destruction they cause. Vitamin E serves as one of the body’s most dominant and potent antioxidants, protecting various cells—including those of the immune system—from the harmful effects of free radicals.
Vitamin E essentially tucks itself into cellular membranes between long chains of fatty acid molecules that play vital roles in cellular health. When free radicals smash into cell membranes, they start a chain reaction of oxidation that vitamin E can stop, as long as adequate levels exist in the diet. The main source of vitamin E in the equine diet is fresh, green grass.
Human research suggests immune system cells are particularly prone to free radical damage, evidenced by the fact that white blood cells contain high levels of vitamin E. Further, studies in humans show that vitamin E plays a key role in the proper functioning of various aspects of immunity.
There is evidence in many domestic animals that vitamin E supplementation can improve immune function in immunocompromised individuals. However, “there are very few studies evaluating the role of vitamin E in immune function in the horse,” said Carrie Finno, DVM, PhD, Dipl. ACVIM, associate professor and Gregory L. Ferraro Endowed Director of the Center for Equine Health at the University of California, Davis, School of Veterinary Medicine.
Research also shows that horses fed diets low in vitamin E and selenium had increased immune responses after vaccination when supplemented with vitamin E. Further, vitamin E supplementation increased specific infection-fighting white blood cells’ bacterial killing capacities.
In sum, the available data imply that antioxidants support horses during times of oxidative stress, but Finno warns against oversupplementation.
“If horses are deficient in vitamin E, supplementation may be beneficial and have some supportive effects on immune function, but vitamin E deficiency needs to be determined using a blood test,” she said. “In addition to its potent antioxidant effects, vitamin E has some nonantioxidant functions. Oversupplementation can lead to prolonged bleeding and inappropriate metabolism of other drugs and vitamins, such as beta-carotene.”
Finno recommends owners establish baseline vitamin E levels for horses and monitor the levels regularly to ensure they stay in the normal range. She also recommends natural formulations over synthetic, because they are the most effective at increasing vitamin E levels in horses.
Horses of different ages and conditions might require different levels of supplementation. It’s important to work with your veterinarian to determine the most appropriate vitamin E levels for your horse prior to adding supplements to the diet.
More information on Finno’s research on vitamin E is available here.