The Role of Antioxidants

Oxidation is a process the body uses for normal energy production and immune function. This is part of the process that enables the body to transform nutrients such as carbohydrates, fats, and proteins into energy. During oxidation (the "burning" of fuels), a few unstable oxygen molecules called free radicals or reactive oxygen species are formed. These molecules have an unpaired electron that makes them quick to react with other molecules, stealing electrons from their outer orbits. This damages the other molecules and can irreparably damage cell walls.

Carey Williams, PhD, equine extension specialist and assistant professor of animal science at Rutgers University, says free radicals can affect several cell components during oxidative stress. "In humans, oxidative stress is associated with heart disease, cancer, Alzheimer's disease, and other neurological disorders," she says. "Free radicals promote breakdown of DNA by inhibiting the normal production process or breaking the strands (mutating normal DNA). They also degrade protein within cells.

"They also break down lipids (fats), which are major parts of cell membranes," Williams continues. "This basically destroys the cell; all the contents leak out and the cell dies. Vitamin E helps maintain the cell by protecting the membrane and keeping it whole.

Being fat-soluble, vitamin E can work within the cell membrane, which has a protective fatty layer of lipids.

"During exercise, there is a much higher than normal level of lipid peroxidation (oxidative deterioration). More membranes are being broken down," she explains. This is one reason muscles become stiff or sore during or following extreme exertion. Free radical production is increased due to the increase in oxygen uptake during exercise.

Free radicals are not always bad; they do serve a purpose in the body. "Since they degrade membranes and break down cells, they are a useful aspect of the immune system, to break down bacterial cells that may be infecting the body, for instance," says Williams. "They are part of the arsenal the body uses in the battle against unwanted cells. They can be used in an 'oxidative burst' within the immune cells, in which bacteria or viruses are being attacked and destroyed by the free radicals," she explains.

Antioxidant activity
During oxidation (the "burning" of fuels), a few unstable oxygen molecules called free radicals or reactive oxygen species are formed. These molecules have an unpaired electron (red, above) that makes them quick to react with other molecules, stealing electrons from their outer orbits. This damages the other molecules and can irreparably damage cell walls. Antioxidants act to stabilize free radicals and halt this destructive process.

Balance is what's important. "Problems occur when things get out of balance," she says. "You don't want a total lack of free radicals; that would be detrimental to the immune system. But you also don't want an overwhelming number of them or they start damaging and destroying good cells. Everything in life has a purpose."

How They Work

Under normal circumstances, substan-ces called antioxidants (which include vitamins E, C, and A, selenium, glutathione, lipoic acid, coenzyme Q10, and isoflavins) act to stabilize free radicals and halt this destructive process. In a healthy, unstressed horse on pasture, some free radicals are produced because this is a natural body process, but antioxidants easily counteract them. If a horse is stressed or exercising hard, however, the oxidation process speeds up and more free radicals are produced. Problems also occur when a horse's feeds don't have enough vitamins.

Countless body cells die off daily during oxygen utilization in body tissues to be replaced by young, healthy cells. This is a normal fact of life. But when free radical numbers are higher than normal due to stress, extreme exertion, inflammation, or other conditions that create more wear on the body, more cells are destroyed than are readily replaced. The horse begins to show signs of fatigue, disease, or damage in whatever body tissue or organ is most affected. The free radicals start a chain reaction of damage, and it is up to the antioxidants to stop it. If the chain reaction succeeds in continuing, excessive cell damage occurs.

"Antioxidants help each other. Once they scavenge free radicals, they become radicals themselves," Williams explains. "Vitamin E may become an alpha-tocopherol radical. Vitamin C may become an ascorbate radical. Now they have an unpaired electron. They are not as reactive and not as damaging as the other free radicals, but they are useless. If there are extra antioxidants in the body, however, these can come along and free up the vitamin E and C radicals and make them active again. You don't need a vitamin E molecule to scavenge a vitamin E radical. Vitamin C can do it, glutathione can do it, etc. They all work together."

Vitamin E--The Biggest Hero

Vitamin E contributes most generously to natural antioxidant defenses, says Kathleen Crandell, PhD, nutrition consultant for Kentucky Equine Research. Vitamin E is a collective term for eight different compounds of plant origin (four tocopherols and four tocotrienols), two of which have strong antioxidant properties. Alpha-
tocopherol is the most biologically active. This fat-soluble compound is embedded in each cell's protective fatty layer, where it can disable free radicals before they damage the cell wall. Alpha-tocopherol has an affinity for fat and is thus attracted to cell membranes, which are composed of unsaturated fatty acids, explains Crandell. If a horse has enough vitamin E, muscle membranes remain more intact during exercise, with less cell breakdown and less leakage of muscle enzymes into the bloodstream.

Tissues that are damaged most are the ones the body uses most. Nerves and muscle cells are commonly damaged by free radicals because they are highly active tissues. Vitamin E can help ward off problems that affect the nerves--degenerative diseases like wobbler syndrome, equine protozoal myeloencephalitis (EPM), equine degenerative myeloencephalopathy (EDM), and various muscle diseases.

"EDM is a degenerative disease of the central nervous system in which horses become uncoordinated, but they respond very well to megadoses of vitamin E (6,000 to 10,000 IU daily; the National Research Council recommends 80 to 100 IU of vitamin E/kg of dietary dry matter in the total diet for foals, pregnant and lactating mares, and working horses)," she says. "Large doses of vitamin E are not toxic to horses; you can't overdose with vitamin E like you can with vitamin A. Horses on a diet low in vitamin E often develop EDM. We're seeing a lot of cases in New England due to the short growing season (horses are not on green pasture very long), and it has become customary to keep them in a pen and feed hay. If people are counting their pennies, they also feed cheap grains that have little or no vitamin E added," says Crandell.


Vitamin E and selenium work together in preventing nerve degeneration, and they also help prevent tying-up and other muscle problems in hard-working horses. Selenium, an important trace mineral, works with vitamin E as an antioxidant; the functions of the two are intricately linked, says Crandell. Deficiencies of one can be compensated for if there is an adequate supply of the other. If vitamin E is present in cell membranes, it can decrease formation of harmful lipid peroxides. If there is selenium in the intracellular fluid, it will remove the lipid peroxides that do form, says Crandell. The amount of either antioxidant needed by the horse depends on the available amount of the other. Optimum amounts of both, however, are necessary to minimize oxidation-caused tissue damage.

Selenium is available in plants when there is an adequate amount in the soil. Some regions, however, are deficient in selenium and others have too much--which can create toxicity when horses eat plants that accumulate this mineral. When using supplements, you need to know whether your feeds are deficient, adequate, or already too high in selenium.

Vitamin C

Ascorbic acid (vitamin C) also plays an important role in neutralizing harmful free radicals. This vitamin is usually not needed in horse diets because they can create their own. In a healthy horse, vitamin C is synthesized in the liver from glucose. In periods of stress, however, vitamin C levels can drop below what is efficient for antioxidant activity.

Vitamin C, being water-soluble, can work inside and outside the cell to combat free radical damage. This vitamin works with vitamin E to protect the cells. After a molecule of vitamin E has inactivated a free radical, it loses its ability to tackle others. But in the presence of vitamin C, the "spent" vitamin E molecule can be regenerated to continue its battle against free radicals.

"Whenever a horse is stressed or ill, vitamin C blood levels drop," Crandell says. "Stress may affect production of this important antioxidant, or perhaps the body is using more and production can't keep up with the need. In studies, however, we've not had much luck in getting obvious response to supplementation with vitamin C. It takes a lot of vitamin C to get any kind of response in the blood levels. The recommended dosage is about 10 grams of vitamin C for the stressed horse to be able to utilize it. Horses are good at producing it, but are not good at absorbing it from the intestine."

Sarah Ralston, VMD, PhD (associate professor in the Department of Animal Science at Cook College, Rutgers University), says horses older than 20 which have problems with recurring infections (such as thrush or skin infections) and horses with pituitary tumors might benefit from 10 grams of vitamin C a day for an average-size horse.

"Any severely stressed horses--heavy show schedule, prolonged transport, etc.-- might benefit from vitamin C supplementation during the period of stress," adds Ralston. "The supplement does not help much if given before stress, or a long time after the stress. Up to five days after the period of stress may be helpful, but this will depend on how stressed the horse is."

Following a long van trip, she recommends giving a horse vitamin C as soon as he arrives at his destination, and for a day or two after. If horses are at a multiple-day show, you can supplement during the show, she says.

"Don't give vitamin C for a prolonged period if horses don't need it," says Ralston. "Unnecessary supplementation seems to adversely affect their own ability to produce it, and it might enhance their excretion of it. If they don't need it--if they are not stressed and don't have pituitary problems--don't give it to them."

Does My Horse Need Supplements?

If a healthy horse is on green pasture, he generally has all the vitamins (and antioxidants) he needs. Many forages contain adequate amounts of vitamin E for a normal horse. Orchardgrass pasture supplies more vitamin E per day than most horses need. Alfalfa, timothy, fescue, and Kentucky bluegrass have less (but adequate) amounts, although hay made from these forages has significantly less than fresh plants, since drying depletes vitamin E content. As much as 80% of vitamin E can be lost in making hay.

Maturity of plants, length of time weathering (or baling forage that's too dry and sun-bleached), and long storage can reduce vitamin E in harvested forage. If a horse does not have access to pasture and is fed harvested or processed feeds, he might not have enough vitamin E, especially if he's in an athletic career that creates more stress and a need for more antioxidants.

Grains are lower in vitamin E than forages. Green forages, properly cured hay, cold pressed oils, and wheat germ oil can be good sources of vitamin E. Processing (heating, grinding, pelleting) and long or improper storage of feeds can decrease vitamin E content. Hay subjected to rain, sun, or high temperatures during curing or storage can lose much of its vitamin E. Stored hay might lose up to 50% of its vitamin E in one month, so year-old hay is usually much more deficient than freshly harvested hay. A horse whose natural diet of grass has been replaced by long-stored hay and highly processed grains (and inexpensive commercial feeds with no added vitamin E) might not be receiving the recommended levels of vitamin E.

Hay grown in a dry climate where it can be cut and dried quickly is often higher in vitamins and other nutrients than hay grown in a wet climate where it takes longer to dry before baling.

"Here on the East Coast, even if hay doesn't get rained on, high humidity may make it slow to dry," says Crandell. "The longer it lies there exposed to the weather, the more nutrients are lost."

And what of other feeds? Drying corn artificially might reduce its alpha-tocopherol level (the most active form of vitamin E) by as much as 50%, she says. Vegetable and soybean oils have significantly more vitamin E than grains, but refining these oils can diminish vitamin E severely. Cold pressed oils keep a much higher amount (although they are can also become rancid if they aren't fresh or stored properly). Since many grains will be low in needed vitamins, a high-quality grain mix that is fortified with vitamins can help ensure the horse is getting adequate amounts of antioxidants.

Adding vitamin C to commercial feed mixes is usually not done; it's neither necessary nor cost-effective. "Older horses, however, might benefit from daily vitamin C supplementation," says Crandell. "Research on older horses has shown better response to vaccinations when they were supplemented with vitamin C."

More Antioxidants for Oldsters

Williams says the levels of oxidative stress in older humans are higher than in young people, and this is probably true in horses as well. "The ability to metabolize oxygen within the cell is less efficient than in the younger body," which means more waste products are created, she says.

Older equines might not absorb nutrients (such as vitamins and minerals) as well as animals in their prime, but this difference might not be as great today because we have more effective deworming practices now, says Crandell. There could be less damage to the gut lining in an older horse today than 50 years ago.

"Still, with age, some parts of the horse's body just don't function as well, and there is some degeneration of the lining of the intestines," Crandell adds.

Prepared senior feeds for horses are high in protein, vitamins, and minerals since older horses might not absorb nutrients as easily. Antioxidants become very important for the older horse, since aging bodies seem more vulnerable to oxidation damage. In human medicine, people over 65 are advised to use more antioxidants, and this is probably good management for elderly horses as well.

If an older horse exercises much, he needs more antioxidants. Today, many horses are staying active in competition for a longer period of their lives. We have extended the active span of their lives with good health care and nutrition--it is not unusual to see horses in their 20s still active in many sports.


Most mixed feeds have some added vitamins that are balanced for the horse's average requirements. But, if a horse is undergoing stress or high-intensity competition, he might need higher levels. Yet you don't want to overfeed supplements, or you could create an unbalanced diet. Some nutrients interact with others, and if you upset the balance, what you are feeding might become more detrimental than beneficial.

"Probably the biggest problem we have with antioxidants is using synthetic forms and hoping they are absorbed," says Kathleen Crandell, PhD, nutrition consultant for Kentucky Equine Research. "Vitamin E has been well tested, and natural forms are found to be better absorbed than synthetic forms. How does the average horse owner know which product is the right form to use? There are many products on the market, so this is the hard part for the horse owner--making the choices."

In periods of stress, adding vitamins E and C to the diet might be beneficial. Even if a horse is getting plenty of vitamin E, it might be poorly utilized in some situations. For example, any debility or illness that interferes with intestinal function (such as damage by parasites or diarrhea) can limit the amount of vitamin E absorbed through the gut lining. If you feed a vitamin E supplement, it should always be given with a meal; it must be mixed with fat molecules in the small intestine in order to be absorbed.

Carey Williams, PhD, equine extension specialist and assistant professor of animal science at Rutgers University, says timing is crucial when giving supplements. "With vitamin E, for instance, it won't give any benefit if you wait until the day before competition to give the supplement. Being fat-soluble, vitamin E is stored by the horse, and you need to build up his stores over at least a three-week period before the stress (i.e., weaning, long van trip, competitive event)," she says. "It's not the same for vitamin C because it is water-soluble."

The biggest single change in the 1989 National Research Council (NRC) recommendations for horses was to increase vitamin E requirements for growth and exercise from 15 International Units (IU) per kilogram of feed (165 IU/day for an 11-kg diet) to 80 IU per kilogram of feed (880 IU/day for an 11-kg diet), a five-fold increase over the 1978 recommendations. The general consensus among equine nutritionists today is that actual requirements for optimal health might be at least 720 IU daily for mares in late gestation, 950 IU for lactation, and 1,000 IU for horses doing intense work. These values could still be too low, and ongoing research will help us determine what is best for the horse.--Heather Smith Thomas


In her doctoral work, Carey Williams, PhD, equine extension specialist and assistant professor of animal science at Rutgers University, studied antioxidants in terms of reducing levels of oxidative stress. "This is a big factor in exercising horses--both high-intensity exertion and endurance exercise," she says. "Many feed companies market vitamin supplements or vitamin E/selenium combinations. Our study tried adding vitamin C to a vitamin E supplement to see if there would be additional benefit. The Arabian endurance horses we worked with were competing in an early-season ride of 50 miles. At this level of work, we found no additional effects by giving vitamin C. Maybe if the horses were working harder (100-mile ride, or a race later in the season with heavier competition) there would be more effect."

In another study she found "significantly reduced muscle enzyme levels (factors in tying-up) in horses supplemented with vitamin E" compared to horses which had no supplements. "Most of the horses that were given vitamin E had much lower levels," Willliams notes. Despite the fact that no horses in the study tied up, muscle enzyme levels are usually monitored by veterinarians since they are an indicator of muscle damage and possible tying-up.

"We found interesting results with lipoic acid, as well," she continues. "It has many of the same effects as vitamin E and can also increase the levels of other antioxidants. When supplementing both vitamin E and lipoic acid, we showed that this increases the levels of glutathione, vitamin C, and antioxidants in general. Antioxidants work in a system to help each other. This is beneficial to the immune system also, and the body stays healthier."

Lipoic acid is found in plants, Williams explains, but its natural form is linked to an amino acid, and it doesn't have as many antioxidant benefits. It doesn't convert to its free form until it's in the body. "We think that a pure lipoic acid supplement may give the horse more than what the body can produce when trying to convert it," Williams says. "Its pure form and reduced form each have different benefits."

She plans to do another study with racehorses to see if they have lower levels of muscle enzymes after high-intensity exercise when supplemented with antioxidants. "We're also looking at different levels of vitamin E to see if a really high level would have a negative effect, versus a normal supplemental dose," she says. "We also want to look at lipoic acid more, since it is a relatively newly discovered antioxidant and has been found to have outstanding benefits with endurance horses. We want to see how extensive these benefits are, and if they could take the place of vitamin E/selenium/vitamin C combinations. Lipoic acid has vast effects because it is both fat-soluble and water-soluble."--Heather Smith Thomas


Heat destroys vitamins, especially the antioxidant vitamin E. The heat in many feed processing methods can destroy much of the vitamin E content, since it is very sensitive to oxidation (heat increases oxidation). "When you buy vitamin E over the counter it is usually an ester, like alpha-tocopherol acetate. The vitamin E is attached to something else to stabilize it," says Kathleen Crandell, PhD, nutrition consultant for Kentucky Equine Research.

"A manufactured feed has gone through a heating process, but the people who produced the vitamin (alpha-tocopherol acetate, which is more stable to begin with), also put a protective layer around the product so it will not be as sensitive to oxidation," says Crandell. "It may look like a tiny bead. The vitamin E is in the middle, with a coating around it. Vitamin E for human use is usually in a gel capsule. The vitamin is an oil, and if it's exposed to air it will oxidize. In commercial feed manufacturing, vitamin E is protected in the same manner, but in a much smaller bead so it can mix well with the feed."--Heather Smith Thomas

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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