Heavy physical exertion can cause cellular damage
Oxidative stress is a condition in which the horse’s cells are exposed to excessive amounts of reactive oxygen species (ROS) and other reactive oxygen compounds that cause damage. During normal metabolism, cells produce oxygen compounds such as free radicals, which can harm cellular structures and functions. Under normal circumstances, the body can neutralize and remove these compounds efficiently through antioxidant systems. However, in certain situations—such as illness or heavy physical exertion—the production of reactive oxygen compounds can exceed the body’s ability to neutralize them. This imbalance leads to oxidative stress, which can damage cell membranes, proteins, DNA, and other cellular structures.
Intense physical training and athletic performance increase oxygen consumption and metabolic reactions, leading to increased production of reactive oxygen compounds. This can result in various types of damage:
Muscle damage: Heavy muscle exertion can cause micro-damage to muscle tissue, triggering inflammation and increasing oxidative stress.
Respiratory oxidative stress: Intense aerobic exercise increases oxygen intake and exposes the body to greater oxidative stress.
Environmental factors: Athletic performance may expose the body to increased levels of oxidizing compounds such as pollutants and ultraviolet radiation.
Insufficient recovery: Inadequate rest between intense training sessions reduces the body’s ability to counter oxidative stress, allowing it to accumulate.
Excessive oxidative stress can negatively affect the health and performance of sport horses, leading to increased inflammation, muscle damage, impaired recovery, and a higher risk of illness and injury. Proper nutrition, hydration, rest days, and recovery are therefore essential for hard-working and competing horses. An antioxidant-rich diet—and antioxidant supplementation when necessary—can help combat oxidative stress and protect the body from its harmful effects.
IMPORTANT ANTIOXIDANTS
Dietary antioxidants such as vitamin C, vitamin E, beta-carotene, flavonoids, and other polyphenolic compounds help neutralize reactive oxygen species and protect cells from oxidative damage. Antioxidants such as vitamin E, vitamin C, glutathione, and selenium all have protective effects.
VITAMIN E
Vitamin E is the most important antioxidant. It is fat-soluble and protects cell membranes from free radical damage. Cell membranes are composed of lipid molecules, which are highly reactive with free radicals, making them especially vulnerable to damage. Vitamin E is essential in the horse’s diet. Deficiency can cause coordination problems and various muscle and nervous system disorders. Without vitamin E, cell membranes become permeable and eventually break down, destroying the cell. Vitamin E has been shown to protect horses against equine degenerative myeloencephalopathy (EDM), a neurological disease. It is particularly important for sport horses, as heavy physical work can cause oxidative stress, lower circulating vitamin E levels, and lead to muscle problems and reduced stamina and performance. Supplementing vitamin E before stressful situations (such as transport or competition) can be beneficial, as it enhances immune function and protects muscle and nerve cells.
Sources of vitamin E:
Fresh pasture grass is rich in vitamin E, but its content decreases as the plant matures and during processing (heat, grinding, etc.). Sun-drying hay reduces its vitamin E content. Because vitamin E is fat-soluble, adding large amounts of oil to the diet increases vitamin E requirements. Vitamin E is often mixed with oil for this reason. Research shows that even high doses of vitamin E are not toxic to horses, although very high amounts should be monitored as they may interact with other nutrients such as beta-carotene.
VITAMIN C
Vitamin C is another important antioxidant and is water-soluble. It is found inside and outside cells, where it neutralizes free radicals and helps prevent cellular damage.
When is vitamin C needed? Healthy horses can synthesize vitamin C in the liver. However, during stress (e.g. aging, intensive training, long-distance transport), endogenous production may not meet demand. Vitamin C supplementation can help reduce the negative effects of stress on the immune system. Being water-soluble, vitamin C poses no risk of overdose, as the liver reduces synthesis when it is not needed.
SELENIUM
Selenium is a trace element found in plants. On its own, selenium has limited antioxidant capacity, but when combined with vitamin E, it becomes a powerful antioxidant. Together, selenium and vitamin E prevent free radical–induced nerve cell damage, helping prevent neurological disorders and muscle degeneration (such as white muscle disease). Selenium is also an essential component of the antioxidant enzyme glutathione peroxidase.
Sources of selenium:
Soil selenium levels vary by region. Forages harvested from alkaline soils, such as rocky mountainous areas, may contain very high selenium levels. Nordic soils are generally low in selenium, so it is often added to fertilizers. Selenium is toxic in high amounts, so intake should be monitored. Horses require about 0.3 mg/kg of diet (approximately 3 mg/day). Chronic toxicity can occur at intakes of around 10 mg/day, with early signs including loss of mane and tail hair and cracked hooves. Acute toxicity symptoms include blindness, behavioral changes, weight loss, excessive salivation, and increased heart and respiratory rates due to heart and liver tissue damage. Toxicity risk increases if the diet is also high in copper.
ANTIOXIDANT ENZYMES
In addition to vitamins and minerals (such as zinc), there are antioxidant enzymes including superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase. These enzymes act inside cells rather than on cell surfaces like vitamin E and selenium.
Superoxide dismutase is found in many organs, with highest activity in the liver, followed by kidneys, brain, heart, and muscles.
Glutathione peroxidase is found mainly in red and white blood cells and prevents oxidation of cell membranes by consuming free radicals. Selenium is critical for its structural integrity.
Glutathione reductase is essential for regenerating glutathione, completing the antioxidant cycle. Catalase, which requires iron, is mainly found in the liver and muscles.
WHAT DOES THIS MEAN FOR THE HORSE?
All stressful conditions—including exercise—require antioxidant activity to counteract increased oxygen consumption and oxidative stress. Horses that travel long distances and compete frequently are more prone to antioxidant deficiencies. A balanced diet and access to fresh pasture or high-quality hay are therefore essential. In some cases, antioxidant supplementation may be necessary, especially for horses not on pasture.
Pregnancy: Pregnancy can also cause oxidative stress, especially in late gestation when fetal growth peaks. Adequate vitamin E, selenium, and essential minerals are crucial during this time.
Aging: As horses age, metabolic efficiency declines, increasing susceptibility to oxidative damage. Older horses—especially those in training—benefit from increased antioxidant intake, particularly vitamins E and C.
Illness: Sick horses are more prone to oxidative stress, often compounded by reduced appetite and nutrient absorption. Additional antioxidants may be needed to support immune function.
Toxins in feed or water can also increase oxidative stress. Poor-quality forage, nitrates, and mycotoxins may all negatively affect health.
Equine motor neuron disease (EMND) is strongly associated with low vitamin E status. Chronic inflammatory and degenerative joint diseases are also linked to excessive free radical production.
As energy demands increase—during heavy exercise or pregnancy—oxidation and free radical production rise. While horses have internal antioxidant defenses, these may not always be sufficient. The best prevention is maintaining overall health through a balanced diet with adequate nutrients, vitamins, and minerals.
Sources:
Chew, B.P. 1996. Importance of antioxidant vitamins in immunity and health in animals. An. Feed Sci. Tech. 59:103-114
Clarkson, P. and Thompson, H. 2000. Antioxidants: What role do they play in physical activity and health? Am. J. of Clin. Nutr. 72 (supp.): 637S-46S.
Thomas, Heather S. 2004. The Role of Antioxidants. The Horse Magazine.
Williams, C.A. 2005. Are you ‘Stressing Out’ your horse? Rutgers Cooperative Extension. FS656.
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