Nutrients are broken down and converted into energy for normal metabolic function. Energy metabolism is generally divided into two types, aerobic and anaerobic. In aerobic metabolism, oxygen is used to produce energy with carbon dioxide and water as an end-product. However 1-2% of the oxygen is not used and results in the formation of free radicals or reactive oxygen species (ROS). These free radicals or ROS can damage DNA and contribute to degenerative changes throughout the body. Antioxidants are responsible for the removal of ROS, scavenging, or inhibiting formation of ROS.

During periods of stress, such as exercise, the rate of oxidation is higher because the body needs to produce energy by rapidly breaking down nutrients (such as protein, carbohydrates, and fat). Natural antioxidant defenses (endogenous antioxidants) help prevent free radical induced damage. However, after strenuous exercise, free radical production may overwhelm the system, and oxidative stress (an imbalance of free radical production and antioxidant defenses) can occur.

Oxidant damage can be reduced through supplementation with natural exogenous antioxidants. Natural antioxidants may be grouped as enzymatic or non-enzymatic. Superoxide dismutase and glutathione peroxidase are examples of enzymatic antioxidants. They are responsible for converting ROS to hydrogen peroxide and to water. Non-enzymatic antioxidants include vitamin E. Vitamin E blocks ROS attacks on fatty acids in cell membranes and the formation of lipid peroxides. Vitamin E also works in tandem with Se to decrease oxidative damage to cells.

It is important to note that all horses are subject to oxidative stress. There are some factors that can increase oxidative stress and the formation of ROS. In addition to horses in intense training, sick or debilitated horses will also have elevated levels of ROS. As the workload increases, dietary energy must be increase to maintain weight and condition thus increasing free radicals and the need for vitamin E and Se. Certain environmental conditions (poor air quality, smog) and dietary ingredients, (high fat) may also increase the need for certain antioxidants.

In summary:
• All horses are subject to free radical formation.
• Increase activity, work load, or environmental conditions can increase the formation of free radicals.
• Dietary ingredients may also influence antioxidant requirement (i.e., increase need for vitamin E in high fat diets).
• Superoxide dismutase and glutathione peroxidase are important parts of the antioxidant system.
• Vitamin E is an important nutrient that functions as an antioxidant.
• Branched chain amino acid supplementation may support optimum muscle metabolism while supplying substrates for muscle synthesis.