We have already discussed reactive oxygen species and their harmful potential in previous articles. These molecules can cause damage to DNA, proteins and lipids, and can affect a wide range of cells and tissues in horses.
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Antioxidant substances play a protective role, preventing oxidation and reducing the risks associated with it.
Among the main antioxidants, the following stand out:
VITAMIN E
Vitamin E is possibly the best known and most studied antioxidant in horses. Vitamin E refers to several molecules (tocopherols and tocotrienols), with the most potent and bioavailable being α-tocopherol.
Vitamin E is a fat-soluble molecule that can be incorporated into the lipid membrane of cells, protecting them from damage associated with oxidation. Since it is not produced by horses, it must be supplied through the diet.
The requirements for this vitamin are 1 to 2 International Units (IU) per kilogram (kg) of live weight (bw) (NRC, 2007), which corresponds to 500 to 1000 IU in a horse weighing 500 kg bw. Some researchers believe that the requirements for vitamin E may be higher, particularly in horses subjected to intense physical activity and with diets with high fat content. In case of doubt, the serum vitamin E level should be assessed and a supplementation protocol established in accordance with it.
Since vitamin E is naturally present in grass, constant access to good quality pasture should alone ensure that these needs are met.
In stabled horses, it is necessary to meet their vitamin E requirements in the form of compound feed, because the processing of the plant and storage result in significant losses of this vitamin, with its content being considered negligible.
Although excess amounts of this vitamin are well tolerated, they should be avoided as they may be detrimental to the absorption of other substances (such as beta-carotene).
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VITAMIN C OR ASCORBIC ACID
Vitamin C, also known as ascorbic acid, helps prevent oxidative damage by capturing free radicals. This molecule works in conjunction with vitamin E. However, since horses have the capacity for endogenous synthesis (i.e., they are capable of producing this molecule), there are no defined requirements for including this vitamin in the diet (NRC, 2007).
Additionally, there is no consensus regarding the hypothesis that oral supplementation results in an increase in serum levels of this vitamin (some authors consider that its absorption after oral administration is poor, others question whether it causes a decrease in its endogenous synthesis).
Supplementation may make sense in senior or immunocompromised horses.
BETA-CAROTENE
Beta-carotene is a precursor of vitamin A, a fat-soluble vitamin that prevents oxidative damage at the membrane level. Like vitamin E, it is a vitamin abundant in pasture that is affected by the processing and storage of the plant, which is why it is normally added to compound feeds.
SELENIUM
Selenium acts synergistically with vitamin E, which is why most vitamin E supplements include it. This mineral is also essential for the antioxidant activity of the enzyme glutathione peroxidase. Insufficient amounts of selenium affect the antioxidant activity of this enzyme. Excesses should, however, be avoided, as this substance poses a significant risk of toxicity.
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OTHER SUBSTANCES
In addition to vitamins and minerals with antioxidant properties, there are enzymes that perform this same function. These enzymes also depend on the presence of certain nutrients, such as iron (in the case of catalase), or copper, zinc and manganese (in the case of superoxide dismutase). The supply of these minerals through a correctly balanced diet thus contributes to adequate antioxidant activity.
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