Oxygen is essential for human life and necessary for energy production, but in some forms it can be damaging to the body as reactive oxygen species (ROS): free radicals. The majority of ROS are produced in the mitochondrial electron transport chain during energy production. Physical exercise augments the production of free radicals and other forms of reactive oxygen species. End-products of oxidative damage are observed in the blood and tissues after acute intensive exercise as well as signs of decreased levels of antioxi-dants in some studies. Strenuous exercise may manifest an imbalance between the production of ROS and antioxidant defences, resulting in an oxidative stress situation in the body . In fact there is some evidence which implicates ROS as an underlying cause of exercise-induced muscle fatigue and damage. During an acute bout of strenuous exercise the immune system is activated and produces a substantial amount of ROS, which may cause an inflammatory process. The immune system produces ROS to kill bacteria and viruses. During b-oxidation of large amounts of fat, as occurs during starvation, there is a substantial production of ROS; this also occurs during oxidation of amino acids through degradation of xanthine to uric acid. In addition there is a ROS production during the autooxidation of catecholamines . The body has created an extensive protective system against these potentially damaging species, the antioxidant system. If the production of free radicals is large enough to overcome the antioxidant defence system, oxidative stress will ensue. Training seems to induce an adaptation with elevation of antioxidant protection through increased levels of the key antioxidant enzymes: the zinc-containing superoxide dismutase, iron-containing catalase and selenium-containing glutathione peroxidase. Training also seems to reduce signs of oxidative stress.
Important dietary sources of antioxidants include vitamins C and E, carotenoids, zinc and selenium, whereas uric acid, bilirubin, ubiquinone (Q10) and the thiol glutathione are important endogenous an-tioxidants. It is not yet fully known whether the body's natural antioxidant defence system is sufficient to counteract the increase of free radical production during intense exercise. There is, however, evidence that antioxidant consumption increases during excessive prolonged exercise, but not to what extent.
Some studies have reported that supplementation with antioxidants, such as vitamins C and E and thiol compounds (e.g. ^-acetyl-cysteine and a-lipoic acid) might have some protective properties against tissue damage induced by oxidative stress . Antioxidant supplements have, however, not been shown to increase performance. The balance between ROS production and availability of antioxidants plays a very important role in maintaining an intact immune system. Anti-oxidant deficiencies have been shown to impair immune function and supplementation has been shown to improve protection against infections in some studies. However, megadoses and unbalanced supplementation with antioxidants may be deleterious as they may cause autoxidation and increased tissue damage and suppress immune functions . Thus recommendations for athletes should give priority to increasing the dietary intake of food items containing naturally occurring antioxidants, such as vegetables and fruits.
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