Altitude training has become more popular among athletes, at least in sports with a high demand for cardiorespiratory fitness. The oxidative stress and antioxidative defenses in hypoxia may be different from that at sea level, and it has been suggested that physical exercise at altitude might result in even more accelerated formation of free radicals and lead to even greater oxidative stress.40 This hypothesis is supported by one study, where the formation of ethane and n-pentane by lipid peroxidation increased as the concentration of oxygen in the ambient air decreased.41 Similarly, athletes had elevated levels of serum diene conjugation after 1 to 2 weeks stay at natural moderate altitude (1650 m above sea level) both before and after a skiing race (25 and 30%, respectively) when compared to the sea level.42 In that study, the serum antioxidant potential tended to be lower before the race at the 1650 m altitude compared to the sea level (1387 p,mol/l vs. 1713 p,mol/l, NS).42 During the race at the 1650 m altitude the serum antioxidant potential increased by 40% (1387 p,mol/l vs. 1943 ^mol/l, p = 0.016), but there was no change at sea level (1713 ^mol/l vs. 1582 p,mol/l, NS).42 The concentration of coenzyme Q was not determined.42
The increased serum antioxidant potential at altitude may be caused by several factors, such as changes in plasma proteins, uric acid, etc. Also, the altered energy metabolism (enhanced metabolism of glycerol and free fatty acids) at altitude may be related to the increased antioxidant potential.43 The changed lipid metabolism in hypoxia may also have raised the concentration of lipid soluble antioxidants like alpha-tocopherol and ubiquinol-10, which are known to increase serum antioxidant potential.30 However, further studies are needed to give any special dietary advice on possible antioxidant supplementation (like lipid soluble alpha-tocopherol and coenzyme Q) during training and racing for longer periods at moderate or high altitude.
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