Abstract
We have determined the urinary 8-hydroxydeoxyguanosine (8-OHdG) levels of 8 professional cyclists during a 4-day and a 3-week stage races. Monitoring of heart rates was used to establish zones corresponding to different intensities of exercise. The urinary 8-OHdG excretion increased significantly in the first day or the first week of each race, respectively, and did not show further increases thereafter. Maximum 8-OHdG levels were reached in parallel to longer times spent at high intensities of exercise. Urinary excretion of creatinine increased with exercise and changes in 8-OHdG levels were not detected when corrected by creatinine excretion. Serum TBARS concentrations did not change significantly at any point during exercise and glutathione concentration was maintained. We conclude that road cycling courses with an oxidative damage to DNA that is sustained as long as the exercise is repeated. Both adaptation of antioxidant defenses and a decreased capacity to maintain a high intensity of effort may contribute to explain absence of progressive increases in 8-OHdG excretion. The results of this study also indicate that in prolonged exercise urinary 8-OHdG is a more sensitive marker of oxidative stress than TBARS and confirm that the correction procedure using the amount of creatinine excreted should not be used when studying effects of exercise on urinary 8-OHdG.

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