The effect of marathon cycling on renal function.

The stress of strenuous long-term exercise may alter renal function. Whether this is also true for marathon cycling is unknown so far. The purpose of this study was to evaluate renal function following competitive marathon cycling. We investigated 38-male, well-trained recreational cyclists credibly not taking any kind of doping who participated in the Otztal Radmarathon. Blood and urine specimens were taken the day before, immediately after and one day after competition. Baseline renal functional parameters--normal before competition--increased significantly afterwards and remained elevated during 24 hours of recovery. The rises in serum creatinine, urea and uric acid were 20, 54 and 42 % (p < 0.001 respectively). The corresponding decline in estimated creatinine clearance was 18 %. In all athletes the serum urea/creatinine ratio rose above 40, fractional sodium excretion and fractional uric acid excretion fell below 0.4 % and 15 %, indicating reduced renal perfusion. The observed effects lasted for at least 24 h despite a stable fluid balance during the race and an expanding plasma volume (PV) in the recovery period. Levels of haematocrit remained unchanged immediately post-race but significantly declined from 0.44 to 0.41 on the following day (p < 0.001). The calculated rise in PV was + 10.8 %. Electrolyte homeostasis was preserved throughout the observation period. Post-exercise proteinuria was small and of the mixed glomerular-tubular type. There was neither evidence for exercise-induced haemolysis, nor for significant skeletal muscle damage. The finding obtained from well-hydrated recreational athletes reveals that the extraordinary strains of marathon cycling influence renal function only on a minimal scale. Though minor, the physiological effects were long-lasting. The results obtained suggest that a reduced renal perfusion is the mechanism responsible for the slight impairment of renal function following exhaustive marathon cycling.