The immediate and delayed effects of marathon running on renal function

Daily blood and 24-hour urine samples from 6 runners were studied for 2 days before and for 5 days after a 42.2 km. marathon footrace run in cool environmental conditions. Although the race caused muscle damage as shown by the increased post-race serum creatine kinase activity and C-reactive protein levels, renal function measured by urine flow rates, creatinine clearance and protein excretion was normal during the race. Sodium and fractional sodium excretion decreased during the race despite a maintained osmolar clearance, and remained low for the next 48 hours, whereas osmolar clearance decreased sharply for the remainder of the race day but it was significantly elevated on days 2 to 4 after the race. Creatinine clearance was increased significantly 24 hours after the race, and reached its peak 3 days after the race, while urine flow rates were elevated from days 2 to 5 after the race. Urea excretion was significantly decreased 3 to 5 days after the race, while creatinine excretion was increased significantly on day 3 after the race. Glomerular proteinuria occurred 24 hours after the race with no associated reduction in tubular reabsorption of the low molecular weight protein beta-2-microglobulin. This study shows previously unrecognized substantial delayed effects of prolonged exercise on renal function. The nature of these changes may reflect catabolic followed by anabolic processes in muscle as well as changes consequent on excess sodium retention and related fluid compartment shifts.