Spontaneous decline in exercise-induced proteinuria during a 100-mile triathlon

To study the effect of prolonged exercise on glomerular permeability and proteinuria, we collected serial urine samples from six athletes during a 100-mile triathlon. Urine collected just before, at the midpoint of, and immediately after the race was analyzed for creatinine by an automated chemistry analyzer, pack method, and for microalbumin by radioimmunoassay. By midrace, the urinary albumin-creatinine ratio increased from the prerace mean +/- SEM of 3.5 +/- 0.5 to 38.3 +/- 11.7 mg/g. The ratio then declined to 12.5 +/- 2.7 mg/g by the end of the race (P less than .04). Similarly, the urinary albumin level increased significantly from 5.9 +/- 0.7 to 80.5 +/- 26.8 micrograms/mL by midrace, followed by a decline to 39.2 +/- 12.9 micrograms/mL. The initial increase in albuminuria was expected and reflects the increase in exercise-induced cardiac output and glomerular permeability. The subsequent decline in albuminuria and albumin-creatinine ratio, despite continued exercise, was unexpected and indicates a decrease in glomerular permeability. Further study is warranted to determine the mechanism of this apparently protective renal response to prolonged exercise.