Renal osmol clearance during prolonged heavy exercise.

Serum and urine osmolality and creatinine were measured in fifty-eight well-trained men, aged 21-57 years, in connection with three 70 km cross-country ski races taking place under essentially the same external circumstances, leading to slight dehydration. Although the race urine flow was low, 0.51 (0.05-1.10) ml/min, the urine osmolality and the urine to serum osmolality ratio were relatively low, averaging 860 (399-1220) mosm/kg H2O and 2.84 (1.32-4.10), respectively. Decreasing urine flow and endogenou- creatinine clearance were associated with decreasing urine to serum osmolality ratio, indicating declining urine concentrating ability. Thus, the data confirm the author's recent suggestion, based on the measurement of a limited number of urine solutes, that during prolonged heavy exercise a decrease in urine flow and glomerular filtration rate is accompanied by a decline in the urine concentrating ability.