Abstract

Renal function including fluid and electrolyte balance was studied during recovery in eight subjects who developed symptomatic hyponatremia (HN; plasma sodium concentration <130 mM) during an 88-km ultramarathon footrace and compared with results for normonatremic runners (NN; n = 18, mean postrace plasma sodium concentration, 138.2 +/- 1.2 (SE) mM). Estimated fluid intake during the race for HN was 12.5 +/- 1.6 (SE) liters over 9 h 41 min (+/-28 min). HN excreted a net fluid excess of 2.95 +/- 0.56 (range 1.2-5.9) liters compared with a fluid deficit of 2.7 +/- 0.3 body weight in NN. The sodium deficit was 153 +/- 35 mmol in HN and 187 +/- 37 mmol in NN. Despite the fluid overload, plasma volume was decreased by 24.1 +/- 5.0 in HN compared with 8.2 +/- 2.6 in NN. Serum renin activity (5.1 +/- 2.0 ng/ml/h), aldosterone concentrations (410 +/- 34 ng/l), creatinine clearances (174.8 +/- 28.2 ml/min), and urine output (6.4 +/- 1.0 ml/min) were markedly elevated in HN during recovery. Thus the hyponatremia of exercise results from fluid retention in subjects who ingest abnormally large fluid volumes during prolonged exercise. Verf.-Referat