Evaluation of renal function and fluid homeostasis during recovery from exercise-induced hyponatremia

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-1·h-1), 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.