Exercise-induced hyponatremia in ultradistance triathletes is caused by inappropriate fluid retention

Objective: To study fluid and sodium balance during overnight recovery foll owing an ultradistance triathlon in hyponatremic athletes compared with nor monatremic controls. Case Control Study: Prospective descriptive study. Setting: 1997 New Zealand Ironman Triathlon (3.8 Km swim, 180 Km cycle, 42. 2 Km run). Participants: Seven athletes ("subjects") hospitalized with hyponatremia (m edian sodium [Na] = 128 mmol L-1). Data were compared with measurements fro m 11 normonatremic race finishers ("controls") (median sodium = 141 mmol L- 1). Interventions: None. Main Outcome Measures: Athletes were weighed prior to, immediately after, a nd on the morning after, the race. Blood was drawn for sodium, hemoglobin, and hematocrit immediately after the race and the following morning. Plasma concentrations of arginine-vasopressin (AVP) were also measured post race, Results: Subjects were significantly smaller than controls (62.5 vs. 72.0 K g) and Lost less weight during the race than controls (median -0.5% vs. -3. 9%, p = 0.002) but more weight than controls during recovery (-4.4% vs. -0. 8%, p = 0.002). Subjects excreted a median fluid excess during recovery(1,3 46 ml); controls had a median fluid deficit (521 mi) (p = 0.009). Estimated median sodium deficit was the same in subjects and controls (88 vs. 38 mmo l L-1, p = 0.25). Median AVP was significantly lower in subjects than in co ntrols. Plasma volume fell during recovery in subjects C-5.9%, p = 0.016) b ut rose in controls (0.76%, p = NS). Conclusions: Triathletes with symptomatic hyponatremia following very prolo nged exercise have abnormal fluid retention including an increased extracel lular volume, but without evidence for large sodium losses. Such fluid rete ntion is not associated with elevated plasma AVP concentrations.