Hyponatremia in ultradistance triathletes

Purpose: Hyponatremia ([plasma sodium] <135 mmol.L-1) is a potentially seri ous complication of ultraendurance sports. However, the etiology of this co ndition is still uncertain. This observational cohort study aimed to determ ine prospectively the incidence and etiology of hyponatremia in an ultradis tance triathlon. Methods: The subjects consisted of 605 of the 660 athletes entered in the New Zealand Ironman triathlon (3.8-km swim, 180-km cycle, a nd 42.2-km run). Subjects were weighed before and after the race. A blood s ample was drawn for measurement of plasma sodium concentration after the ra ce. Results: Complete data on pre- and postrace weights and plasma sodium c oncentrations were available in 330 race finishers. Postrace plasma sodium concentrations were inversely related to changes in body weight (P = 0.0001 ). Women (N = 38) had significantly lower plasma sodium concentrations (133 .7 vs 137.4 mmol.L-1; P = 0.0001) than men (N = 292) and lost significantly less relative weight (-2.7 vs -4.3%; P = 0.0002). Fifty-eight of 330 race finishers (18%) were hyponatremic; of these only 18 (31%) sought medical ca re for the symptoms of hyponatremia (symptomatic). Eleven of the 58 hyponat remic athletes had severe hyponatremia ([plasma sodium] <130 mmol.L-1); sev en of these ii severely hyponatremic athletes were symptomatic. The relativ e body weight change of the 11 severely hyponatremic athletes ranged from - 2.4% to +5%; eight (73%) of these athletes either maintained or gained weig ht during the race. In contrast, relative body weight changes in the 47 ath letes with mild hyponatremia ([plasma sodium] 130-134 mmol.L-1) were more v ariable, ranging from -9.25% to +2.2%. Conclusions: Hyponatremia is a commo n biochemical finding in ultradistance triathletes but is usually asymptoma tic. Although mild hyponatremia was associated with variable body weight ch anges, fluid overload was the cause of most (73%) cases of severe, symptoma tic hyponatremia.