To evaluate the role of fluid and Na ؉ balance in the development of exercise-associated hyponatremia (EAH), changes in serum Na ؉ concentrations ([Na ؉ ]) and in body weight were analyzed in 2,135 athletes in endurance events. Eighty-nine percent of athletes completed these events either euhydrated (39%) or with weight loss (50%) and with normal (80%) or elevated (13%) serum [Na ؉ ]. Of 231 (11%) athletes who gained weight during exercise, 70% were normonatremic or hypernatremic, 19% had a serum [Na ؉ ] between 129 -135 mmol͞liter, and 11% a serum [Na ؉ ] of <129 mmol͞liter. Serum [Na ؉ ] after racing was a linear function with a negative slope of the body weight change during exercise. The final serum [Na ؉ ] in a subset of 18 subjects was predicted from the amount of Na ؉ that remained osmotically inactive at the completion of the trial. Weight gain consequent to excessive fluid consumption was the principal cause of a reduced serum [Na ؉ ] after exercise, yet most (70%) subjects who gained weight maintained or increased serum [Na ؉ ], requiring the addition of significant amounts of Na ؉ (>500 mmol) into an expanded volume of total body water. This Na ؉ likely originated from osmotically inactive, exchangeable stores. Thus, EAH occurs in athletes who (i) drink to excess during exercise, (ii) retain excess fluid because of inadequate suppression of antidiuretic hormone secretion, and (iii) osmotically inactivate circulating Na ؉ or fail to mobilize osmotically inactive sodium from internal stores. EAH can be prevented by insuring that athletes do not drink to excess during exercise, which has been known since 1985.endurance ͉ exchangeable Na ϩ stores ͉ fluid overload ͉ overdrinking ͉ syndrome of inappropriate ADH secretion

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NSAIDs are commonly used by athletes competing in endurance events and are a risk factor for hyponatremia and altered renal function. Notwithstanding high rates of NSAID use, the incidence of hyponatremia was low. We attribute this to changes in fluid replacement guidelines and drink station availability that reduce the risk of overdrinking, the principal cause of this condition.