Hyponatremia, Exercise-Associated

Causes

In simple terms, low blood sodium concentrations are caused by either a depletion of sodium from excessive salt loss or by a dilution of sodium from excess fluid intake. Current evidence suggests that the majority of EAH cases are caused by sustained fluid intake beyond the body's capacity to excrete any buildup of excess fluid (dilutional EAH). Although excessive sodium loss through sweat and urine may contribute to the development of EAH, this particular mechanism appears less common. The contribution of sodium loss in the development of hyponatremia may become more significant in endurance events lasting over 24 hours in hot environmental conditions; however, more research is necessary to fully clarify the role of sodium losses in EAH. Therefore, it should be emphasized that EAH is primarily a salt imbalance from the ingestion combined with the retention of too much fluid rather than from the ingestion of too little salt. Recent evidence suggests that the body's main antidiuretic hormone (arginine vasopressin) is stimulated during exercise so that any fluid excess cannot be appropriately excreted. At rest, any body fluid excess can promptly be excreted, so fluid overload hyponatremia is unlikely in healthy subjects under nonexercising conditions.

Risk Factors

Recognized risk factors have been identified in athletes developing EAH. Athlete-related (internal) risk factors include excessive drinking behavior, weight gain during exercise, low body weight, being female, slow performance pace, event inexperience, and ingestion of nonsteroidal anti-inflammatory medications. Event-related (external) risk factors include high availability of drinking fluids, exercise lasting over 4 hours, and unusually hot or extremely cold environmental temperatures. Although none of these factors can conclusively predict the development of EAH, a constellation of such risk factors, combined with any sign or symptom of hyponatremia, warrants urgent blood sodium measurement at a nearby medical facility.

...