Too much water can be life-threatening for marathoners

Runners who consume too much water or sports drinks during a marathon can develop a life-threatening condition called exercise-associated hyponatremia (EAH). Beyond drinking, however, researchers at Harvard-affiliated McLean Hospital report in the May 2007 issue of the American Journal of Medicine that this complication during endurance exercise is also the result of a hormonal stress response, which decreases urine formation and prevents the excretion of excess water.

“This is a major paradigm shift for those who think that EAH is due primarily to salt loss or overconsumption of fluids,’’ said Arthur J. Siegel, chief of the department of internal medicine at McLean Hospital. “It’s also an inside job. Avid drinking may be a precondition but dysregulation of the anti-diuretic hormone or arginine vasopressin (AVP), which governs water balance, emerges as the root cause.”

Secretion of AVP governs water balance and is normally suppressed when an excess is present in body fluids. “Failure of appropriate suppression prevents excretion of excess water, which results in the rapid fall of blood sodium levels,’’ Siegel said. “Continued ingestion of dilute fluids, including sports drinks, leads to potentially fatal acute cerebral edema under such circumstances.”

Siegel and colleagues from across Partners HealthCare came to this understanding after testing blood samples from asymptomatic and collapsed runners at the 2001, 2002, 2003, and 2004 Boston Marathons, including investigation of two fatal cases in 2002 with permission of the families. The findings confirmed inappropriate secretion of AVP as the proximate cause, meeting criteria for a condition first described in the American Journal of Medicine in 1967. This connection pointed to using hypertonic solutions, such as 3 percent saline, as a treatment for life-threatening cases of EAH, as previously validated for this syndrome.