Three decades ago, the top warning sounded by race officials was dehydration. Athletes were told to hydrate constantly after several studies found a link between dehydration and a rise in body temperature, which can lead to heatstroke.
But now researchers are taking a second look at the risks of drinking too much fluid during exercise.
Hyponatremia, or so-called water intoxication, was thrust into the spotlight after the back-to-back deaths of two female runners in 2002, including one who ran in the Boston Marathon. In both cases, the women drank excessive amounts of fluids.
Hyponatremia happens when the body's sodium level falls below normal. People lose salt through their sweat, and overdrinking dilutes the sodium in the bloodstream, causing the brain to swell and push against the skull.
Symptoms of hyponatremia include nausea, vomiting, weakness and, in severe cases, seizures, coma and death.
Research has shown that exercise-related hyponatremia occurs only during periods of exertion lasting four hours or longer, such as in marathons or triathlons, during which athletes are more likely to drink a lot.
How much people should drink during prolonged exercise has touched off a debate among fitness experts.
They agree that hyponatremia is a serious problem, but some fear that stressing the dangers of overdrinking will cause athletes to dehydrate themselves, and lead race officials to curb the number of water stops during long-distance competitions.
Last year, race officials at the Houston Marathon halved the number of water stops after finding an unusually large number of hyponatremic runners at the medical tent in past years.
Although runners were initially upset with the decision, doctors saw fewer hyponatremic cases, said Dr. Joseph Chorley, the marathon's assistant medical director.
One of the leading international voices on hyponatremia is Tim Noakes, a sports science professor at the University of Cape Town in South Africa, who has published scores of articles on the perils of overdrinking.
Noakes' advice is to drink only when thirsty, because the body will instinctively know when it needs water.
Choosing sports drinks instead of water can reduce the risk of hyponatremia but may not eliminate it. "Exercisers must be warned that the overconsumption of fluid (either water or sports drinks) before, during, or after exercise is unnecessary and can have a potentially fatal outcome," Noakes wrote in an editorial in the July 2003 issue of the British Medical Journal.
