Full-blown cases of hyponatremia (sometimes called water intoxication) are relatively rare, roughly 0.1% to 4% of people who sweat steadily for hours in grueling, long-distance events, says Scott Montain, a research physiologist at the U.S. Army Research Institute of Environmental Medicine. The incidence of hyponatremia appears to be highest in events lasting more than four hours, especially at high temperatures. But the prevalence of warning symptoms is much higher--up to 27% of athletes who seek attention in a medical tent during a long race.

That can be disastrous. Normally, the body tries to keep positively and negatively charged electrolytes in balance to keep cells electrically neutral, says Dr. Ronenn Roubenoff, associate professor of medicine and nutrition and director of human studies at the Jean Mayer USDA Human Nutrition Research Center at Tufts University. Though sodium is probably the most important electrolyte for endurance athletes to worry about, he says, "an imbalance of any one of the electrolytes can be harmful."

Normally, sodium is plentiful in the blood and relatively low inside cells. But when the concentration in the blood gets too low compared to the amount inside cells--either because a person drank too much water, took in too little sodium, or both--water rushes into cells. "Water follows sodium as day follows night," Roubenoff says.