Long distance runners and other endurance athletes have long been educated to believe that drinking lots of fluids during a long distance or endurance event was critical. And if you didn't drink enough water, you ran the risk of dehydration.

Researchers - studying 488 runners in the 2002 Bost Marathon found that the bigger danger is in drinking too much fluid (water or sports drinks) rather than not enough. They found that 62 of the 488 runners, more than one in eight, had a serious fluid and salt imbalance after the event. And three of them were in the danger zone.

Hyponatremia - a condition where drinking too much water or other fluids to the point where the salt level in the body drops too much - can develop during marathon races where the runners drink constantly to stave off dehydration.

Runners who actually gained weight (anywhere from 4-11 pounds) during the event and very thin runners are most at risk. Runners who drink sports drinks with very little salt in them are least at risk. The goal of drinking during a race is to replace water that is lost, not to take in more than you are losing.

A good way to learn how much is right for you is to weight yourself before a heavy training session. Then drink and record how much liquid you consume during the trainging. Then weight yourself again. If you find that you weight more after the training than before, you should cut down on your liquid intake. By performing this exercise, you will learn how much liquid you really need to injest during an event and be able to pace yourself accordingly.

Hyponatremia can begin with confusion and lethargy and can progress rapidly to more severe symptoms. They can include twitching, siezures, stupor, coma and even death.

In recent years, hyponatremia has killed several amateur marathon runners as well as competitors in the Marine Corps Marathon.