Long distance runners or other people exercising for long periods of time risk putting themselves at risk of a condition called hyponatremia if they drink too much water during their run according to a new study published today in the New England Journal of Medicine.

Hyponatremia is caused when the body's sodium content becomes too low. Because runners lose sodium when they sweat and water does not replenish this essential nutrient, the sodium content of the body can become drastically reduced by drinking too much while exercising.

The symptoms of hyponatremia include becoming lethargic or disoriented or even seizure and respiratory distress. The condition can even cause death.

The study examined 488 runners who competed in the 2002 Boston Marathon. They gave blood samples both before and after the race. 13 percent of the runners drank so much during the 26 mile race that they developed hyponatremia. Three of the runners had rates so low they were in danger of dying.

Most of the runners who had problems were slower runners who took a longer time to finish the race, usually as many as four hours. They therefore had more time to drink at water stops. Runners consumed as many as 13 cups of water or sports drink during the race.

'Those who are running to finish the race very fast don't have time to drink a lot of water along the way,' said Dr. Benjamin Levine of the University of Texas Southwestern Medical Center in Dallas. 'Those who are not running the race competitively tend to stop at every water station and take a drink -- in a long race, they can dilute themselves.'

Doctors and long distance runners long suspected this was the case but this is the first study that confirms it medically.

'Before this study, we suspected there was a problem,' said Dr. Marvin Adner, the medical director of the Boston Marathon, which is next Monday told the New York Times. 'But this proves it.'

Runners are advised to drink before the race and to wait until they begin to urinate before drinking large amount of water. This shows their body is no longer retaining excess water.