Runners: Beware Too Much Water

(CBS)  Conventional wisdom has always been to avoid dehydration during exercise. But for some marathon runners, excessive water intake can be dangerous and even fatal, cautions The Early Show medical correspondent Dr. Emily Senay.

She explains that drinking too much water during long-distance races, marathons and other endurance exercises can cause a potentially fatal condition called hyponatremia, in which salt levels in the body are diluted by the excess water and fall to dangerously low levels, threatening vital bodily functions.

And researchers say hyponatremia may be a bigger problem than previously thought.

In the 2002 Boston Marathon, one female runner died because her body lost too much salt.

What's more, a new study in the New England Journal of Medicine shows that many other runners in that race ran the same risk.

Researchers took blood samples and other information from almost 500 women and men who ran that race.

They found that 13 percent of the runners had low sodium levels, including three runners with critically low sodium levels, and the one death.

The researchers estimate that, since 15,000 people ran that race, almost 2,000 had low sodium levels at the end of the race, and about 90 probably had dangerously low sodium levels.

Most experts agree that running a marathon is a reasonably safe endeavor, but race officials now warn runners about the dangers of excessive fluid intake.

This year's 26-mile Boston Marathon is scheduled for Monday.

Low sodium can result in headache, confusion, seizures, and death. Symptoms can include nausea, grogginess and confusion.

The researchers found that the strongest predictor of hyponatremia was weight gain during a race. Drinking three or more liters, or about six and a half pints during the race, drinking every mile, running at a slower pace, being a woman, and being lean increased the chances that a runner would gain weight by the end of the race.

The researchers didn't say how much fluid runners should consume during such a race, because people vary in size and the rate at which they sweat.

But runners can estimate how much they should drink by weighing themselves before and after training runs to see how much they lose, and so, how much water they should replace. Other experts suggest drinking only while moving and avoiding excessive amounts of water after a race.

In addition, notes Senay, the researchers say it didn't seem to make a difference whether runners drank pure water or sports drinks, even though some sports drinks do contain sodium because sports drinks contain a lot more water than salt.