WEDNESDAY, April 12, 2005 (HealthDay News) -- Marathon runners who gulp down too much water during a tough race are doing their bodies no favors, researchers report.
A new study of competitors in the 2002 Boston Marathon found too many drank excessive amounts of water during the grueling 26-mile race, causing their blood salt levels to dip to potentially dangerous levels in a condition called hyponatremia.
In fact, one runner in that marathon, a 28-year-old woman, died from hyponatremia after finishing the race.
Although runners usually cited a fear of dehydration as the reason for their excessive water intake, the study "showed that there is a point where drinking too much can be critical," said lead researcher Dr. Christopher S.D. Almond, a pediatric cardiology fellow at Children's Hospital Boston. His team report their findings in the April 14 issue of the New England Journal of Medicine.
Almond and his colleagues enrolled 766 marathoners in their trial and got blood samples from 488 of them at the finish line. Of those, 62 had abnormally low blood salt levels -- three of them low enough to be called "critical."
Abnormally low salt levels were more common in women (37 of 166, or 22 percent) than men (25 of 322, or 8 percent). Many of the affected runners drank more than three liters of water during the race, enough to raise their body weight despite the more than 26 miles they ran.
Based on that sample, 1,900 of the nearly 15,000 Boston Marathon runners that year had hyponatremia, Almond estimated.
Thinner runners -- those with a body mass index of about 20 -- were more likely to end up with low salt levels, the study found. That might help explain the higher incidence among women, Almond said, since it's believed women runners tend to drink as much water as men, even though their average weight is lower.
A racing time of more than four hours was also associated with abnormally low blood salt levels, the researchers reported.
Hyponatremics were not the champion runners, who completed the course in about two hours, noted Dr. Benjamin D. Levine, a professor of medicine at the University of Texas Southwestern Medical Center and Presbyterian Hospital in Dallas, and co-author of an accompanying editorial.
"Really fast people don't have time to drink water," Levine said. "The slower the athlete, the greater the risk, and the predominant reason for that behavior is that you have more time to drink."
Up to a point, drinking water during a race makes sense, Levine said. "These people are not idiots," he said. "They are following rules that say you have to drink a lot of water to stay ahead of your thirst. But athletes lose water at different rates and lose salt at different rates, so the general rule may not apply to an individual."
Detailed guidelines about water intake during long-distance races have been issued by USA Track & Field, an organization that covers the subject in depth, Levine said. Serious runners who build their lives around racing tend to be aware of those guidelines, he said.
For less devoted racers, "using your thirst as a guideline may be the best way to handle it," Levine said. "Don't worry about getting a little dehydrated. Drink if you are thirsty, but don't drink for the sake of drinking."
Almond said his study was not designed to produce recommendations about avoiding problems caused by excess water intake, but simply to make runners aware that those problems exist.
"If I could say one thing over and over, it would be safety, safety, safety," Almond said.
More information
Water intake guidelines for athletes are set out by USA Track & Field.
