Researchers who studied 488 runners in the 2002 Boston Marathon found that 62, or more than one in eight, had a serious fluid and salt imbalance from drinking too much water or sports drinks. Three of them had extreme imbalances.
One 28-year-old woman died after the race from the condition, called hyponatremia, in which the excess water dilutes the salt level in the body too much.
"More is definitely not better when it comes to fluids, but it's a hard message to get across," said Leslie Bonci, director of sports nutrition at Pittsburgh Medical Center.
Endurance athletes have long been warned about getting dehydrated, and many tend to drink more on race day than they do during training.
The study was reported in Thursday's New England Journal of Medicine.
The researchers, led by Dr. Christopher Almond, a cardiologist at Harvard Medical School and Children's Hospital Boston, tested Boston Marathon runners' blood after the race and collected information on their condition, race time and liquid intake.