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.
They found hyponatremia was most serious in runners who gained substantial weight — 4½ pounds to 11 pounds — from drinking lots of water along the route. Extremely thin runners also were at high risk. Runners who drank sports drinks, which contain very little salt, were not less likely to develop hyponatremia.
Bonci and Almond said a good way to prevent problems is for athletes to weigh themselves before and after training sessions. If they gain significant weight, they should cut back on water intake until they find the right balance — long before race day.
The goal is simply to replace water lost to sweating.
Hyponatremia can begin with confusion and lethargy and progress rapidly to twitching, seizures, stupor, coma and death.
Severe cases are believed to have become more common with the growing popularity of endurance sports. In recent years, hyponatremia has killed several amateur marathon runners, as well as competitors in the Marine Corps Marathon.
This year's Boston Marathon will be on Monday.