As many as one in eight marathon runners may risk falling ill by drinking too much water during races, researchers said in a study released days before the Boston Marathon.

A study of 488 competitors at the 2002 Boston Marathon released on Wednesday concluded that 13 percent probably consumed so much fluid that their blood salt levels fell dangerously low -- a condition known as hyponatremia.

One of the runners that year, 28-year-old Cynthia Lucero, died of hyponatremia four miles from the finish line. Race organizers have since mounted an educational campaign to warn runners about the dangers of excessive drinking.

"Hyponatremia -- and, particularly, severe hyponatremia -- may be a greater problem than previously recognized," said the team of researchers, led by Christopher Almond of Children's Hospital in Boston.

The 26-mile Boston race, due to be run on Monday, is the world's oldest annually contested marathon.

Writing in this week's edition of the New England Journal of Medicine, Almond and his colleagues declined to say how much fluid runners should consume during such a race because individuals vary in size and the rate at which they perspire.

But they said runners should find their best level of hydration by weighing themselves before and after training runs. If they have gained weight at the end, they have probably taken in too much fluid, the researchers said.

"The strongest single predictor of hyponatremia was considerable weight gain during the race," they concluded.

Drinking three or more liters (6-1/2 pints) during the race, drinking every mile, running at a slower pace, being a woman, and being lean -- with a body-mass index of less than 20 -- increased the likelihood that a runner would gain weight by the end of the race.

The Almond team also found that hyponatremia loomed as a problem no matter which type of fluid the runners drank.

"It is important to recognize that currently available 'sports drinks' are not protective," said Benjamin Levine and Paul Thompson in a commentary in the Journal. "Most are hypotonic and provide far more water than salt."

Levine, of the University of Texas Southwestern Medical Center in Dallas, and Thompson, of Hartford Hospital in Connecticut, said the advice of the Almond team "is sensible and practical and should be considered seriously by all competitors."

Overall, Levine and Thompson said, running a marathon "is a reasonably safe sport, with less than one death per 50,000 participants."