Conventional wisdom says athletes need to replenish fluids by drinking a lot of water. But it can also be dangerous to get too much fluid. The resulting sodium deficiency in the blood, or hyponatremia, has claimed the lives of a handful of runners in the U.S.
In 2002, Cynthia Lucero, 28, collapsed during the Boston Marathon and died of overhydration two days later.
The event was a wake-up call for marathon organizers, said Dr. Chris Woollam, a sports medicine specialist and director of the Toronto Marathon.
"We would take these people who came in somewhat dry, or not feeling well, and we'd give them an IV," Woollam recalled. "We'd put fluid into them, and that was perhaps the biggest mistake we could do."
Drinking too much is dangerous because it dilutes the blood's sodium content, which is already lowered during strenuous exercise since salt is lost in sweat.
In extreme cases, hyponatremia causes the brain to swell, leading to fainting, coma and even death.
It's not top marathon runners who are most at risk, since they are less inclined to stop and drink a lot of water. Rather, it's the slow, back-of-the-pack runners who tend to overhydrate.
As the risks of hyponatremia have become better understood, marathon directors have adjusted the number of water stations along courses to try to remove the temptation to drink too much.
Overhydration generally affects athletes engaged in prolonged, strenuous exercise. Of these, four per cent show symptoms of hyponatremia, which is not nearly as common as dehydration.
Both conditions are risky.
"Like so many things in our life today, it's a balance, and it's learning to have that balance of not overdoing it but having adequate hydration," said John Stanton of the Running Room.