The NY Times has an article (warning: requires subscription) describing a study published in the New England Journal of Medicine (abstract of study) that found that drinking too much water during intense exercise lead to hyponatremia, low sodium levels, in 13 percent of the runners tested. The research was conducted in part because medical teams at races were finding high incidences of hyponatremia, and the researchers wanted to learn more about the types of racers who were at risk. The study found that slower (i.e. non-elite) athletes were more likely to get the condition, largely because they were taking the time to drink more fluids. Here's an extended quotation. It cites runners because that's where much of the problem lies, but cyclists are also at risk, as mentioned later in the article.

"Hyponatremia is entirely preventable, Dr. Adner and others said. During intense exercise the kidneys cannot excrete excess water. As people keep drinking, the extra water moves into their cells, including brain cells. The engorged brain cells, with no room to expand, press against the skull and can compress the brain stem, which controls vital functions like breathing. The result can be fatal.

But the marathon runners were simply following what has long been the conventional advice given to athletes: Avoid dehydration at all costs.

"Drink ahead of your thirst," was the mantra.

Doctors and sports drink companies "made dehydration a medical illness that was to be feared," said Dr. Tim Noakes, a hyponatremia expert at the University of Cape Town.

"Everyone becomes dehydrated when they race," Dr. Noakes said. "But I have not found one death in an athlete from dehydration in a competitive race in the whole history of running. Not one. Not even a case of illness."

On the other hand, he said, he knows of people who have sickened and died from drinking too much.

Hyponatremia can be treated, Dr. Noakes said. A small volume of a highly concentrated salt solution is given intravenously and can save a patient's life by pulling water out of swollen brain cells.

But, he said, doctors and emergency workers often assume that the problem is dehydration and give intravenous fluids, sometimes killing the patient. He and others advise testing the salt concentration of the athlete's blood before treatment.

For their part, runners can estimate how much they should drink by weighing themselves before and after long training runs to see how much they lose - and thus how much water they should replace.

But they can also follow what Dr. Paul D. Thompson calls "a rough rule of thumb."

Dr. Thompson, a cardiologist at Hartford Hospital in Connecticut and a marathon runner, advises runners to drink while they are moving.

"If you stop and drink a couple of cups, you are overdoing it," he said.

Dr. Adner said athletes also should be careful after a race. "Don't start chugging down water," he said.

Instead, he advised runners to wait until they began to urinate, a sign the body is no longer retaining water. " GINA KOLATA, NY Times, 04/14/05