This New York Times article reports on a just released New England Journal of Medicine study that indicates athletes who drink as much liquid as possible during intense exercise to avoid dehydration face an even greater health risk than dehydration.
The study reports that an increasing number of people who engage in intense exercise or recreation are severely diluting their blood by drinking too much water or sports drinks, risking serious illness and, in some cases, death.
The condition — called Hyponatremia — occurs because, during intense exercise, the kidneys cannot excrete excess water. Accordingly, as intense exercisers continue to exert themselves and drink more fluid, the extra water moves into their cells, including brain cells. The expanded brain cells eventually have no room to expand further and press against the skull and compress the brain stem, which controls vital functions such as breathing.
Indeed, the mantra from docs to intense exercisers over the past generation — i.e., avoid dehydration at any cost — may be part of the culprit. As the Times article notes:

“Everyone becomes dehydrated when they race,” [said one of the researchers involved in the study]. “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.

To make matters even more complicated, Hyponatremia can be treated,
but doctors and emergency workers often pressume that a person feeling ill after intense exercise is simply suffering from dehydration. Thus, they give the exerciser intravenous fluids, which makes the Hyponatremia worse and can kill the patient.
I guess those old high school football coaches of mine back in the late 1960’s who didn’t allow my teammates and I so much as a drink during two-a-days in the summer heat knew more than they were letting on? ;^)