PHOENIX—Here’s the latest must-have component of a running program – a scale. While we’ve been telling our members, clients and ourselves to drink lots of water before, during and after exercise, it seems though that the message may have been followed too much by some. Drinking too much water while exercising intensely for long periods of time is thought to be a cause of hyponatremia, or abnormally low sodium levels, and can be fatal.

In fact, a recent New England Journal of Medicine study found that many runners in the 2002 Boston Marathon drank excessive amounts of water during the 26.2-mile race. Of the 488 runners studied, 62 of them had low blood sodium levels and three of them had levels so low that they were in danger of dying.

Tom Clark, assistant program director and of the Village Racquet and Health Club in Phoenix talks to his running club about when it’s appropriate to drink and when to lay off the agua. He admits that drinking loads of cool water can be enticing in the 100-degree-plus heat of June. To avoid that temptation, his 30 or so runners begin training for their four- to six-mile runs as early as 4:30 a.m. to avoid the heat.

“For hydration I put out coolers along our route, filled with both water and with a sports drink,” he says. “I recommend to my runners that they get a drink at every available drinking station, as there are several along the route. However, they are told to only drink a minimal amount of about 4-6 oz. at each station. That way, they are getting enough fluids without overdoing it.”

Here’s how hyponatremia happens: during intense exercise a person’s kidneys can’t excrete excess water that’s coming in. As a person keeps drinking, the extra water moves into the cells. The brain cells, enlarged from the excessive amount of water, press against the skull and can push against the brain stem, which controls vital functions like breathing.

According to the study, hyponatremia tends to be more common in slower runners who take more than four hours to finish the marathon – giving them plenty of time to chug water. Slower runners drank an average of three liters of water, causing them to actually gain weight. As more and more “average” people attempt marathons and other demanding athlete events, researchers are seeing an increase in this life-threatening condition.

Clark, a U.S.A Track and Field certified coach, makes sure his runners are aware of how much fluid they are taking in. And here’s where the scale comes in handy, he also heeds experts’ advice to weigh runners before and after races.

“Although this phenomena is fairly rare, it is still something worth being mindful of,” he says. “Since a runner can lose as much as six pounds of sweat per hour by running in this Arizona heat, I have them weigh themselves occasionally before and after the runs, just to make sure that they aren't losing too much water weight. This also helps to ensure that they are taking in the correct amount of replacement fluids.”