"TOO MUCH OF A GOOD THING CAN BE BAD, EVEN TOO MUCH WATER." – Randy Eichner, M.D.

While most runners and cyclists know the importance of hydrating, some don't realize overhydrating can dangerously lower blood sodium levels – a condition known as hyponatremia.

Hyponatremia seems most common in female and beginning marathoners and triathletes for two reasons, says Randy Eichner, M.D., professor of medicine and team physician at the University of Oklahoma and volunteer physician at the Hawaii Ironman Triathlon. One, they are on the race course for many hours, losing lots of sodium in sweat. Two, they are hypervigilant about staying hydrated. "They drink lots of water in the days before the race, then stop at every fluid station along the course," he says.

"Drinking is important, but the problem
is they drink too much water."

In a hot marathon or triathlon, if you drink too little, you can dehydrate and risk early fatigue and heat illness. But if you drink too much, you risk low sodium, Eichner says. Keys to avoiding hyponatremia are 1) don't drink more than you sweat; and 2) favor sports drinks, which have some sodium, over water, which has none. Also, focus on a salty diet in the days before the race and eat some pretzels in the last half of the race.

"You're no camel," Eichner says, cautioning against overhydrating. "Know your body well enough to know how much drinking is too much."

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