Runners have always been told to drink plenty of water to prevent hydration. Now I hear that drinking too much water can be harmful. Which is it

"If you’re exercising for extended periods, it’s important to drink enough water to prevent dehydration. But new research indicates that drinking too much water during prolonged exercise can dilute the sodium content of your blood, leading to a potentially fatal condition called hyponatremia," says Brenda Townes, MD, who practices internal medicine with Baptist Medical Associates. She is also a marathon runner.

Previous recommendations encouraged runners to drink as much as possible to "stay ahead" of their thirst. This often meant that athletes were drinking more fluid than they were losing through sweating.

To prevent hyponatremia, drink only as much fluid as you lose due to sweating during a race. Experts recommend that athletes drink no more than 31 ounces (or about 800 milliliters) of water per hour.

Hyponatremia occurs more often in women than in men. In addition, you may be at increased risk of hyponatremia if you are on a low-sodium diet or you aren’t acclimated to warm weather.

Symptoms of hyponatremia include:

• Nausea
• Headache
• Muscle cramps
• Confusion
• Seizures

If untreated, hyponatremia can lead to rapid swelling of the brain, resulting in coma and death.

"Keep in mind that dehydration during prolonged exercise is still far more common than is hyponatremia. So, it’s important to start exercise well hydrated and to drink plenty of fluids during exercise," Dr. Townes adds.