When Jennifer Zanni crossed the line of her first Ironman competition last year, she was concerned that she might be dehydrated. She had experienced problems with dehydration while she was training, so she made sure she was drinking plenty of fluids, especially during the 26.2-mile run, the last and arguably most brutal event in an Ironman race. Within 15 minutes of finishing however, Zanni had begun speaking erratically and her next memory is of waking up a hospital bed. The diagnosis? The problem wasn't dehydration; it was hyponatremia, also known as water intoxication.

Hyponatremia, or overhydration, occurs when water is consumed more quickly than the body can process it, throwing off the body's balance of fluids and electrolytes in the bloodstream. The excess water in the blood causes cells to swell, and if that happens to cells in the brain, the results can be lethal.

The condition is most often associated with endurance athletes, marathoners or Ironman participants, but it is also showing up regularly on school athletic fields. That's what happened to 17-year-old football player Zyrees Oliver, who drank more than four gallons of water and Gatorade during practice to combat muscle cramps and feelings of dehydration but wound up collapsing — and later dying — from hyponatremia.

The worst thing about hyponatremia is that the symptoms often mimic those of dehydration: cotton mouth, inability to pee, lack of sweat. But the condition is still 100 percent preventable if you remember one guideline: Drink according to your thirst.

To prevent dehydration, parents, coaches and athletes have been pushing fluids at practice and during games. But experts say athletes should use their thirst as a guide when deciding when or how much to drink. Like many athletes, Zanni was under the impression that if you feel thirsty, you're already dehydrated. She learned her lesson the hard way, and has since been working to remind athletes about the dangers of overhydrating during exercise.

And contrary to popular belief, muscle cramps and fatigue aren't caused by dehydration but from pushing too hard. If an athlete starts to experience these symptoms, he should be allowed to sit it out for a few minutes. Then, he should only drink up if he's also feeling thirsty.

Hyponatremia: When too much water can kill
Experts recommend that athletes follow this 1 simple rule when they drink water.