During strenuous exercise you should drink all the water you can, right? Wrong.
Drinking too much water can be as dangerous as drinking too little. Be careful not to fall for the lure of sports drink commercials that suggest that a little bit of exercise requires a whole lot of fluid consumption.
Professional athletes, particularly runners, move at a pace that doesn't allow them to take excess water breaks. However, a first-time marathon runner may take five hours to finish a race. At this slow pace, amateur runners often drink more than necessary over the course of 26 miles. While the odds are slim that one will drink so much water as to create a dangerous situation, it can happen. In these rare cases of hyperhydration, the blood has too much water and too little sodium. The brain can swell, which can lead to seizures and death.
Of course, this doesn't mean you should toss out your sports water bottle and forget about dehydration. The best advice is to pay attention to your own body. It's true that you shouldn't wait until you feel thirsty to drink water, but that doesn't mean you need to force water down your throat every minute. A trick you can try is to weigh yourself before exercise and immediately after. If you lose weight, you should probably drink more water next time. If you gain weight, you should probably drink less.
Also, monitor your sweat. People who sweat faster will need to replenish their fluids more often.