Hydrate Before You're Thirsty
The latest thinking on dehydration is that you should drink to thirst, not to capacity. This is especially true for endurance athletes. If you're going for more than three hours, listen to your body rather than chugging like crazy. Basically, mild dehydration isn't great but it's not deadly. Hyponatremia, a low blood sodium concentration due to over-hydration, is.
"It's really more of an issue for the people in the four-hour marathon club," says Dr. Beckerman, meaning recreational runners. Because these runners have been out on course for longer, they've likely lost more electrolytes than the two-and-a-half-hour finishers. They're also more likely to take a few cups at each water stop and guzzle. The result is that serum sodium levels can drop to dangerous levels, causing nausea, dizziness, and in extreme cases, death.