Water is the most important, yet most frequently forgotten, nutrient. Don't wait until you feel thirsty to drink water. The recommended eight glasses of water per day is probably adequate for a sedentary person, but is not enough for a very active person. Nor is eight glasses per day enough in hot temperatures.
Water is the most commonly overlooked endurance aid.
Proper hydration is essential for good performance in exercise. A person who is exercising vigorously may start to become dehydrated before the thirst mechanism kicks in. Cold water is the best fluid for exercisers. Avoid sugar drinks. It's rare, but too much water can be a problem.