A study from the University of North Carolina at Chapel Hill shows that drinking fluids earlier can improve performance more than taking them later. Seven highly-trained male triathletes, aged 18 to 35 years, were tested during two simulated Olympic-distance triathlons. They took a full glass of water at 8, 16, 24, and 32 kilometers, and this was compared to taking the same drink 2, 4, 6 and 8 kilometers later in the event (at 10, 20, 30, and 40 kilometers).
As you would expect, opening swim times for 1500 meters were similar between trials; as were the second event (40-km cycling) times, but the third event (10-km run) times were faster when the athletes took food and drink earlier. Dehydration does not harm an athlete's performance until he lacks a large amount of water and his blood volume is depleted significantly. That explains why the athletes' performance was not harmed until the third event of the three-event competition.
When you exercise hard or in hot weather, you sweat and breathe off huge amounts of fluid. Losing fluid reduces blood volume to make you tired. Anyone who exercises vigorously can increase their endurance by taking in fluids, and competitive athletes can increase their endurance by taking in extra fluids just before the start of their event and drinking fluids regularly during events that last more than an hour. Be sure to replace salt as well as fluid you lose when you sweat, and don’t force yourself to drink large amounts of water. More on hyponatremia ; journal reference ; recommended books on fitness and nutrition