The Facts of Hydration
Staying hydrated during the course of exercise is crucial because water is what delivers oxygen to the muscles, fueling them in the course of your work-out. Without adequate fluids, the cardiovascular system is strained, the probability of heat injury skyrockets, and performance is impaired.

The most serious effect of dehydration during exercise is the inability to adequately sweat and cool off. This is more serious than it sounds since the body's chief method of heat loss in a hot-dry temperature can be life threatening. In fact, 80% of heat loss happens through perspiration.

Add to this the fact that it is common for a person to dehydrate by 2%-6% of their body weight during exercise and that as little as 1% weight loss during exercise begins to impair performance. Unfortunately, feeling thirsty isn't an adequate indicator that it's time to drink. By the time a person feels thirsty they are usually dehydrated. Studies show that the average person does not drink nearly the amount of fluids required to prevent dehydration. If water or some other fluid is available, most people will drink only enough to replace 2/3 of the fluids lost.