During a vigorous workout, the body may have lost important water- before any signs of thirst are given. Imagine, a body so dehydrated that it cannot even sweat. And, a dedicated athlete who is pushing on. This could be a deadly mix.
According to an article posted by the Gatorade Sports Science Institute, at the 2000 National Athletic Trainers' Association (NATA) they published a report, "Fluid Replacement for Athletes" that states that "the onset of significant dehydration is preventable and also that minimizing dehydration is the simplest and most effective step athletes can take to protect both health and performance."
Athletes can regularly experience dehydration, but those training in warmer climates are at higher risk for dehydration and therefore must take greater precautions when working out. What's very misleading about dehydration is that there are no real early warning signs. Thirst is not always experienced. Fatigue can be mistaken for just being tired. Sweat loss is underestimated. For these reasons, being well-educated on the matter and not waiting for symptoms to set in is the best way to avoid dehydration. How does dehydration work? This condition places cardiovascular and thermoregulatory functions under severe stress-- plasma decreases and therefore, so does cardiovascular function. This is because there is competition between muscle and skin for limited blood supply-- which can quickly overwhelm the body's cardiovascular capacity to cope. More seriously, this can also create the negative physiological circumstances that have tragically claimed the lives of athletes, which we sometimes see on high school and college fields during football camps. When the body cannot cope, fatigue can set in. If one continues the work-out after the body's warning signs- it can turn into leg cramps, heat exhaustion, or heat stroke. When dehydrated, sweat may not form properly, which means the body cannot even give a cry for help.
That said, water is crucial to maintaining core body temperature and perhaps the best intervention for the aforementioned ailments. In fact, sometimes it's not even good enough to down a bottle of water after the exercise, and in fact, it is recommended that water be replenished throughout the workout, perhaps in 15-20 minute intervals. Just like the routine workout- a water-break routine should also be routine. Knowing one's own sweat rate will help tremendously, as replacing 100% of the lost fluid is the healthiest thing to do.
While most trainers and doctors would say water is indeed the best sports drink, adequately formulated drinks as Gatorade and Powerade can also play a role in helping with dehydration. GSSI says these drinks encourage voluntary fluid intake (for those who may not prefer "tasteless" water), stimulate fast absorption, promote rapid and complete hydration and improve performance. In addition, GSSI states that the thirst mechanism in the brain is stimulated by the osmotic effects of electrolytes in body fluids, effective sports drinks will also contain the correct electrolyte profile to maintain the osmotic drive for drinking.
So, in summary, it is critical to know how much water you lose during a workout, training, or competition. And, it is critical to replace that amount during exercise. Healthy means hydrated and is the only way to protect your body during a vigorous work-out. As stated above, the body's warning signs aren't too easy to read.