Using energy for physical exercise creates heat. If the body did not release this heat, the body's temperature would rise- in other words, it would overheat very quickly. Extreme environmental conditions can make this situation potentially worse, since such things as hot and humid weather will compound the problems.
The solution here is, of course, water. Water makes up 50-60 percent of the human body's overall weight and as much as 90 percent of the blood plasma by weight. The role of water is to transport nutrients to cells and to carry away waste products. It also distributes heat throughout the body and enables heat to be released from the body.
The overall effect of sweating is to cool the body, but the cost is dehydration. The body also loses water through urination, through the feces, and through breathing (water vapour). For this reason, especially under physically active conditions, a constant replacement of fluids is required for the system to continue to operate at peak performance.
Since the assimilation of water by the body under active conditions cannot keep up with its use, it is important that replenishment take place before, during and after the physical activity. The rule of thumb is that one should not wait to become thirsty, since at that point it is impossible to catch up with one's fluid requirements.
The precise content of the fluids is also important. The level of carbohydrates in undiluted fruit juices, high-sugar drinks, and carbonated beverages is too high and ingestion may cause cramping. Tea, coffee, and soft drinks act as diuretics and will dehydrate the body further. Plain water may be best.
Before Exercise. Athletes should begin training or a competitive event well hydrated. This can be accomplished by consuming two to three cups of fluid containing carbohydrates 2-3 hours prior to exercising and one cup 10-20 minutes prior. Ingesting drinks that contain carbs 2-3 hours before exercise will aid in maximizing the athlete's glycogen (energy) stores. During Exercise. A sports beverage containing carbs should be consumed if the activity is longer than 50 minutes in duration. The ideal carb concentration is from 6 to 8 percent, as it is better absorbed than at higher concentrations. Thirst is increased as voluntary fluid intake if the beverage contains salt, which replenishes that lost through persperation. For optimal fluid replacement, the beverage will contain a 6 percent electrolyte-carb solution. Generally, an athlete should drink at least one-half cup of cool fluid (10-15 C) after each 10 minutes of exercise. Cool water is more easily absorbed by the body. After exercise. Fluid losses during exercise should be regained within 2 hours by drinking fluids containing carbs to rebuild glycogen stores and electrolytes (sodium and potassium) to speed rehydration. Bodily fluids can also be replaced with foods high in water content, such as watermelon, oranges, grapes and tomatoes.
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