Proper hydration in important for all athletes in order to:
  1. Replace water lost as a result of sweating 
  2. Avoid marked decreases in performance that result from dehydration
  3. Help maintain core body temperature within acceptable limits during exercise  
Dehydration negatively affects performance and is associated with premature fatigue. This is particularly the case for prolonged aerobic exercises such as distant running or cycling, but athletes competing in team sports or events of short duration can also be affected by dehydration. 

Ironically, dehydration reduces the capacity of the digestive system to absorb water. Athletes should not wait until they are dehydrated before they drink, as this slows rehydration and causes gastric cramping. 

It is well established that the sensation of thirst is not a good indicator of an individuals level of dehydration. When thirst manifests itself, approximately 2% of body mass has already been lost. Consequently, one cannot gauge dehydration by referring to the sensation of thirst. Therefore during exercise, it is important to drink on a schedule rather than according to thirst. 

Athletes should drink plenty of fluid everyday, particularly before a practice session or competition. Athletes who are well hydrated have the following characteristics. 
  1. Sweating begins sooner, and is more abundant 
  2. An enhanced rate of absorption of the fluids consumed during exercise
In practical terms, this means drinking 1.5 - 2.5 cups of fluid 2 to 3 hours before exercise. This allows time for excess to be excreted as urine before the exercise starts.