If you wait to drink until you are thirsty and stop drinking when your thirst is satisfied, you’ll remain 25 percent to 50 percent dehydrated.
According to the International Olympic Committee, dehydration impairs an athlete’s performance in most events: endurance sports, team sports, power and sprint sports, winter sports and sports with weight classes.
Athletes need to be well-hydrated before and during exercise and competition.
Your body needs adequate water for all activities and cannot adapt to dehydration. Internally, you’re about 65 percent water. Without water, you can only survive a few days. Dehydration makes your blood thicker, increasing your heart rate and decreasing the amount of blood your heart can pump with one beat and causing your blood pressure to fall.
Dehydration makes it harder for fat to get into your muscles to be used for fuel, so your muscles burn the limited sugars (glycogen) already there.