Because 70 percent of all muscle tissue and 10 percent of fatty tissue dehydration can have a huge impact on your performance. Dehydration affects your body's ability to regulate heat and heart rate to rise. This causes you to feel more tired during exercise. Dehydration also weakens your mental function.
Because the body is made up of so much water there are many ways you lose water such as breathing, sweating and digestion.
For most people, water is all that is needed. If you will be exercising at a high intensity for longer than an hour, a sports drink containing potassium and other nutrients can provide energy and electrolytes to help you perform for a longer period of time.
American Council on Exercise recommends "drinking 17 to 20 ounces of water two to three hours before a workout. Then, about 20 to 30 minutes beforehand, drink another 8 ounces. While exercising, continue to drink 7 to 10 ounces about every 10 to 20 minutes. Within 30 minutes of completing the workout routine, drink another 8 ounces and continue to gulp down 16 to 24 ounces for each pound you lost during the workout to regain the water you sweated out."