When starches or sugars are eaten, the body changes them all to glucose, the only form of carbohydrate used directly by muscles for energy. Whether carbohydrates are in the form of starches (in vegetables and grains), sucrose (table sugar), fructose (found in fruits and juices) or lactose (milk sugar), carbohydrates are digested and ultimately changed to glucose.

The body uses this glucose in the blood for energy. Most glucose is stored as glycogen in the liver and muscles. During exercise glycogen is broken down in the muscles and provides energy. Usually there is enough glycogen in muscles to provide fuel for 90-120 minutes of exercise.

Most exercise and sports games do not use up glycogen stores so eating carbohydrates during the activity usually isn't needed. But for some athletes, eating or drinking carbohydrates during exercise helps maintain their blood glucose and energy levels.

Most athletes need not be concerned with "carbohydrate loading," the special technique of eating a lot of carbohydrates for several days before an endurance event. Instead, focus on getting enough carbohydrates everyday. The best way to ensure plenty of energy for exercise is to eat a nutritious, balanced diet that is high in carbohydrates and low in fat with lots of different foods .