When glucose isn't needed for immediate energy, the body stores it as glycogen in the liver and skeletal muscle. Glycogen stores are especially important when working out, as the body's energy demands may exceed the amount supplied by glucose in the bloodstream after prolonged exercise or when exercising with insufficient carbohydrate intake. According to Iowa State University, during low-intensity workouts, glycogen stores can be used to increase glucose levels in the bloodstream for as long as 90 minutes -- whereas during high-intensity exercise, glycogen stores provide energy for only about 20 minutes. Active people therefore need to replenish glucose regularly by eating plenty of carbohydrates. However, in less-active people with adequate glycogen stores, additional glucose is stored as fat.