It takes about 90 minutes of low-intensity exercise to deplete glycogen stores, according to Iowa State University. Running for Fitness advises people to take food or sports drinks on runs that last for more than two hours.
Iowa State University points out that the amount of time it takes to deplete glycogen stores depends on the intensity of the exercising. While glycogen stores last for up to 90 minutes in low-intensity exercise, such as distance running, those same stores last for only 20 minutes during continuous, high-intensity workouts.
Running for Fitness explains that energy from glycogen stored in muscles lasts for about an hour of exercise. Then, the body draws energy from the glycogen stored in the liver, called blood sugar. After the depletion of blood sugar levels, the human body needs more glycogen. If athletes need a quick infusion of energy in the middle of a run, they can take gels, which are chewy sachets of syrup that provide quick energy.
Glyocgen is attained through the ingestion of carbohydrates, Iowa State University says. Those who have diets rich in carbohydrates have more glycogen during exercise for energy, and they replenish their stores of glycogen after exercise more easily. The best type of glycogen for athletes is starch, a complex carbohydrate that is easily broken down and stored in muscles and the liver. Bread, cereal and pasta are just a few examples of starches.