During prolonged exercise, the body's primary source of energy is derived from carbohydrates and fat. As exercise continues beyond approximately 20 minutes, the body increasingly relies on stored fat for energy. Initially, glycogen stored in muscles and the liver serves as the main fuel source during high-intensity efforts. However, as intensity decreases or duration increases, fat becomes the dominant fuel source. This adaptation is crucial for endurance athletes, allowing them to sustain energy levels over long events, helping to improve overall performance and endurance.