The primary fuel source for the body during prolonged exercise is fat. As physical activity continues, especially in endurance exercises, the body increasingly relies on fat stores for energy. Initially, carbohydrates are used as the primary fuel; however, as glycogen stores deplete, fat becomes a more significant energy source due to its higher energy density. This shift allows athletes and individuals engaging in endurance activities to sustain energy levels for longer durations. Effective training can enhance the body's capacity to utilize fat and improve overall exercise endurance.