During prolonged exercise, the body adjusts its energy system by relying more on fat stores once glycogen levels become depleted. Initially, during high-intensity activities, the body draws on available glucose for quick energy. However, as the duration increases, transitioning to fat oxidation becomes critical for sustained energy output. These shifts ensure that athletes can maintain performance over extended periods without the risk of depleting glycogen stores. Proper fueling before and during exercise is essential for supporting these adjustments effectively.