During prolonged physical activity, the body uses stored fat as a primary energy source once glycogen stores are depleted. This process, known as fat oxidation, occurs through a metabolic pathway that breaks down fatty acids to produce energy. Endurance activities, such as long-distance running or cycling, rely heavily on this fat-burning process, especially as the duration of the activity increases. Proper nutrition, including carbohydrate intake before exercise, supports initial energy needs, while training adaptations over time can enhance the body’s ability to efficiently utilize fat for sustained energy during extended physical efforts.