During prolonged exercise, the body primarily utilizes carbohydrates for energy, especially at higher intensities. Glycogen stored in muscles and the liver acts as the first source of energy. As exercise continues and glycogen stores begin to deplete, the body gradually shifts to burning fat for fuel. This transition is essential for endurance activities, as fat provides a more sustained energy source. However, maintaining carbohydrate availability through proper nutrition and hydration is crucial for optimizing performance and delaying fatigue during prolonged exercise sessions.