During exercise, the body primarily uses carbohydrates for energy. They are broken down into glucose, which fuels muscle activity. Carbohydrates are stored as glycogen in muscles and the liver; during high-intensity activities, these stores are quickly mobilized to meet energy demands. As exercise continues, the body gradually shifts to using more fat stores for energy, especially during prolonged or less intense activities. This transition allows for sustained energy availability, optimizing performance and endurance throughout the workout.