International Sports Sciences Association (ISSA) Trainer Practice Exam

During endurance exercise, the primary fuel source is fatty acids. This is primarily due to the body's adaptation to prolonged physical activity, where it tends to utilize fat for energy once glycogen stores are sufficiently depleted. When engaging in steady-state aerobic activities, such as long-distance running or cycling, the body shifts to burning fat because it is a more efficient and sustainable energy source for extended periods. While glucose is also used, mainly during higher intensity exercise when immediate energy is required, the reliance on glucose decreases as the duration of exercise increases. Amino acids can serve as a fuel source, particularly during intense endurance efforts or when carbohydrate availability is low, but they are not the primary fuel. Creatine, while essential for short bursts of energy and high-intensity workouts, does not play a significant role in endurance activities as it is more associated with ATP replenishment during short-duration efforts. Ultimately, during long-duration exercise, fatty acids provide a substantial energy yield and enable the body to conserve its limited glycogen reserves, making them the primary fuel source in these situations.