During prolonged endurance activities, what is primarily utilized for fuel after glycogen stores are depleted?

During prolonged endurance activities, once glycogen stores are depleted, the body primarily turns to fats as a fuel source. This shift occurs because fat provides a high yield of ATP per gram, making it a valuable energy source for extended periods of exercise.

As exercise continues and the intensity remains moderate, the body employs a metabolic pathway known as beta-oxidation to break down fatty acids into acetyl-CoA, which then enters the Krebs cycle for energy production. This process is particularly crucial for athletes engaged in aerobic activities that last longer than a couple of hours, where glycogen stores become insufficient.

While proteins can also be used as a fuel source, this occurs to a much lesser extent and primarily in situations of prolonged fasting or severe caloric deficit. Carbohydrates are the body's preferred energy source during high-intensity efforts and are depleted more quickly than fats. ATP, while vital for immediate energy, is not a fuel source itself; rather, it is a product of metabolism that provides energy to cells.

Thus, understanding the reliance on fats for prolonged endurance activities emphasizes the importance of adequate fat stores and the body’s ability to efficiently utilize them for sustained energy output.