What is the primary role of protein as a fuel source during exercise?

The primary role of protein as a fuel source during exercise is that it serves as a last resort through gluconeogenesis. In the body's energy hierarchy, carbohydrates and fats are typically utilized first to provide fuel during exercise. When glycogen stores are depleted, particularly during prolonged or intense exercise, the body may turn to protein. This process involves converting amino acids from protein into glucose, which can then be used for energy. Protein is not a preferred energy source due to the metabolic costs associated with its use and the importance of maintaining amino acids for other physiological functions such as muscle repair and growth.

In contrast, the other choices do not accurately represent protein's role during exercise. While protein can provide energy, it is not the first choice for energy, nor is it designated solely for high-intensity workouts. Instead, carbohydrates and fats remain the primary fuel sources, with protein being mobilized primarily in circumstances of energy deficit or when other fuel sources are insufficient.