What is the primary macronutrient utilized for energy during prolonged moderate-intensity exercise?

During prolonged moderate-intensity exercise, fat serves as the primary macronutrient utilized for energy. As the duration of the exercise increases and the body gets adapted to the activity, it shifts from using carbohydrates to utilizing a higher proportion of fat stores for energy. This shift occurs because fat oxidation is more efficient for sustaining energy over extended periods compared to carbohydrates, which tend to be mobilized for immediate energy needs.

Additionally, while carbohydrates are crucial for quick bursts of energy and are primarily used in higher intensity exercise, the reliance on fat becomes more significant as intensity decreases to a moderate level that can be sustained for a longer time. Proteins generally play a supporting role in energy metabolism and are not a primary source during prolonged exercise. Fiber, although important for digestive health, does not serve as an energy source during exercise. Therefore, fat is indeed the primary macronutrient utilized for energy during this type of activity.