What is the primary fuel used in the body during prolonged periods of exercise?

During prolonged periods of exercise, the body mainly relies on fats as its primary fuel source. This is because, as exercise duration increases, the availability of carbohydrates—stored as glycogen in muscles and the liver—begins to decrease. Initially, during high-intensity activities, carbohydrates might be the preferred energy source due to their ability to provide rapid energy. However, as the duration of exercise extends into moderate and lower intensities, the body shifts to utilizing fats more significantly.

Fats are a more abundant energy source compared to carbohydrates and can supply a vast amount of ATP (adenosine triphosphate) during prolonged activities. This transition is facilitated by the body’s adaptations to efficient fat oxidation, allowing for sustained energy delivery over longer periods without leading to rapid depletion of glycogen stores. Hence, during prolonged exercise, especially in endurance activities, fatty acid oxidation becomes increasingly important for maintaining energy levels.