What is the primary fuel source for the body during prolonged, moderate-intensity exercise?

During prolonged, moderate-intensity exercise, the body primarily utilizes fat as its main fuel source. As exercise duration extends, the body shifts from using glycogen (stored carbohydrates) to fat, because fat oxidation provides a more sustained energy output. This transition occurs because at lower intensity levels, the body has adequate oxygen supply to oxidize fatty acids, which helps spares carbohydrates for more intense exercise where they are more efficiently utilized.

Carbohydrates serve as a primary fuel source during high-intensity exercise due to their faster breakdown and quicker energy release, which is why they might be prominent in short bursts of activity. Proteins are generally not a primary energy source during exercise; instead, they are utilized more for muscle repair and recovery. Glycogen is indeed a form of carbohydrate storage, but during prolonged exercise, the direct utilization of fatty acids becomes more significant for energy production. Thus, the correct answer reflects the body’s nutritional strategy of conserving glycogen and relying on fat for energy in sustained activities.