Which of the following substances is primarily used as fuel during endurance exercise?

During endurance exercise, the body primarily relies on fatty acids as a fuel source. This is because, as exercise duration increases and intensity becomes more moderate, the utilization of fat becomes more efficient. Fatty acids provide a significant amount of energy and are abundant in the body’s fat stores, allowing for sustained energy release over extended periods of activity.

While liver glycogen and muscle glycogen also serve as energy sources, they tend to be utilized more prominently during high-intensity exercise when quick energy is necessary. In contrast, during lower intensity and longer duration activities, the body shifts towards burning fat because it can maintain energy production without depleting glycogen stores too quickly.

Electrolytes are vital for hydration and proper muscle function during exercise, but they do not serve as fuel sources. Instead, they help maintain the balance of fluids and support nerve function, which is crucial during endurance activities but does not provide energy in the way that fatty acids do.