Which of the major energy systems utilizes fat as an energy source?

The oxidative energy system is the correct choice because it is primarily responsible for producing ATP (adenosine triphosphate) using oxygen and relies on carbohydrates and fats as fuel sources. This system is engaged during longer-duration, lower-intensity activities, such as running at a steady pace or cycling for extended periods.

Fat metabolism occurs through a process called beta-oxidation, where fatty acids are broken down into acetyl CoA, which then enters the Krebs cycle and the electron transport chain to produce ATP. This makes the oxidative system particularly important for endurance athletes, as it allows for sustained energy production over long periods.

In contrast, the ATP-PC system relies on phosphocreatine stored in the muscles for short bursts of high-intensity activity, lasting only up to about 10 seconds, and does not utilize fat. The glycolytic system produces energy from glucose through anaerobic means, primarily for activities that last from approximately 10 seconds to a couple of minutes, and also does not use fat as a primary energy source. Lastly, creatine phosphate is specifically involved in the ATP-PC system and similarly does not involve fat metabolism.

Thus, the oxidative system's capacity to utilize fat as an energy source during prolonged physical activity makes it the