Which energy system is primarily responsible for energy during long-term, endurance exercise through aerobic metabolism of glucose and fatty acids?

• A. Aerobic respiration of glucose and fatty acids
• B. Lipid oxidation only
• C. Phosphagen system
• D. Anaerobic glycolysis

Answer: (A)

In sustained, endurance exercise the body relies primarily on aerobic metabolism to meet energy demands. This means using oxygen to fully oxidize substrates in the mitochondria—most notably glucose and fatty acids—to generate ATP. The phosphagen system provides quick energy for only a few seconds, and anaerobic glycolysis can power short bursts but isn’t sustainable for long-duration activity due to fatigue from lactate accumulation. Fatty acid oxidation becomes more prominent as exercise continues, with glucose also contributing, but the key is that energy comes from aerobic respiration of both glucose and fatty acids. This aerobic process uses oxygen to produce more ATP over time, supporting prolonged activity.