What is the relationship between exercise intensity and the body's energy source?

The correct choice highlights a fundamental principle of exercise physiology: as exercise intensity decreases, the body shifts its primary energy source from carbohydrates to fats. At lower intensities, the metabolic pathways that break down fat become more efficient and are favored due to their ability to sustain energy over longer durations. Fat oxidation is more prominent because it requires a slower, more prolonged process, which aligns with lower-intensity activities like walking or light jogging.

As exercise intensity increases, especially during activities like sprinting or heavy resistance training, the body tends to rely more on carbohydrates for quick energy. This is due to carbohydrate metabolism being more efficient for rapidly accessible energy, which is crucial during higher intensity work.

Therefore, at lower exercise intensities, fat serves as the primary energy substrate, providing a vast energy reserve, while carbohydrates become a more significant energy source as intensity rises to meet the increased demands of the body. This transition between fuel sources is a key consideration for athletes and individuals engaged in fitness training, helping to inform their dietary and exercise choices.