“How Intense is Intense?”
As well as duration, intensity plays an important role in the efficiency of fat-metabolism during exercise. In general, the percentage of energy contributed by fat diminishes as the intensity of exercise increases.
Fat can only be broken down in the presence of oxygen. Oxygen serves as the catalyst that enables proteins and enzymes of the body to burn fat during an exercise metabolism.
The heart and lungs can provide only so much oxygen -- so fast. When muscle exertion is so great that the demand for energy outstrips the oxygen supply, the body cannot process oxygen fast enough. Therefore the body cannot burn fat. Instead, it reverts back to anaerobic metabolism and burns more glucose.