Additionally, if the fuel is coming from the blood, it also has to get across the muscle membrane. Glycogen is the high-performance fuel that can be burned at high rates to fuel very intense exercise. But even with glycogen supercompensation, we rely heavily on the other three sources of energy during training and racing. To perform our best, we need muscles that can use both fat and carbohydrate well. We need more enzymes for fuel metabolism.
Enzyme enhancement is stimulated in two ways.
The first is exercise. Training increases mitochondria in the exercising muscles. The second is diet. If you increase the fat content of your diet, your muscles respond by making more copies of the enzymes necessary for fat utilization. If this machinery is well developed, fat can support very high rates of exercise.