The fat in the diet provides the full gas tank reserve for endurance. Part of the fatty acids are converted to easily digested sugar forms that are stored in the liver. The fatty acids that are not held in reserve in the liver provide for part of the present energy needs of the body or they are deposited in and about the muscular system. The complete absence of fat in the diet means slow starvation even if the other nutrients are ample. There is another source of energy but a smaller portion of the amount eaten goes into the ready reserve.
Carbohydrates are the high octane energy source.
Compare fats and carbohydrates to the relative combustion qualities of oil and gasoline respectively. Both will burn but gasoline is ignited easier and burns quicker. Starches and basic sugars are converted into a more readily useable glucose at a relatively faster rate than fats. In this form, they are absorbed by the blood system and supplied to the muscular tissue to furnish needed energy. There is no question about the energetic effect it has on some people.