Fat is your body's main source of energy. That is why the body stores it in such great amounts from the food we eat. Fat breakdown is slow and thus provides a better fuel for long term at rest activities like standing, sitting ,and routine walking.
What Is Your Body's Primary Fuel Source At Rest?
FAT is the primary source of energy at rest, but as the intensity of your activity increases it shifts from fats to carbs.
Protein
Food is primary source of energy....but energy is released when food is oxidized.