Sugars, fats, and proteins all contain large quantities of chemical energy. When they are broken down by oxidation, these chemical energies are released for life activities. They all provide energy to the body.
The breakdown of sugars accounts for about 70% of the total energy in normal conditions, so sugars are the main energy source for life activities. Fats are served as energy storage material that contain more than twice as much energy as sugar. Proteins are the raw materials of living organisms and performers of life activities, therefore they do not serve as the primary energy source.
They are broken down to produce energy in the following order: glucose, glycogen, fat and protein.
Glucose in the blood is oxidized to provide energy. When blood glucose level is low, glycogen is broken down into glucose to be used by cells. If there is not much sugar, the body breaks down fats into glycerol and fatty acids. They are oxidized to water and carbon dioxide to provide energy. When the sugars and fats are depleted, proteins are broken down into amino acids. They are oxidized to water and carbon dioxide to supply energy. The breakdown of large amounts of amino acids produces too much ammonia, which may be toxic to humans. The body does not readily use proteins as energy source. When this happens, it means the person is seriously ill or in the famine.
The energy required by cells is directly provided by ATP hydrolysis. Energy from sugars, fats and proteins is transferred to ATP in order to be used for all life activities.