Just as much heat is created when fat is burned in the body as when it is burned outside of the body. The heat from “burning” wood is produced by the union of the oxygen from the air with hydrogen and carbon, forming carbon dioxid and water. The light in the burning of wood is caused by the rapid combustion. Combustion occurs within the body more slowly, hence no light is produced. The exact process by which the potential energy latent in food is converted into heat and energy is not known. It is partly released during the digestive process, through the chemical action produced when the elements of the food come into contact with oxygen and with the digestive juices. This combustion gives to the digestive organs the necessary warmth to enable them to do effective work. A certain amount of heat is necessary for the chemical changes, and digestive juices flow more freely when the body is warm. Heat is necessary, also, to aid the peristaltic movements of the digestive organs. It has been estimated that about one-sixth of the heat liberated evaporates through the skin, the lungs, and the excreta, while five-sixths is required to maintain the body heat. If the digestive forces are not working perfectly and if the food is not properly prepared, some of the fuel is not utilized. But, in normal conditions, if the food is supplied in proportion to the energy required, the heat and energy given off should exactly equal the latent heat and energy consumed. If more food is taken than is necessary to produce heat and energy, the excess of material is stored and if the excess continues the bodily machinery may be clogged. The relief lies in consuming the excess through exercise. More oxygen is required to put the excess in condition for use, and the extra amount of oxygen is gained by means of the deep breathing occasioned by exercise. It is to be noted, also, that no force within the body is lost. In the very process of the removal of waste, heat and energy are created, so that the parts no longer needed are utilized by the system, while they are being removed from it. Here is a lesson in economy of force. A small portion of the heat of the body is gained from the sun or from artificial heat, but by far the greater part is generated within the body. As mentioned before, the fuel for the body consists of _fats, starches, and sugars_, which, in combination with oxygen, create force. From the foregoing, it follows that the fuel value of any food depends on the amount of fats, starches, and sugars it contains.