Foods must primarily furnish an adequate supply of food material, but they must also furnish this material in such form as can be utilized by the system. This process of food utilization is really one of combustion, similar to that in a stove or furnace where the fuel material combines with oxygen and gives off heat. It takes place in the cells of all tissue and is very slow. Food material, like any other fuel, if burned in the air, can be entirely consumed, leaving nothing but ashes, but when utilized in the body the process of combustion is slower and less complete.

Before combustion can take place in the body all food must be put into the form in which it can be carried by the blood and utilized by the cells. The first step in this process is to change solid foods into fluids. For this, plenty of water must be supplied either between meals or with the meals. The insoluble foods that cannot be put directly into solution must be changed into chemical compounds that can be dissolved. These processes are partly mechanical and partly chemical.