Like most machines, the human machine requires the impartation of energy. Similarly, also, this is supplied by the combustion of certain carbonaceous substances. It needs constant repair. These and its other needs are all furnished in the daily food supply. The life of this machine can be greatly lengthened by intelligent care, or shortened by neglect and abuse. Its efficiency may be similarly affected. While one cannot hear the pounding of the engine or the rattling of the machinery, yet the machine is damaged if run under too high a pressure and at too great speed. There are seven classes of the essential elemental food substances,--proteins, fats, carbohydrates, vitamines, salts, cellulose, and water. The ideal diet is one in which these seven elements are regularly supplied to the body in the amounts required to meet its daily needs. A person living close to nature, receiving his food first-handed, direct from nature's health food factory, and eating it with only the cooking and seasoning necessary, and with a reasonable variety, would probably find his diet sufficient, and the elements in about the proper proportions; and with an honest appetite, steadied by a little temperate-in-all-things ballast, he probably would not go far astray as to the proper amounts. But unfortunately, the average individual is not living close to nature. Much that is artificial has come in. Our appetites are capricious, deceitful, and unreasonable. Our foods come to us processed, cartonned, and tinned, often embalmed, devitalized, or adulterated. They are often served to us so disguised that we cannot tell whether their nutritive substance has been concentrated or diluted, or indeed whether or not the body will recognize it as having any nutritive value at all, despite its pleasing flavor. Therefore, in order that the ideal may be approximated to a reasonable and practical degree, we must have some knowledge not only of the needs of the body, but also of these food elements, and how their values may be estimated in the various food substances. The foods that enter into the make-up of the body and supply its heat and energy are three,--protein, fat, and carbohydrate. While the salts to a certain extent enter into the body structure, they have but little to do with heat and energy production. The remaining food classes are adjuncts, their use being simply to make possible the utilization, by the body, of the tissue and fuel foods. The cellulose assists mechanically in digestion; the water furnishes the necessary fluid; and the vitamines provide the battery, as it were, which sets the whole apparatus in motion. _The Heat Unit_