Fats, like carbohydrates, are free from nitrogen, but differ from them in containing a much larger percentage of carbon, and hence have greater fuel value per pound. Fats contain on an average 76.5 per cent of carbon, 11.9 per cent of hydrogen, and 11.5 per cent of oxygen. With their larger content of carbon and smaller proportion of oxygen, fats are less easily oxidizable than sugars, requiring a larger intake of oxygen for their combustion, but when oxidized they yield more heat per pound than carbohydrates. Fats and carbohydrates are thus seen to be the natural fuel foodstuffs of the body. They cannot serve for the upbuilding or renewal of tissue, but by oxidation they constitute an economical fuel for maintaining body temperature and for power to run the bodily machinery. It should be remembered, however, that anything capable of being burned in the body may serve as fuel material; hence proteid food, though of specific value as a tissue builder, may likewise by its oxidation yield energy for heat and work, but its combustion, owing to the content of nitrogen, is never complete. Further, its use as fuel is uneconomical and undesirable for reasons to be discussed later, but it is well to know that its oxidation, though incomplete, is accompanied by the liberation of energy, as in the oxidation of non-nitrogenous foods. A portion of the carbon, hydrogen, and oxygen of the proteid molecule will burn within the body to gaseous products, as do sugars and fats, but there remains a nucleus of nitrogen, with some carbon, hydrogen, and oxygen, which resists combustion and must be gotten rid of by the combined labors of liver and kidneys. Fats and carbohydrates, on the other hand, undergo complete combustion to simple gaseous products, carbon dioxide and water, which are easily removed by the lungs, skin, etc. These three classes of foodstuffs exist in a great variety of combinations or admixtures in nature. In many cases, noticeably in milk, all three occur together in fairly large quantities. In animal foods, such as meats, fish, etc., proteid and fat alone are found, while in perfectly lean meat proteid only is present, excepting a small amount of fat. Again, the white of the egg contains proteid alone. Hence, a meat and egg diet would be essentially a proteid diet. In vegetable foods, as in the cereals, there is found an admixture of proteid and starch, the latter predominating in many cases, as in wheat flour. The following table,[2] showing the chemical composition of various food materials, may be of service in throwing light on the relative distribution of the three classes of foodstuffs in natural products. [2] The data composing this table are taken from Bulletin 28 (Revised Edition), United States Department of Agriculture, Office of Experiment Stations. THE CHEMICAL COMPOSITION OF SOME COMMON FOOD MATERIALS