_Carbohydrate, protein and fat._--The food supply of mankind is limited to the tissues of plants and animals, and the fuel in all such foods is either a carbohydrate, a fat, or a protein. Carbohydrates are starches and sugars; fats are oils, lards and complex materials such as may be found in brain, and egg yolk; protein is abundant in lean meat and cheese. Compared to the protein, the carbohydrates and fats are relatively simple chemical substances, fairly stable and, hence, easy to study chemically. Proteins are excessively changeable, to which character they owe their name. Some proteins are called albumins. Egg albumin, egg white, is a typical protein possessing all of the essential protein characteristics. The carbohydrates and fats serve chiefly as fuels, while the proteins, besides providing energy, build or rebuild the living tissues of the body. A diet may be deficient in either fat or carbohydrate, and still remain adequate. It must, however, provide a modicum of protein, since parts of the protoplasmic machinery of the tissues are constantly wearing out and requiring replacement. _Foods are disintegrated by catalysts._--While the conversion of the energy in carbohydrates, fats and proteins into the life flame is a process of combustion or oxidation, it is not a simple burning or explosion, as in the case of the fuel ignited in the cylinder of the gasoline motor, but proceeds by successive steps or stages. The earliest of these steps is taken in the kitchen where food is prepared by cooking. There the starches are softened and partly broken up, and the proteins are coagulated. The next stages are passed during digestion, beginning in the mouth, where the ptyalin, a ferment in the salivary juice, attacks starches, and continuing as the food passes successively through the stomach and the intestine. Ferments are also called catalysts. At each and every stage of digestion and assimilation some catalyst facilitates and accelerates the process of decomposition in a manner similar to the action of ptyalin on starch, or of yeast in brewing beer. Consequently, the food, before it has descended more than half way down the length of the small intestine, has been disintegrated chemically into fragments that are smaller and, chemically, much less complex than the original foodstuffs. The starches are split into sugars, the chief of which is glucose; the fats are broken into fatty acid and glycerin; the proteins are torn apart and comparatively simpler structures, the so-called amino-acids, result.