Dame Nature is very discriminating; she demands a definite form of nitrogenous compound, some peculiar or specific grouping of the nitrogen element with other elements in the food that can make good the waste of proteid tissue. In the inactive and fibrous tissues of animals, such as are found in bones, tendons, and ligaments, there is present a substance known as collagen, which, when boiled with water, as in the making of soups, is transformed into gelatin. This body, because of its close chemical relationship to proteid or albuminous substances, is known as an albuminoid. Yet, though it has essentially the same chemical composition as ordinary albuminous substances and shows many of the reactions characteristic of the latter, it cannot take the place of true proteid in building up or repairing the tissues of the body. To quote again from Dr. Curtis: “Tissue is nitrogenous, so that, of course, only nitrogenous food can serve for its making; but of the two kinds of nitrogenous principles, proteids and albuminoids, behold, proteids only are of avail! Why this is so is unknown, since albuminoid is equally nitrogenous with proteid; but so it is--proteid and proteid alone can fulfil the high function of furnishing the material basis of life. Gelatin cannot even go to make the very kind of tissue of which itself is a derivative. Alongside of its brother proteid, gelatin stands as a prince of the blood whose escutcheon bears the ‘bend sinister.’ Such a one, though of royal lineage, may never aspire to the throne.” It is thus quite clear that the true proteid foods are tissue builders in the broadest sense of the term, and it is equally evident that they are absolutely essential for life, since no other kind or form of foodstuff can take their place in supplying the needs of the body. Every living cell, whether of heart, muscle, brain, or nerve, requires its due allowance of proteid material to maintain its physiological rhythm. No other foodstuff stands in such intimate relationship to the vital processes, but so far as we know at present any form of true proteid, whether animal or vegetable, will serve the purpose. Carbohydrates include two closely related classes of compounds, viz., sugars and starches. They are entirely free from nitrogen, containing only carbon (44.4 per cent), hydrogen (6.2 per cent), and oxygen (49.4 per cent), and hence are classified as non-nitrogenous foods. Obviously, they cannot serve as tissue builders, but by oxidation they yield energy for heat and work. They constitute an easily oxidizable form of fuel, and when supplied in undue amounts they may undergo transformation within the body into fat, which is temporarily deposited in tissues and organs for future needs.