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 lias 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 pro-teid 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.

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., pro-teid 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 admix* ture of proteid and starch, the latter predominating in many cases, as in wheat flour. The following table,1 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.

The Chemical Composition Of Some Common Food Materials

1 The data composing this table are taken from Bulletin 28 (Rerised Edition), United States Department of Agriculture, Office of Experiment Stations.

In commenting on these figures, reference to which will be made from time to time in other connections, it may be wise to emphasize the large amount of water almost invariably present in natural foodstuffs. Further, it is to be noted that, in animal products especially, the variations in proteid-content are in large measure coincident with variations in the amount of water present. In other words, foods of animal origin if freed entirely of water would, as a rule, show essentially the same percentage of proteid matter. Fat is naturally variable, according to the condition of the animal at the time it was slaughtered. Among the vegetable products, carbohydrate, mainly in the form of starch, becomes exceedingly conspicuous, though proteid is by no means lacking. Indeed, in some cereals, as in oatmeal, in dried peas and beans, the content of proteid will average as high as in fresh beef, while in addition 50-70 per cent of the entire substance is made up of carbohydrate. Again, in the edible nuts, the content of proteid runs high, in some cases higher than in fresh beef, while at the same time carbohydrate and fat are noticeably large. Further, it is to be noted that in nuts there is here and there Borne striking individuality, as in pine nuts and Brazil nuts, both of which show a noticeable lack of carbohydrate as contrasted with peanuts, almonds, and walnuts; a fact of some importance in cases where a vegetable food rich in proteid is desired, but with freedom from starch.