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.
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.
THE CHEMICAL COMPOSITION OF SOME COMMON FOOD MATERIALS
| Food Materials. | Proteid. | Carbo-hydrate. | Fat. | Water. | Mineral Matter. | Fuel Value per pound. |
|---|---|---|---|---|---|---|
| per cent | per cent | per cent | per cent | per cent | calories | |
| Fresh beef, loin, lean, edible portion | 24.2 | 0 | 3.7 | 70.8 | 1.3 | 615 |
| Fresh beef, round, lean, edible portion | 22.3 | 0 | 2.8 | 73.6 | 1.3 | 540 |
| Fresh Porterhouse steak, edible portion | 21.9 | 0 | 20.4 | 60.0 | 1.0 | 1270 |
| Fresh beef liver | 21.0 | 1.7 | 4.5 | 71.2 | 1.6 | 605 |
| Fresh beef tongue | 19.0 | 0 | 9.2 | 70.8 | 1.0 | 740 |
| Fresh sweetbreads | 16.8 | 0 | 12.1 | 70.9 | 1.6 | 825 |
| Fresh beef kidney | 16.9 | 0.4 | 4.8 | 76.7 | 1.2 | 520 |
| Cooked beef, roasted | 22.3 | 0 | 28.6 | 48.2 | 1.3 | 1620 |
| Cooked round steak | 27.6 | 0 | 7.7 | 63.0 | 1.8 | 840 |
| Broiled tenderloin steak | 23.5 | 0 | 20.4 | 54.8 | 1.2 | 1300 |
| Dried beef, canned | 39.2 | 0 | 5.4 | 44.8 | 11.2 | 960 |
| Stewed kidneys, canned | 18.4 | 2.1 | 5.1 | 71.9 | 2.5 | 600 |
| Fresh corned beef, edible portion | 15.3 | 0 | 26.2 | 53.6 | 4.9 | 1395 |
| Fresh breast of veal, lean | 21.2 | 0 | 8.0 | 70.3 | 1.0 | 730 |
| Fresh leg of lamb, edible portion | 19.2 | 0 | 16.5 | 63.9 | 1.1 | 1055 |
| Lamb chops, broiled | 21.7 | 0 | 29.9 | 47.6 | 1.3 | 1665 |
| Roast leg of lamb, edible portion | 19.4 | 0 | 12.7 | 67.1 | 0.8 | 900 |
| Roast leg of mutton, edible portion | 25.9 | 0 | 22.6 | 50.9 | 1.2 | 1420 |
| Fresh lean ham | 25.0 | 0 | 14.4 | 60.0 | 1.3 | 1075 |
| Smoked ham, fat, edible portion | 14.8 | 0 | 52.3 | 27.9 | 3.7 | 2485 |
| Chicken, broilers, edible portion | 21.5 | 0 | 2.5 | 74.8 | 1.1 | 505 |
| Turkey, edible portion | 21.1 | 0 | 22.9 | 55.5 | 1.0 | 1360 |
| Roast turkey, edible portion | 27.8 | 0 | 18.4 | 52.0 | 1.2 | 1295 |
| Fricasseed chicken, edible portion | 17.6 | 2.4 | 11.5 | 67.5 | 1.0 | 855 |
| Fresh cod, dressed | 11.1 | 0 | 0.2 | 58.5 | 0.8 | 215 |
| Fresh mackerel, edible portion | 18.7 | 0 | 7.1 | 73.4 | 1.2 | 645 |
| Fresh halibut, steaks | 18.6 | 0 | 5.2 | 75.4 | 1.0 | 565 |
| Fresh shad, edible portion | 18.8 | 0 | 9.5 | 70.6 | 1.3 | 750 |
| Fresh smelt, edible portion | 17.6 | 0 | 1.8 | 79.2 | 1.7 | 405 |
| Cooked bluefish, edible portion | 26.1 | 0 | 4.5 | 68.2 | 1.2 | 670 |
| Broiled Spanish mackerel, edible portion | 23.2 | 0 | 6.5 | 68.9 | 1.4 | 715 |
| Salt codfish, edible portion | 25.4 | 0 | 0.3 | 53.5 | 24.7 | 410 |
| Salt mackerel, edible portion | 22.0 | 0 | 22.6 | 42.2 | 13.2 | 1345 |
| Canned salmon, edible portion | 21.8 | 0 | 12.1 | 63.5 | 2.6 | 915 |
| Canned sardines, edible portion | 23.0 | 0 | 19.7 | 52.3 | 5.6 | 162 |
| Fresh round clams | 6.5 | 4.2 | 0.4 | 86.2 | 2.7 | 215 |
| Fresh oysters, solid | 6.0 | 3.3 | 1.3 | 88.3 | 1.1 | 230 |
| Fresh hen’s eggs | 13.4 | 0 | 10.5 | 73.7 | 1.0 | 720 |
| Boiled hen’s eggs | 13.2 | 0 | 12.0 | 73.2 | 0.8 | 765 |
| Butter | 1.0 | 0 | 85.0 | 11.0 | 3.0 | 3605 |
| Full cream cheese | 25.9 | 2.4 | 33.7 | 34.2 | 3.8 | 1950 |
| Whole cow’s milk | 3.3 | 5.0 | 4.0 | 87.0 | 0.7 | 325 |
| Corn meal, unbolted | 8.4 | 74.0 | 4.7 | 11.6 | 1.3 | 1730 |
| Oatmeal | 16.1 | 67.5 | 7.2 | 7.3 | 1.9 | 1860 |
| Rice | 8.0 | 79.0 | 0.3 | 12.3 | 0.4 | 1630 |
| Wheat flour, entire wheat | 13.8 | 71.9 | 1.9 | 11.4 | 1.0 | 1675 |
| Boiled rice | 2.8 | 24.4 | 0.1 | 72.5 | 0.2 | 525 |
| Shredded wheat | 10.5 | 77.9 | 1.4 | 8.1 | 2.1 | 1700 |
| Macaroni | 13.4 | 74.1 | 0.9 | 10.3 | 1.3 | 1665 |
| Brown bread | 5.4 | 47.1 | 1.8 | 43.6 | 2.1 | 1050 |
| Wheat bread or rolls | 8.9 | 56.7 | 4.1 | 29.2 | 1.1 | 1395 |
| Whole wheat bread | 9.4 | 49.7 | 0.9 | 38.4 | 1.3 | 1140 |
| Soda crackers | 9.8 | 73.1 | 9.1 | 5.9 | 2.1 | 1925 |
| Oyster crackers | 11.3 | 70.5 | 10.5 | 4.8 | 2.9 | 1965 |
| Ginger bread | 5.8 | 63.5 | 9.0 | 18.8 | 2.9 | 1670 |
| Sponge cake | 6.3 | 65.9 | 10.7 | 15.3 | 1.8 | 1795 |
| Lady fingers | 8.8 | 70.6 | 5.0 | 15.0 | 0.6 | 1685 |
| Apple pie | 3.1 | 42.8 | 9.8 | 42.5 | 1.8 | 1270 |
| Custard pie | 4.2 | 26.1 | 6.3 | 62.4 | 1.0 | 830 |
| Squash pie | 4.4 | 21.7 | 8.4 | 64.2 | 1.3 | 840 |
| Indian meal pudding | 5.5 | 27.5 | 4.8 | 60.7 | 1.5 | 815 |
| Tapioca pudding | 3.3 | 28.2 | 3.2 | 64.5 | 0.8 | 720 |
| Fresh asparagus | 1.8 | 3.3 | 0.2 | 94.0 | 0.7 | 105 |
| Fresh lima beans | 7.1 | 22.0 | 0.7 | 68.5 | 1.7 | 570 |
| Dried lima beans | 18.1 | 65.9 | 1.5 | 10.4 | 4.1 | 1625 |
| Dried beans | 22.5 | 59.6 | 1.8 | 12.6 | 3.5 | 1605 |
| Cooked beets | 2.3 | 7.4 | 0.1 | 88.6 | 1.6 | 185 |
| Fresh cabbage, edible portion | 1.6 | 5.6 | 0.3 | 91.5 | 1.0 | 145 |
| Green corn, edible portion | 3.1 | 19.7 | 1.1 | 75.4 | 0.7 | 470 |
| Dried peas | 24.6 | 62.0 | 1.0 | 9.5 | 2.9 | 1655 |
| Green peas | 7.7 | 16.9 | 0.5 | 74.6 | 1.0 | 465 |
| Raw potatoes, edible portion | 2.2 | 18.4 | 0.1 | 78.3 | 1.0 | 385 |
| Boiled potatoes | 2.5 | 20.9 | 0.1 | 75.5 | 1.0 | 440 |
| Fresh tomatoes | 0.9 | 3.9 | 0.4 | 94.3 | 0.5 | 105 |
| Baked beans, canned | 6.9 | 19.6 | 2.5 | 68.9 | 2.1 | 600 |
| Apples, edible portion | 0.4 | 14.2 | 0.5 | 84.6 | 3.0 | 290 |
| Bananas, yellow, edible portion | 1.3 | 22.0 | 0.6 | 75.3 | 0.8 | 460 |
| Fresh cranberries | 0.4 | 9.9 | 0.6 | 88.9 | 0.2 | 215 |
| Oranges, edible portion | 0.8 | 11.6 | 0.2 | 86.9 | 0.5 | 240 |
| Peaches, edible portion | 0.7 | 9.4 | 0.1 | 89.4 | 0.4 | 190 |
| Fresh strawberries | 1.0 | 7.4 | 0.6 | 90.4 | 0.6 | 180 |
| Dried prunes, edible portion | 2.1 | 73.3 | 0.0 | 22.3 | 2.3 | 1400 |
| Almonds, edible portion | 21.0 | 17.3 | 54.9 | 4.8 | 2.0 | 3030 |
| Peanuts, edible portion | 25.8 | 24.4 | 38.6 | 9.2 | 2.0 | 2560 |
| Pine nuts, edible portion | 33.9 | 6.9 | 49.4 | 6.4 | 3.4 | 2845 |
| Brazil nuts, edible portion | 17.0 | 7.0 | 66.8 | 5.3 | 3.9 | 3265 |
| Soft-shell walnuts, edible portion | 16.6 | 16.1 | 63.4 | 2.5 | 1.4 | 3285 |
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 some 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.
Another generality, to be thoroughly understood, is that while the figures given for proteid express quite clearly and with reasonable degree of accuracy the relative amounts of proteid matter present in the foodstuffs in question, there may be important differences in availability of which the percentage figures give no suggestion. In other words, the analytical data deal solely with the total content of proteid, while there is needed in addition information as to the relative digestibility, or availability by the body, of the different kinds of proteid food. For example, roast mutton, cream cheese, and dried peas contain approximately the same amount of proteid. Are we then to infer that these three foods have the same nutritive value so far as proteid is concerned? Surely not, since no account is taken of the relative digestibility of the three foods. It is one of the axioms of physiology that the true nutritive value of any proteid food is dependent not alone upon the amount of proteid contained therein, but upon the quantity of proteid that can be digested and absorbed; or, in other words, made available for the needs of the body. The same rule holds good for both fats and carbohydrates, but as proteid is the more important foodstuff, and is as a rule taken more sparingly, the question of availability has greater import with the proteid foods.
The availability or digestibility of foods can be determined only by physiological experiment. By making a comparison for a definite period of time of the amount of a given food ingredient consumed and the amount that passes unchanged through the intestine, an estimate of its digestibility can be made. The result, to be sure, is not wholly free from error, since we cannot always distinguish between the undigested food and so-called metabolic products coming from the digestive juices and from the walls of the intestine; but the errors are not large, and results so obtained are full of meaning. In a general way it may be stated that with animal foods, such as meats, eggs, and milk, about 97 per cent of the contained proteid is digested and thereby rendered available for the body. With ordinary vegetable foods, on the other hand, as they are usually prepared for consumption, only about 85 per cent of the proteid is made available. This is partially due to the presence in the vegetable tissue of cellulose, which in some measure prevents that thorough attack of the proteid by the digestive juices which occurs with animal foods. With a mixed diet, i. e., with a variable admixture of animal and vegetable foods, it is usually considered that about 92 per cent of the proteid contained therein will undergo digestion.