The fats of food include both the animal fats and the vegetable oils. The carbohydrates include such compounds as starches, sugars and the fibre of plants or cellulose, though the latter has but little value as food for man. The more important function of both these classes of nutrients is to supply energy to the body to meet its requirements above that which it may obtain from the proteids. It is not improbable that the atoms of their molecules as well as those from the proteids are built up into the protoplasmic substance of the tissues. In this sense, these nutrients may be considered as being utilized also for the formation of tissue; but they are rather the accessory ingredients, whereas the proteids are the essential ingredients for this purpose. The fats in the food in excess of the body requirements may be stored as body fat, and the surplus carbohydrates may also be converted into fat and stored.
To a certain extent, then, the nutrients of the food may substitute each other. All may be incorporated into the protoplasmic structure of body tissue, though only the proteids can supply the essential nitrogenous ingredients; and apart from the portion of the proteid material that is indispensable for this purpose, all the nutrients are used as a source of energy. If the supply of energy in the food is not sufficient, the body will use its own proteid and fat for this purpose. The gelatinoids, fats and carbohydrates in being utilized for energy protect the body proteids from consumption. The fat stored in the body from the excess of food is a reserve of energy material, on which the body may draw when the quantity of energy in the food is insufficient for its immediate needs.
What compounds are especially concerned in intellectual activity is not known. The belief that fish is especially rich in phosphorus and valuable as a brain food has no foundation in observed fact.
2. Metabolism of Matter and Energy.—The processes of nutrition thus consist largely of the transformation of food into body material and the conversion of the potential energy of both food and body material into the kinetic energy of heat and muscular work and other forms of energy. These various processes are generally designated by the term metabolism. The metabolism of matter in the body is governed largely by the needs of the body for energy. The science of nutrition, of which the present subject forms a part, is based on the principle that the transformations of matter and energy in the body occur in accordance with the laws of the conservation of matter and of energy. That the body can neither create nor destroy matter has long been universally accepted. It would seem that the transformation of energy must likewise be governed by the law of the conservation of energy; indeed there is every reason a priori to believe that it must; but the experimental difficulties in the way of absolute demonstration of the principle are considerable. For such demonstration it is necessary to prove that the income and expenditure of energy are equal. Apparatus and methods of inquiry devised in recent years, however, afford means for a comparison of the amounts of both matter and energy received and expended by the body, and from the results obtained in a large amount of such research, it seems probable that the law obtains in the living organism in general.
The first attempt at such demonstration was made by M. Rubner[3] in 1894, experimenting with dogs doing no external muscular work. The income of energy (as heat) was computed, but the heat eliminated was measured. In the average of eight experiments continuing forty-five days, the two quantities agreed within 0.47%, thus demonstrating what it was desired to prove—that the heat given off by the body came solely from the oxidation of food within it. Results in accordance with these were reported by Studenski[4] in 1897, and by Laulanie[5] in 1898.
The most extensive and complete data yet available on the subject have been obtained by W. O. Atwater, F. G. Benedict and associates[6] in experiments with men in the respiration calorimeter, in which a subject may remain for several consecutive days and nights. These experiments involve actual weighing and analyses of the food and drink, and of the gaseous, liquid and solid excretory products; determinations of potential energy (heat of oxidation) of the oxidizable material received and given off by the body (including estimation of the energy of the material gained or lost by the body); and measurements of the amounts of energy expended as heat and as external muscular work. By October 1906 eighty-eight experiments with fifteen different subjects had been completed. The separate experiments continued from two to thirteen days, making a total of over 270 days. In some cases the subjects were at rest; in others they performed varying amounts of external muscular work on an apparatus by means of which the amount of work done was measured. In some cases they fasted, and in others they received diets generally not far from sufficient to maintain nitrogen, and usually carbon, equilibrium in the body. In these experiments the amount of energy expended by the body as heat and as external muscular work measured in terms of heat agreed on the average very closely with the amount of heat that would be produced by the oxidation of all the matter metabolized in the body. The variations for individual days, and in the average for individual experiments as well, were in some cases appreciable, amounting to as much as 6%, which is not strange in view of the uncertainties in physiological experimenting; but in the average of all the experiments the energy of the expenditure was above 99.9% of the energy of the income,—an agreement within one part in 1000. While these results do not absolutely prove the application of the law of the conservation of energy in the human body, they certainly approximate very closely to such demonstration. It is of course possible that energy may have given off from the body in other forms than heat and external muscular work. It is conceivable, for example, that intellectual activity may involve the transformation of physical energy, and that the energy involved may be eliminated in some form now unknown. But if the body did give off energy which was not measured in these experiments, the quantity must have been extremely small. It seems fair to infer from the results obtained that the metabolism of energy in the body occurred in conformity with the law of the conservation of energy.
3. Composition of Food Materials.—The composition of food is determined by chemical analyses, the results of which are conventionally expressed in terms of the nutritive ingredients previously described. As a result of an enormous amount of such investigation in recent years, the kinds and proportions of nutrients in our common sorts of food are well known. Average values for percentage composition of some ordinary food materials are shown in Table I. (Table I. also includes figures for fuel value.)
Table I.—Percentage Composition of some Common Food Materials.
| Food Material. | Refuse. | Water. | Protein. | Fat. | Carbo- hydrates. | Mineral Matter. | Fuel Value per ℔ |
| % | % | % | % | % | % | Calories. | |
| Beef, fresh (medium fat)— | |||||||
| Chuck | 16.3 | 52.6 | 15.5 | 15.0 | · · | 0.8 | 910 |
| Loin | 13.3 | 52.5 | 16.1 | 17.5 | · · | 0.9 | 1025 |
| Ribs | 20.8 | 43.8 | 13.9 | 21.2 | · · | 0.7 | 1135 |
| Round | 7.2 | 60.7 | 19.0 | 12.8 | · · | 1.0 | 890 |
| Shoulder | 16.4 | 56.8 | 16.4 | 9.8 | · · | 0.9 | 715 |
| Beef, dried and smoked | 4.7 | 53.7 | 26.4 | 6.9 | · · | 8.9 | 790 |
| Veal— | |||||||
| Leg | 14.2 | 60.1 | 15.5 | 7.9 | · · | 0.9 | 625 |
| Loin | 16.5 | 57.6 | 16.6 | 9.0 | · · | 0.9 | 685 |
| Breast | 21.3 | 52.0 | 15.4 | 11.0 | · · | 0.8 | 745 |
| Mutton— | |||||||
| Leg | 18.4 | 51.2 | 15.1 | 14.7 | · · | 0.8 | 890 |
| Loin | 16.0 | 42.0 | 13.5 | 28.3 | · · | 0.7 | 1415 |
| Flank | 9.9 | 39.0 | 13.8 | 36.9 | · · | 0.6 | 1770 |
| Pork— | |||||||
| Loin | 19.7 | 41.8 | 13.4 | 24.2 | · · | 0.8 | 1245 |
| Ham, fresh | 10.7 | 48.0 | 13.5 | 25.9 | · · | 0.8 | 1320 |
| Ham, smoked and salted | 13.6 | 34.8 | 14.2 | 33.4 | · · | 4.2 | 1635 |
| Fat, salt | · · | 7.9 | 1.9 | 86.2 | · · | 3.9 | 3555 |
| Bacon | 7.7 | 17.4 | 9.1 | 62.2 | · · | 4.1 | 2715 |
| Lard, refined | · · | · · | · · | 100.0 | · · | · · | 4100 |
| Chicken | 25.9 | 47.1 | 13.7 | 12.3 | · · | 0.7 | 765 |
| Turkey | 22.7 | 42.4 | 16.1 | 18.4 | · · | 0.8 | 1060 |
| Goose | 17.6 | 38.5 | 13.4 | 29.8 | · · | 0.7 | 1475 |
| Eggs | 11.2 | 65.5 | 13.1 | 9.3 | · · | 0.9 | 635 |
| Cod, fresh | 29.9 | 58.5 | 11.1 | 0.2 | · · | 0.8 | 220 |
| Cod, salted | 24.9 | 40.2 | 16.0 | 0.4 | · · | 18.5 | 325 |
| Mackerel, fresh | 44.7 | 40.4 | 10.2 | 4.2 | · · | 0.7 | 370 |
| Herring, smoked | 44.4 | 19.2 | 20.5 | 8.8 | · · | 7.4 | 755 |
| Salmon, tinned | · · | 63.5 | 21.8 | 12.1 | · · | 2.6 | 915 |
| Oysters, shelled | · · | 88.3 | 6.0 | 1.3 | 3.3 | 1.1 | 225 |
| Butter | · · | 11.0 | 1.0 | 85.0 | · · | 3.0 | 3410 |
| Cheese | · · | 34.2 | 25.9 | 33.7 | 2.4 | 3.8 | 1885 |
| Milk, whole | · · | 87.0 | 3.3 | 4.0 | 5.0 | 0.7 | 310 |
| Milk, skimmed | · · | 90.5 | 3.4 | 0.3 | 5.1 | 0.7 | 165 |
| Oatmeal | · · | 7.7 | 16.7 | 7.3 | 66.2 | 2.1 | 1800 |
| Corn (maize) meal | · · | 12.5 | 9.2 | 1.9 | 75.4 | 1.0 | 1635 |
| Rye flour | · · | 12.9 | 6.8 | 0.9 | 78.7 | 0.7 | 1620 |
| Buckwheat flour | · · | 13.6 | 6.4 | 1.2 | 77.9 | 0.9 | 1605 |
| Rice | · · | 12.3 | 8.0 | 0.3 | 79.0 | 0.4 | 1620 |
| Wheat flour, white | · · | 12.0 | 11.4 | 1.0 | 75.1 | 0.5 | 1635 |
| Wheat flour, graham | · · | 11.3 | 13.3 | 2.2 | 71.4 | 1.8 | 1645 |
| Wheat, breakfast food | · · | 9.6 | 12.1 | 1.8 | 75.2 | 1.3 | 1680 |
| Wheat bread, white | · · | 35.3 | 9.2 | 1.3 | 53.1 | 1.1 | 1200 |
| Wheat bread, graham | · · | 35.7 | 8.9 | 1.8 | 52.1 | 1.5 | 1195 |
| Rye bread | · · | 35.7 | 9.0 | 0.6 | 53.2 | 1.5 | 1170 |
| Biscuit (crackers) | · · | 6.8 | 9.7 | 12.1 | 69.7 | 1.7 | 1925 |
| Macaroni | · · | 10.3 | 13.4 | 0.9 | 74.1 | 1.3 | 1645 |
| Sugar | · · | · · | · · | · · | 100.0 | · · | 1750 |
| Starch (corn starch) | · · | · · | · · | · · | 90.0 | · · | 1680 |
| Beans, dried | · · | 12.6 | 22.5 | 1.8 | 59.6 | 3.5 | 1520 |
| Peas, dried | · · | 9.5 | 24.6 | 1.0 | 62.0 | 2.9 | 1565 |
| Beets | 20.0 | 70.0 | 1.3 | 0.1 | 7.7 | 0.9 | 160 |
| Cabbage | 50.0 | 4.2 | 0.7 | 0.2 | 4.5 | 0.4 | 100 |
| Potatoes | 20.0 | 62.6 | 1.8 | 0.1 | 14.7 | 0.8 | 295 |
| Sweet potatoes | 20.0 | 55.2 | 1.4 | 0.6 | 21.9 | 0.9 | 440 |
| Tomatoes | · · | 94.3 | 0.9 | 0.4 | 3.9 | 0.5 | 100 |
| Apples | 25.0 | 63.3 | 0.3 | 0.3 | 10.8 | 0.3 | 190 |
| Bananas | 35.0 | 48.9 | 0.8 | 0.4 | 14.3 | 0.6 | 260 |
| Grapes | 25.0 | 58.0 | 1.0 | 1.2 | 14.4 | 0.4 | 295 |
| Strawberries | 5.0 | 85.9 | 0.9 | 0.6 | 7.0 | 0.6 | 150 |
| Almonds | 45.0 | 2.7 | 11.5 | 30.2 | 9.5 | 1.1 | 1515 |
| Brazil nuts | 49.6 | 2.6 | 8.6 | 33.7 | 3.5 | 2.0 | 1485 |
| Chestnuts | 16.0 | 37.8 | 5.2 | 4.5 | 35.4 | 1.1 | 915 |
| Walnuts | 58.1 | 1.0 | 6.9 | 26.6 | 6.8 | 0.6 | 1250 |
It will be observed that different kinds of food materials vary widely in their proportions of nutrients. In general the animal foods contain the most protein and fats, and vegetable foods are rich in carbohydrates. The chief nutrient of lean meat and fish is protein; but in medium fat meats the proportion of fat is as large as that of protein, and in the fatter meats it is larger. Cheese is rich in both protein and fat. Among the vegetable foods, dried beans and peas are especially rich in protein. The proportion in oatmeal is also fairly large, in wheat it is moderate, and in maize meal and rice it is rather small. Oats contain more oil than any of the common cereals, but in none of them is the proportion especially large. The most abundant nutrient in all the cereals is starch, which comprises from two-thirds to three-fourths or more of their total nutritive substance. Cotton-seed is rich in edible oil, and so are olives. Some of the nuts contain fairly large proportions of both protein and fat. The nutrient of potatoes is starch, present in fair proportion. Fruits contain considerable carbohydrates, chiefly sugar. Green vegetables are not of much account as sources of any of the nutrients or energy.