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DIETETICS
are used like those of food. An important use of the carbohydrates and fats is to protect body tissue (muscle, &c.) from consumption. What compounds are especially ■concerned in the production of intellectual or nervous energy is not known. The idea that fish is especially rich in phosphorus and valuable as brain food has no foundation in observed fact. Heat and muscular work represent forms of energy. The energy is developed as the food is consumed in the body, and is measured in the laboratory by means of an apparatus called the calorimeter. The unit commonly used is the calorie, the amount of heat which would raise the temperature of a kilogram of water 1 degree centigrade, or approximately 1 pound of water 4 degrees Fahrenheit. By heat of combustion is meant the amount of heat which a given substance yields upon combustion with oxygen outside the body. Thus a gram of cane-sugar yields 3'96 ■calories, and a gram of proteid, in the form of white of egg, -5’69 calories. By fuel-value is here meant the total •energy which a given substance can yield the body; in other words, it is the heat of combustion of that part of the food which is capable of oxidation within the body. 2. Metabolism of Matter and Energy.—In so far as its material phenomena are concerned, life consists of transformations of matter and energy, to which the term metabolism is applied. The processes of metabolism are thus of two definite and closely allied kinds—the metabolism of matter, in which the changes are chemical, and that of energy, in which the changes are physical. To say that the chemical transformations in the body obey the law of the conservation of matter is simply to say that the body can neither ■create nor destroy matter—a principle long since demonstrated and universally accepted. It would seem that the metabolism of energy must in like manner obey the law of the conservation of energy, but owing to the great experimental difficulties the actual demonstration of this principle has been long delayed. For this demonstration it is necessary to prove that the income and expenditure of •energy are equal. The body receives energy in the compounds of the food and drink. In the processes of nutrition the potential energy of these compounds is metabolized, and appears as kinetic energy in the heat given off from the body, and in the external muscular work performed, although a small amount remains unchanged, mostly in the unoxidized materials excreted’by the kidneys, intestines, and otherwise. The minute quantities given off as electricity or light by some animals are here neglected. The apparatus and methods of experimenting lately devised make it possible to measure the energy of income in food .and drink on the one hand, and that of expenditure in body heat, external work, and unoxidized excretory products on the other. The potential energy of income and expenditure is measured by the heat of combustion of food and excretory products as determined by burning with oxygen in the bomb calorimeter. The kinetic energy, which is found only in the expenditure, is measured by the respiration calorimeter—an apparatus in which the subject of the experiment (a man, for instance) remains for a number of days and nights under conditions which permit of comparison between the matter and energy taken in and given ■off. This involves weighings, measurements, and analyses of the food and drink, and of respiratory and other •excretory products, and measurements of the heat given off by the body, and of the external mechanical work, allowance being made for the energy of the body material gained or lost during the experiment. Partial measurements of income and expenditure of energy were published by Rubner 1 in 1894. The experiments were made with 1
Ztschr. Biol. 30, 73.
dogs without external muscular work. The income of energy was computed from the excretory products, while the outgoings in the form of heat (since no external muscular work was done) were measured directly. In the average of 8 experiments, which continued during 45 days, the two quantities agreed within 0'47 per cent., thus demonstrating, what the author desired to prove, that the heat given off from the body came solely from the oxidation of food within it.2 Results in accord with these were reported by Studenski and by Laulanie in 1898.3 Results of experiments with men have been4 lately published by Atwater, Rosa, and Benedict and associates, who used a respiration calorimeter as above mentioned, and made detailed analyses and determinations of heats of combustion of food, drink, and excretory products, and measurements of heat given off by the body and the heat equivalent of the external muscular work done. In all, 30 experiments had been reported by November 1801, occupying from 2 to 4 days each, or a total of 93 days of experimenting. In some of these experiments the subject was at rest; in others he rode a stationary bicycle belted to a dynamo, so that the external muscular work was used to generate an electric current which was measured and afterwards transformed into heat within the calorimeter by passage through a resistance, the heat being measured with that given off by the body. The diet was generally not far from sufficient to maintain nitrogen and carbon equilibrium. In these experiments the energy actually measured, in the form of heat given off from the body and that from the external muscular work done in work experiments, accord on the average very closely with the estimated potential energy of the material oxidized in the body. It is to be observed, however, that the variations for individual days, and in the averages for the individual experiments as well, were considerable, sometimes amounting to 6 per cent., or even more. This is perhaps not strange in view of the physiological conditions of the experiments. In the average of all the experiments the measured energy of expenditure was 99'8 per cent, of the estimated energy of income ; in other words, they agreed within two parts in one thousand. 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 been given off from the body in other forms than heat and external muscular work. Thus it is conceivable that intellectual activity may involve a transformation of physical energy, and that this energy 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 these results that the metabolism of energy in the body occurred in conformity with the law of the conservation of energy. 3. Digestibility and Availability of Food.—The value of a food for nutriment depends not only upon the kinds and amounts of nutrients which it contains, but also upon the proportions of these nutrients which the body can digest and make available, and upon the ease and convenience with which the materials are utilized. By digestion in the narrower sense is understood the changes, chiefly chemical, but partly physical, which the food undergoes in the alimentary canal in order to prepare it for absorption. By digestibility of food, as the term is popularly understood, several things are, or may be, meant. Some of these (as the ease with which a given food material is digested, the time required for the process, the influence of different methods of preparation, including cooking, and the influence of condiments) are so difficult of measurement, and others (as bodily conditions, and the question as to whether a given food agrees or disagrees with a given person) are so dependent upon the peculiarities of different individuals, as to make it impossible to lay down hard and fast rules. The term digestibility is also applied to the proportion of the several kinds of nutrients which the body can digest from different kinds of food materials. These proportions are best found by actual experiment in which 2 In Russian. Cited in Bulletin No. 45 of the Office of Experiment Stations, U.S. Department of Agriculture: “A Digest of Metabolism Experiments,” by W. 0. Atwater and C. F. Langworthy. 3 4 Archiv. de biol. norm, et path., 1898, 4. Office of Experiment Stations, U.S. Department of Agriculture, Bulletin No. 63, “Description of a New Respiration Calorimeter,” by W. O. Atwater and E. B. Rosa; and Bulletin No. 69, “ Experiments on the Metabolism of Matter and Energy in the Human Body,” by W. 0. Atwater and F. G. Benedict.
