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Science has proved that the human body is composed of certain chemical elements and that food materials are combinations of like elements; it has likewise proved that the body will utilize her own structure for fuel to carry on the work of her various functions unless material is supplied for this purpose from an outside source, namely, food, which in chemical composition so closely resembles that of the human body.

Amount and Type of Food.—The next point of investigation would logically be the amount and kind of food necessary to best accomplish this purpose. To be able to do this it was necessary to have some standard unit by which to measure the amount of heat each food was capable of producing when burned outside the body, after which it was more or less simple to calculate the heat production of each of the food combinations within the organism. An apparatus known as the “Bomb Calorimeter”[15] was devised by Berthelot, and adapted for the examination of food materials by Atwater and Blakesley. The food material to be tested was placed within the bomb, which was charged with a known amount of pure oxygen. The bomb was then sealed and immersed in a weighed amount of pure water, into which a very delicate thermometer was inserted. The food within the bomb was ignited by means of an electric fuse, and the heat given off by the burning of the material was communicated to the surrounding water and was registered upon the thermometer. It was evident that some definite name had to be devised by which these heat units might be known. Hence the name “calorie,” which represents the amount of heat required to raise the temperature of 1 kilogram of pure water 1 degree centigrade, or about 4 pounds of water 1 degree Fahrenheit.

Transformation of Foods into Available Fuel.—A comparison has been made between the human body and steam engine, but this comparison is not adequate, since the food does not produce heat within the body originally, but energy of which heat is a by-product. Each food combination has a certain amount of dormant energy within its structure and this energy does not become active nor can it be utilized by the body until the food, of which it is a part, is changed within the organism to substances more nearly like its own. This liberated active energy is then used as a motive power to carry on the internal and external work of the body, and the heat, which is invariably the consequence of any active energy (motion), leaves the body as such. It will be seen, then, that the human body acts not as a steam engine, but rather as a transforming machine by means of which the dormant energy of the food is transformed into an active agent of which heat is a natural result.

In the calorimeter it was found that the carbohydrates and fats burned to the same end products, namely, carbon dioxide and water, while the proteins, upon oxidation, produced carbon dioxide, water and nitrogen gas. In the body it was found that the carbohydrates and the fats acted in exactly the same manner as in the calorimeter, producing the same end products. But this was not the case with the proteins; the oxidation process of this chemical combination was found to be not nearly so complete within the body as in the calorimeter, and instead of the free nitrogen as produced in the apparatus there were urea and other nitrogenous substances eliminated which, while combustible, represented a less complete oxidation of the proteins.

The following table represents the amount of heat produced as the result of a complete oxidation of the foodstuffs in the calorimeter.

TABLE[16]

Carbohydrates	4.10 cal. per gram
Fats	9.45 cal. per gram
Protein (nitrogen × 6.25)	5.65 cal. per gram

The loss of potential energy due to the incomplete oxidation of the proteins in the body is approximately 1.3 calories to each gram of protein in food; consequently in calculating the fuel value of protein foods, due allowance must be made for these losses. Allowance must also be made for the incomplete digestion, or losses occurring in the digestion, of the foodstuffs. These losses, as well as the approximate amount of each constituent absorbed, are represented in the following table.[17]

	Lost	Absorbed
Carbohydrates	2 per cent.	98 per cent.
Fats	5 per cent.	95 per cent.
Proteins	8 per cent.	92 per cent.

The physiological fuel factors of food, or the amount of heat produced as the result of combustion of 1 gram of organic food material after the above-mentioned losses have been accounted for, may be obtained as follows.[18]

Carbohydrates	4.10 × 98% = 4 cal. per gram
Fats	9.45 × 95% = 9 cal. per gram
Proteins	4.35 × 92% = 4 cal. per gram