Food may be considered as a store-house of latent or potential energy.
[Sidenote: Intake and outgo of energy accurately determined]
Because of the law, the conservation of energy, which shows that no
energy in the universe can be lost, it is possible to study, with great
accuracy, the energy produced in, and given off by, the human body.
The method by which energy is measured in accurate scientific
experiments is by means of a device called the respiratory calorimeter.
[Sidenote: How energy is measured]
[Sidenote: Energy yielded from one gram each of proteids, carbohydrates
and fats]
This device is a small room, the walls of which are impervious to the
transmission of both heat and air. In this room a man or an animal may
be kept for a period of several days. The air breathed, the food eaten,
the body-heat given off, the waste-products excreted, and the mechanical
work done, are all measured with the greatest scientific accuracy. Many
interesting results have been obtained from the investigations conducted
with this wonderful scientific device. These experiments will not be
given in detail in this work, but it might be remarked that experiments
within the respiratory calorimeter have proved absolutely that the law
of "the conservation of energy" works in the human body in the same
manner as in the scientist's laboratory. Moreover, such experiments
have confirmed the results of the oxidation of various foods in the
laboratory, and have given us data from which to compute the stored
energy in various food substances. It has thus been found that the
amount of energy yielded to the body from one gram of proteid is 4.1
calories, and from one gram of carbohydrates 4.1 calories, while one
gram of fat oxidized in the body yields 9.3 calories, which is more than
twice that yielded by the proteids and the carbohydrates.
[Sidenote: Simple method of finding number of calories in any food]
Since it has been proved that the laws established in the laboratory
also apply to the human body, it is not necessary to conduct expensive
experiments upon the human subject in order to ascertain the amount of
energy in some new food. The food may be analyzed chemically, and the
energy computed according to the above figures, or a sample of the food
may be burned with an oxidizing agent in the laboratory, and the heat
measured. This latter process consists simply of oxidizing a gram of
the food in a closed steel cylinder which is immersed in a known amount
of water at a known temperature. The increase in the temperature of the
water, multiplied by the weight of the water in grams, gives the number
of calories contained in the substance tested.