Fig. 74.—The effect of food on growth. Reprinted from publication of the Carnegie Institution. Courtesy Professor Lafayette B. Mendel.
In Chapter I we learned that the body is a working machine whose first requirement is fuel. Hence the first consideration in the diet is to have the proper amount of fuel for each day, to provide energy for the constant internal work that keeps the body alive, and for the variable external work
which may be so light as to consist of the few movements that one makes lying in bed, or sitting quietly; or so hard as to exercise many muscles, as playing tennis, bicycling, or swimming.
Fig. 75.—Respiration calorimeter, open. From the “Journal of Biological Chemistry.” Courtesy of Professor Graham Lusk.
Energy requirements of adults.—We have also learned something about the foods which supply this energy; we must now find out how much fuel (in the form of food) it takes to do different amounts of work, just as the owner of an automobile wants to know how much gasoline per mile or per hour is required to run his machine under different conditions. Very careful experiments have been made on many men in different ways to measure their energy output,
the most accurate and interesting being those made in a respiration calorimeter, a device so delicate as to be able to measure the extra heat given off when one changes from lying perfectly quiet to sitting up equally still, thus adding the work of holding the upper part of the body upright. A respiration calorimeter large enough to hold a child is shown in Figs. 75 and 76. You can see that it consists of a chamber with thick walls to prevent loss of heat. In Fig. 75 the door is open. When an experiment is going on the door is closed, as in Fig. 76, air being furnished through special tubes. The walls are fitted with delicate thermometers and every device
which will help to get the exact amount of heat given off from the body is employed.
Fig. 76.—Respiration calorimeter, closed. From the “Journal of Biological Chemistry.” Courtesy of Professor Graham Lusk.