§ 4. Food considered exclsively as Source of Heat.
We have seen that food is, in the first place, a
source of chemical energy; and, in the second place,
a source of vital energy—finally, and consequently, a
source of thermal energy. It is this last point of
view which has exclusively struck the attention of
certain physiologists, and hence has arisen a peculiar
manner of conceiving the rôle of food. It consists in
looking on food as a source of thermal energy.
This conception is easily applied to warm-blooded animals, but to them exclusively—and this is where it first fails. The animal is warmer than the environment in general. It is constantly giving out heat to it. To repair this loss of heat it takes in food in exact proportion to the loss it sustains. When it is a question of cold-blooded vertebrates, which live in water and in most cases have an internal temperature which is not distinguishable from that of the environment, we see less clearly the thermal rôle of food. It seems then that the production of heat is an episodic phenomenon, not existing for itself.
However that may be, food is in the second place a source of thermal energy for the organism. Can it be said, inversely, that every substance which we introduce into the economy, and which is there broken up and gives off heat, is a food? This is a moot point. We dealt just now with purely thermogenic foods. However, most physiologists are inclined to give a positive answer. In their eyes the idea of food cannot be considered apart from the fact of the production of heat. They take the effect for the
