alimentary energetics. 139 



§ 4. Food considered exclusively as 

 Source of Heat. 



We have seen that food is, in the first place, a • 

 s,ource oi chejnical energy ; and, in the second place, 

 a source o{ 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 role 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 environ- 

 ment 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 tem- 

 perature which is not distinguishable from that of 

 the environment, we see less clearly the thermal role 

 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 in- 

 troduce 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 pro- 

 duction of heat. They take the effect for the 



