78 THE UTILIZATION OF FOOD 



stuffs are present in abundance. The digestive enzyms 

 and other manufactured products of living protoplasm 

 are for the most part of protein composition, and so re- 

 quire amino-acids for their production. One could, if 

 necessary, nourish himself indefinitely on a diet consist- 

 ing of proteins, as the Eskimos frequently have to do, 

 because proteins can serve both as tissue-formers and as 

 energy-yielders. Carbohydrates and fats are extremely 

 valuable foods because they furnish abundant energy in 

 readily usable form, but by themselves they are in- 

 sufficient because they have no tissue-building capacity. 

 Metabolism in Animals is often Wasteful. — It is 

 a curious and important fact, and one in which animals 

 seem to differ markedly from plants, that in the former 

 much of the metabolism that occurs consumes a great 

 deal more material than is theoretically necessary. For 

 example, we can calculate the energy necessary for doing 

 a certain amount of work, one of the simplest calculations 

 the engineer has to make. We can measure very accu- 

 rately the energy yielded by the burning of a given 

 amount of food material. But when that food material 

 is burned in the body in connection with the doing of 

 a given amount of work, we find that much more food 

 is burned than is necessary to account for the work that 

 is done. It is a familiar principle of engineering that 

 the energy consumed in doing a particular job is greater 

 than the energy that can be recovered in the doing of the 

 work. Since we have the law known to physics as the 

 law of the conservation of energy, by which we mean 

 that the total amount of energy neither increases or de- 

 creases — although it may be changed from one form 

 into another — this energy which fails to appear in the 

 doing of the work must appear in some other form; and 

 the form in which it appears is always heat. This law 

 applies as well to the energy transformations of living 

 things as to the energy transformations going on in dead 

 machines. Since we find that we do not get as much 



