THE FOOD AND DIGESTION 375 



retained, and weight is gained. As already indicated, the 

 power of storing proteids is generally small. 



Proteid Diet. Proteids contain all the chemical elements 

 required for the building and repair of the tissues, and from 

 the complexity of their molecules they also supply latent 

 energy. It is therefore theoretically possible for an animal 

 to sustain life on proteids, and certain animals can be fed 

 exclusively upon them. Thus Pfliiger kept a dog for many 

 months upon a purely proteid diet without injury to its 

 health. But to supply the necessary energy in proteids 

 alone requires the consumption of excessively large quanti- 

 ties. For a man to get the energy equivalent to 3000 Calories 

 a very moderate expenditure per diem he would have to 

 eat more than seven times the usual amount of proteid. 

 Further, it has been shown that, when large quantities are 

 taken, a portion is broken up in the intestinal wall and 

 formed into urea by the liver and excreted by the kidney, 

 and thus excessive work is thrown upon these excretory 

 organs. While these organs usually form and excrete 

 about 33 grms. of urea per diem, on such a diet they would 

 have to deal with no less than 231 grms. 



It is therefore not advantageous to adopt a too purely 5 

 proteid diet. The great use of proteids is as muscle-builders. 

 When the muscles are in a state of constant activity they 

 have a certain power of laying on proteid as they grow. 

 Hence the value of proteids in muscular training. 



Gelatin, although undergoing digestion and absorption like 

 the proteids, is not available as a muscle-builder. Its sole 

 use is as an energy yielder, and in this respect it has a value 

 equal to the proteids. 



Carbohydrate Diet. Carbohydrates are of equal value 

 with proteids as a source of energy, but they contain no 

 nitrogen, and they are not available for building up and 

 repairing the protoplasm of muscles and other tissues. Car- 

 bohydrates alone will not support life, but when added to 

 proteids they enable the animal to do with smaller quantities 

 of the latter. They are thus sometimes termed proteid 



