THE FOOD AND DIGESTION 315 



required are also contained in these substances; and, as 

 will be presently shown, they have a considerable energy 

 value. Hence Proteids form THE essential organic con- 

 stituent of the food. Theoretically it should be perfectly 

 possible for an animal to live on proteids alone, with a 

 suitable addition of water and salts. 



In estimating the actual energy value of proteids in the 

 body a difficulty arises in the fact that, instead of being 

 decomposed to C0 2 , H 9 0, NH 3 , S0 3 , as they are during com- 

 bustion, in the body the nitrogenous part is not broken down 

 further than urea CON 2 H 4 , 3 grms. of proteid yielding 1 

 grm. of urea. If the energy value of the complete combus- 

 tion of a definite amount of proteids is first ascertained, and 

 then the energy value of the amount of urea derived from 

 the same amount of proteid is determined, by subtract- 

 ing the latter from the former, the energy value of proteids 

 in their decomposition in the body is found (see p. 318). 

 The combustion of 1 grm. of proteid to urea yields 4*1 

 Calories of Energy. 



2. Modified Proteids (Albuminoids). In studying the 

 chemistry of the formed material of the various protecting, 

 supporting, and connecting tissues, these substances have 

 been considered (p. 29). 



Keratin, elastin, and mucin seem to be of no importance 

 as articles of food. If taken in the food they pass through 

 the alimentary canal practically unchanged. 



While raw collagen seems also to be of little use, gelatin, 

 formed by boiling collagen, has a certain value. Although 

 it cannot take the place of proteids, because it cannot be 

 used for building up the living tissues of the body, it is 

 nevertheless decomposed into urea, and in decomposing it 

 yields the same amount of energy as the proteids. It has, 

 therefore, a definite though restricted value as a food 

 stuff. 



3. Fats. The chemistry of the fats has already been con- 

 sidered (p. 31). From the fact that they contain so little 

 oxygen in proportion to their carbon and hydrogen, a large 

 amount of energy is liberated in their combustion, and 

 therefore they have a high energy value as food-stuffs. 



One gramme of fat yields twice as much energy as the 



