FOOD AND DIGESTION* 329 



and sulphur required in the construction and repair of the 

 living tissues are obtained. The carbon and hydrogen required 

 are also contained in these substances; and, as will be pre- 

 sently shown, they have a considerable energy value. Hence 

 Proteins form THE essential organic constituents of the food. 

 Theoretically it should be perfectly possible for an animal 

 to live on proteins alone, with a suitable addition of water 

 and salts. 



In estimating the actual energy value of proteins in the body 

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

 to C0 2 , H 2 0, NH 3 , S0 3 , as they are during combustion in the 

 calorimeter, in the body the nitrogenous part is not broken 

 down further than to urea CO(NH 2 ) 2 , 3 grms. of protein 

 yielding 1 grm. of urea. If the energy value of the com- 

 plete combustion of a definite amount of proteins is first 

 ascertained, and then the energy value of the amount of 

 urea derived from the same amount of protein is determined, 

 by subtracting the latter from the former, the energy value 

 of proteins in their decomposition in the body is found 

 (see p. 331). 



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

 Calories of Energy. 



2. Selero-proteins. In studying the chemistry of the formed 

 material of the various protecting, supporting, and connecting 

 tissues, these substances have been considered (see p. 12). 



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 proteins, because it cannot be used 

 for building up the living tissues of the body, it is neverthe- 

 less decomposed into urea, and in decomposing it yields the 

 same amount of energy as the proteins. 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. 29). 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 there- 

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



