FOODS AND THE THEORY UF DIGESTION 119 



in different grains. Again, certain vegetables, as beans, 

 peas, and peanuts, are rich in a kind of proteid which is 

 called legnmen. 



Proteids are compounds of carbon, hydrogen, oxygen, 

 nitrogen, and a small per cent of sulphur. Certain ones 

 (the nucleo-proteids from grains) also contain phosphorus. 

 All of the proteids are highly complex compounds and 

 form a most important class of nutrients. 



Purposes of Proteids. The chief purpose of proteids 

 in the body is to rebuild the tissues. Not only do they 

 supply all of the- main elements in the tissues, but they 

 are of such a nature chemically that they are readily built 

 into the protoplasm. They are absolutely essential to life, 

 no other nutrients being able to take their place. An 

 animal deprived of them exhausts the proteids in its body 

 and then dies. In addition to rebuilding the tissues, 

 proteids may also be oxidized to supply the body with 

 energy. 



Albuminoids form a small class of foods, of minor importance, which 

 are similar to proteids in composition, but differ from them in being 

 unable to rebuild the tissues. Gelatin, a constituent of soup and 

 obtained from bones and connective tissue by boiling, is the best 

 known of the albuminoid foods. On account of the nitrogen which 

 they contain, proteids and albuminoids are often classed together as 

 nitrogenous foods, 



Carbohydrates. While the carbohydrates are not so 

 essential to life as are the proteids, they are of very 

 great value in the body. They are composed of carbon, 

 hydrogen, and oxygen, and are obtained mainly from 

 plants. There are several varieties of carbohydrates, but 

 they are similar in composition. All of those used as 

 food to any great extent are starch and certain kinds 

 of sugar. 



