MILK AS A FOOD. 



457 



repairing the waste of the tissues ; and second, its value as a 

 source of energy. As a food for infants it is required not only to 

 repair waste of tissues, but actually to build them up. 



Composition of Constituents. The following table gives 

 the percentage composition of the three main constituents of 

 milk : 



TABLE CLIV. 



It is seen that fat is the richest in carbon and hydrogen, protein 

 next, while sugar occupies the lowest place. Neither fat nor 

 sugar can replace proteins, as these compounds form the only 

 source of nitrogen. Fat and sugar being composed of the same 

 three elements may replace each other, but it is evident that 

 in building up tissues containing high percentages of carbon 

 and hydrogen, fat is a far more advantageous food than sugar. 

 As a food for infants the value of milk depends largely on the 

 fat present, and it is doubtful whether fat can be replaced by 

 sugar without detriment to anabolic processes. 



As a food for adults, where the tissues are ready formed, milk 

 may be regarded chiefly as a source of energy. From this point 

 of view fat may be replaced by the iso-dynamic quantity of milk- 

 sugar. 



Heat of Combustion of Constituents. The following 

 values for the heat of combustion of the constituents of milk are 

 due to Strohmer : 



TABLE CLV. 



Fats Butter fat, . 



Other fats, . 



Sugar Milk-sugar, 



Cane-sugar, 



Proteins Casein, . 



Albumin, 



9,231-3 calories per gramme. 



9-500 



3,950 



3,955 



5,858-3 



5,735-2 



These values assume that complete combustion takes place, 

 and that carbon dioxide, water, and nitrogen are produced. 

 In the case of fat and sugar it may be assumed fairly that an 

 approach to complete combustion takes place in the human 

 body, and that carbon dioxide and water are excreted. The 



