46 MILK AND ITS PRODUCTS AS FOODS 



It was but natural that biological students of the problems 

 of nutrition should give their attention to milk and endeavor to 

 ascertain wherein its different constituents excelled the corre- 

 sponding constituents of other foods. Knowing that the young 

 grow and thrive on a diet composed exclusively of milk, they 

 realized that this food must contain nutritive material of an 

 exceptionally high order. 



Proteins 



The proteins of plants differ from those of animals and of each 

 other. Animals do not take the proteins of their foods, whether 

 of plant or animal origin, and use them as such, without any 

 change. In the processes of digestion and assimilation^ animals 

 break proteins up into the simpler substances, amino-acids, and 

 from these build up the proteins of the muscle and other tissues 

 of the body. 



There are eighteen amino-acids that are considered in nutri- 

 tion problems. The protein of wheat (gluten) is able to supply 

 only a limited number of these in sufficient quantity, and some 

 not at all. The proteins of the corn kernel (zein, etc.) are 

 able to supply others, and so on with the proteins of the different 

 foods ; but none of these is a complete protein food for the animal 

 body. The chief protein of milk (casein) is, however, an excep- 

 tion, being a very complete and well-balanced protein, which is 

 able to supply the different amino-acids in sufficient quantities 

 and right proportions to build up muscle and other protein 

 tissue of the body. Thus we would say that milk furnishes pro- 

 tein of a quality quite superior to that of almost any other food. 

 This we see exemplified in the fact that the young animal lives, 

 grows and thrives on milk alone. 



Ash or Mineral Matter 



In the ash or mineral matter of the food there must be a 

 sufficiency of such elements as sodium, potassium, calcium, 

 magnesium, iron, etc., in the form of inorganic salts. These 

 perform very important functions. Each has its work to do, 

 and they are not interchangeable. 



