330 HOW DOES MAN DETERMINE THE VALUE OF FOOD? 



type of protein, because it does not contain all the amino acids 

 necessary for tissue building or tissue repair. On the other hand, 



Professor Hopkins showed the value of milk as a good food. Explain his experiment and re- 

 sults as given in the above diagram. 



the proteins in milk contain the amino acids necessary for growth. 

 It is estimated that there are eighteen of these amino acids com- 

 monly found in proteins, and that all of those essential for growth 

 are found in lean meat, cheese, milk, eggs, and a few grains and 

 nuts. So it happens that probably most of us, without realizing, 

 have used for food the proteins containing all the essential amino 

 acids. 



Vitamins. These health-regulating substances must be a part 

 of every diet. We know that they occur in milk and in certain 



vegetables. We shall 

 learn more about 

 them in the second 

 problem. 



The value of water. 

 It has long been 

 known that water and 

 the various mineral 

 salts it contains are 

 essential to life. The human body, by weight, is about two thirds 

 water. Over 80 per cent of the blood is water. Water is abso- 

 lutely essential in passing off the wastes of the body. Water 



There are eighteen different amino acids. Where any num- 

 ber of these are bound up chemically they form a protein. 



