NUTRITION 231 



and Osborne, the idea that different proteins may have very 

 different values in nutrition. They therefore fed rats and 

 mice on diets of abundant fuel value and containing all the 

 non-protein constituents of the diet, but containing only one 

 protein. It was found that some proteins failed entirely to 

 nourish the animal, as shown by the steady loss of weight ; 

 others would keep an adult animal in good condition with 

 no loss of weight, but did not provide the material for the 

 normal growth of young animals ; other proteins not only 

 maintained the normal weight of the adult but a young 

 animal fed on them would grow in a perfectly normal man- 

 ner. We must therefore distinguish between (1) proteins 

 which may provide for both growth and maintenance, (2) pro- 

 teins which will provide for maintenance but not for growth, 

 and (3) proteins which will provide for neither maintenance 

 nor growth. 



Further study showed that the nutritional limitations of 

 the last two classes of proteins are due to the fact that they 

 are lacking in certain amino-acids or else contain them in 

 very small amounts; for if these amino-acids were added to 

 the diet, growth and maintenance became normal. The rea- 

 son for this becomes clear on the assumption already made 

 in our discussions of digestion and nutrition, that the value 

 of protein as food lies in the fact that it yields a great 

 variety of amino-acids, each necessary to some chemical 

 manufacturing process of the living cell. 



18. The value of the mixed diet. As a matter of fact no 

 one tries to live on a single protein. Meat contains at least 

 two ; eggs, three or more ; milk, two ; the cereals, two or 

 more each. By taking a mixture of these in our food, the 

 deficiency of one protein in amino-acids is made up by the 

 excess of the same acid in another. For this reason we can 

 completely meet the protein needs of the body on a mixed 

 diet with a far smaller total intake of protein than if the 

 diet contained only one protein. 



