130 A. I. RIXGER 



will lose in body weight quite sharply; after adding tryptophan, the curve 

 of body weight becomes more horizontal. For an adult to just maintain 

 his body weight is perfectly normal. But merely maintaining body weight 

 for a child or growing animal is a decided abnormality. They have to 

 grow, and growth dues n;t occur until lysin is added to the diet. 



The records of Rats 1773 and 1900 are corroborative of the first two. 



From all the above data, the conclusion must be reached that the pro- 

 teins in the dietary of all animals fulfill a series of functions which are 

 not fulfilled by any of the other foodstuffs. They supply ammo acids 

 which the body itself cannot manufacture. Tyrosin, tryptophan and lysin 

 are indispensable ammo acids without which nutritional equilibrium can- 

 not be established. Only plant cells have the power of synthesizing these. 



For a protein, therefore, to be physiologically adequate, it must con- 

 tain all of these ami no acids and in sufficient quantities. 



The study of the protein metabolism really resolves itself into a study 

 of the metabolism of the amino acids. When we speak of a minimum 

 protein requirement, we may in reality translate that into a minimum re- 

 quirement of indispensable amino acids and the "wear and tear" quota 

 may really represent that amount of protein which contains all the indis- 

 pensable amiiio acids that are necessary for our maintenance. 



The Influence of Protein on Metabolism 



The Specific Dynamic Action of Protein 



The final stage of all the metabolic processes in the animal body is 

 one of oxidation, whereby energy is. liberated in the form of heat. The 

 amount of heat produced depends entirely upon the amount of material 

 that is oxidized. When an animal is at rest and fasting, the oxidation 

 processes are at a low ebb, the heat production is at a correspondingly low 

 level. (We speak of its l>osal metabolism.) If the subject becomes more 

 active, the oxidative processes and heat production increase in definite 

 proportion, so that by doing fairly hard physical work the metabolism may 

 reach a point double and triple the basal level. 



A most remarkable phenomenon was observed by Voit in his early 

 respiratory metabolism experiments. He found that even though at per- 

 fect physical rest, the heat metabolism of an individual increases after 

 the ingestion of food ; to a slight extent, after carbohydrates, to a greater 

 extent after fat, and to a most marked extent after protein. In other 

 words, if we determine the starvation caloric requirements of an individual, 

 and put him on a protein diet sufficient to cover those requirements, the 

 individual's metabolism will increase as a result of ingesting the food and 

 produce more heat than before. 



