CHAPTER I. 



DIGESTION AND ABSORPTION OF PROTEIN. 

 General Methods of Investigation. 



THE elucidation of the problems of protein metabolism has been 

 attempted in various ways. The usual and most commonly employed 

 method is that of studying the output of nitrogen in the urine, parti- 

 cularly the variations which occur in the partition of the different nitro- 

 genous constituents. Since the careful and painstaking work of Folin 

 (129) and the publication of his analyses of the composition of the 

 normal urine upon different diets the conclusions have assumed an ac- 

 curacy and weight which previously were lacking. 



The variations in the output of nitrogen have been investigated 

 under different conditions, thus (i) during complete starvation, (2) after 

 definite alterations in the amount of one or more constituents present in 

 the diet, as, for example, the production of nitrogen hunger on a diet 

 which supplies the requisite number of calories, but which is lacking in 

 protein or other substances containing nitrogen, (3) a diet which, although 

 rich in nitrogen, lacks either carbohydrate or fat, (4) a diet in which the 

 normal protein of the food is replaced by some nitrogen-containing 

 substitute, i.e. by the addition of cleavage products of protein formed 

 either by the action of a ferment, or an acid. 



Observations have also been made when the protein has been intro- 

 duced parenterally by intravenous, intraperitoneal or subcutaneous in- 

 jection. The influence of various extraneous factors like work has also 

 been investigated. 



The problem has again been attacked in another fashion by means 

 of perfusion of different organs and tissues with substances of known 

 constitution or which give a well-defined reaction ; thus, for example, 

 repeated experiments have been carried out on the intestine and the 

 liver in an attempt to discover the fate of the products of digestion 

 after they leave the lumen of the intestine. 



The subject has also been investigated by studying the fate of 



