156 NUTRITIONAL PHYSIOLOGY 



sion of these facts will be undertaken in the chapter on The 

 Hygiene of Nutrition (Chapter XXII). 



Folin, of the Harvard Medical School, has classified the 

 facts of protein metabolism in a particularly clear and 

 helpful form. He distinguishes two lines of transforma- 

 tion, the endogenous and the exogenous. Under the head 

 of endogenous metabolism he traces the various steps in 

 the history of those building-stones which are erected into 

 the proteins of the blood and other tissues. The narrative 

 is continued in the account of the rather gradual and steady 

 crumbling which such tissue-proteins undergo. What is 

 loosely called the wear and tear of the cells gives rise to 

 definite end-products, one of which Folin holds to be of 

 particular value in estimating the extent of such decompo- 

 sition. This is the substance creatinin, which accompanies 

 urea out of the body and which is far less subject to fluctua- 

 tion. The urea excreted rises and falls with the protein 

 ration, but the creatinin is not markedly responsive to 

 dietetic variations. It is believed, therefore, that endog- 

 enous metabolism, a necessary feature of animal life, is 

 relatively independent of feeding, at least while nutrition 

 is satisfactory. 



Exogenous metabolism is an expression to cover all the 

 reactions affecting the uncombined amino-acids. Hence 

 it includes the formation of urea, dextrose, and whatever 

 substances may be formed from the nitrogenous cleavage 

 products in the liver. It may be extended to take in the 

 secondary production of glycogen or of fat from surplus 

 sugar originating in this way. In contrast with endogen- 

 ous metabolism its amount varies widely. With a low 

 protein diet the exogenous changes will be but a fraction 

 of what they will become with abundant protein. One 

 may be tempted to conclude that in fasting the metabolism 

 will be wholly endogenous. The insight of a German writer 

 has served to show us that this is not so. We have spoken 

 at length of the assembling of amino-acids fresh from the 

 intestine to form the standard proteins of the blood. Now 

 there are differences of constitution between the blood- 



