788 NUTRITION AND HEAT REGULATION. 



ucts, but remains in the more or less complex form indicated by the 

 term polypeptid. This portion may serve as a nucleus for the 

 reconstruction of a body proteid suitable for assimilation into the 

 living structure of the cells. On the other hand, it is known that 

 some of. the split products of the digestion of proteid, the am- 

 monia, the leucin, etc., when circulated through the liver, give 

 rise to urea. Since the split products of proteid digestion are car- 

 ried at once to the liver, it is possible that this fate overtakes them, 

 and that the nitrogen contained in them is at once converted to urea 

 and prepared for elimination, while a portion of the rest of the mole- 

 cules from which the nitrogen is thus removed is retained in the 

 body to be subsequently oxidized and furnish heat energy. This 

 non-nitrogenous residue may first be converted to sugar or fat before 

 its final oxidation. The characteristic feature of this view is the 

 belief that a large part of the nitrogen of the proteid food is promptly 

 converted to urea and is eliminated before becoming a part either 

 of the living proteid or the circulating proteid of the body. This 

 view seems to be opposed to our conceptions of the importance of 

 proteid foods, but it is in harmony with the surprising fact that the 

 digestive enzymes are adapted to split the proteid molecule into 

 what we may call its ultimate products, the relatively simple amido- 

 bodies, and moreover that most of the proteid food taken into our 

 bodies reappears, so far as its nitrogen is concerned, in a few hours 

 as urea in the urine. 



Folin* has called attention to the fact that the proportions 

 of the different nitrogen compounds in the urine vary with the 

 amount of proteid food, Upon an average diet containing 16 to 

 17 gms. of nitrogen (100 to 106 gms. of usable proteid) the urea 

 forms 87 to 88 per cent, of the total nitrogen of the urine, while when 

 the proteid intake is reduced to 3 or 4 gms. of nitrogen the urea forms 

 only 61 to 62 per cent, of the total nitrogen of the urine. On the 

 other hand, the creatinin and the purin bodies (uric acid, xanthin, 

 etc.) are not diminished in amount with a decrease in the proteid 

 food. He suggests, therefore, that the latter bodies, creatinin and 

 purin bases and perhaps a part of the other nitrogenous waste prod- 

 ucts, represent the waste of the breaking down of the living tissues, 

 the catabolism or wear and tear of the living machinery. The urea, 

 on the other hand, represents in large part the nitrogen of that por- 

 tion of the proteid food which, from the present point of view, is 

 hydrolyzed during digestion into the split products and is changed 

 to urea in the liver. 



The Amount of Proteid Necessary for Normal Nutrition 

 Luxus Consumption. As was stated above, nitrogen equilibrium 

 may be maintained on different amounts of proteid food. It is 



* Folin, "American Journal of Physiology," 13, 45, 66, and 117, 1905. 



