GENERAL METHODS HISTORY OF PROTEIN FOOD. 813 



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 protein 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 protein the am- 

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

 rise to urea. Since the splk products of protein 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 (see paragraph below on " The Inter- 

 mediate Stages in Protein Metabolism"), while the rest of the mole- 

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

 body to be subsequently oxidized and to 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 protein food is promptly 

 converted to urea and is eliminated before becoming a part either 

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

 view is in harmony with the fact that the digestive enzymes are 

 adapted to split the protein molecule into what we may call its 

 ultimate products, the relatively simple amino-bodies, and moreover 

 that most of the protein 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 protein food. Upon an average diet containing 16 to 

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

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

 the protein 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 protein 

 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 that portion of the 

 protein food which, from the present point of view, is hydrolyzed 

 during digestion into split products and whose nitrogen is converted 

 to urea in the liver. 



The Amount of Protein Necessary for Normal Nutrition 

 Luxus Consumption. As was stated above, nitrogen equilibrium 

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

 important, from a scientific and from an economic standpoint, to 



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



