THE ABSORPTION AND USE OF FOODS 449 



the absorption of a meal, or any increase of nitrogen in it, such 

 as would occur if protein derivatives were absorbed. We are 

 therefore compelled to believe that the portal blood, which 

 cannot all be collected and analyzed, since such a procedure 

 would bleed the animal to death, and which can, therefore, only 

 be examined in occasional samples, must receive the absorbed 

 protein. 



The modern ideas as to the way in which the proteins are ab- 

 sorbed have arisen chiefly as the result of a somewhat changed 

 conception of the use of protein in the Body. It will be recalled 

 that protein as a food is supposed to have a twofold function: as 

 a repairer of the waste of living tissues, and as part of the fuel 

 supply of the Body (Chap. XXV). 



It was formerly taken for granted that the wear and tear on 

 the living cells is so great as to require the bulk, if not the whole, 

 of the ordinary protein intake for the restoration of the broken- 

 down tissues. Of recent years, however, physiologists have come 

 more and more to believe that the cells work with comparatively 

 little injury to themselves, and that therefore only a small part of 

 the usual protein intake is required for tissue repair; the rest 

 serving for fuel. 



The fuel value of protein lies, not in its nitrogen content, but 

 in its carbon and hydrogen, since these are the elements which 

 oxidize readily with evolution of heat. It is therefore quite un- 

 necessary that the protein that is to serve as fuel be present in the 

 tissues as protein, having its elements combined into the complex 

 structure of the protein molecule; all that is necessary is to have 

 its carbon and hydrogen in available form for ready oxidation. 

 For tissue repair, however, it seems to be essential that the com- 

 plex protein molecule be preserved. 



The theory of protein absorption that we shall consider here 

 accounts for our inability to demonstrate the presence of ab- 

 sorbed protein in the portal blood on the basis of the possible 

 chemical alteration in the fuel protein suggested above. Accord- 

 ing to this theory the amino acids which are in the intestine as 

 the end products of protein digestion, all undergo chemical al- 

 teration during their passage through the walls of the intestine; 

 those that are destined for tissue repair being rebuilt into the 

 blood proteins; and those that are to serve for fuel being so split 



