CHEMISTRY OF DIGESTION AND NUTRITION. 



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due to a loss or a gain in body-fat in other words, to changes in-the carbon 

 equilibrium. 



Nutritive Importance of the Proteids. The digestion and absorp- 

 tion of proteids have been considered in previous sections. We believe that 

 the digested proteid is absorbed into the blood in a slightly modified form, with 

 the exception of the variable quantity which suffers decomposition into the 

 simpler amido- compounds while in the intestine as a result of putrefaction or of 

 the prolonged action of trypsin. Subsequently this proteid material passes into 

 the lymph and is brought into contact with the tissues. Its main nutritive 

 importance lies in its relations to the tissues, and, speaking generally, we 

 may say that the final fate of the proteid molecule is that it undergoes a phys- 

 iological oxidation whereby the complex molecule is broken down to form 

 the simpler and more stable compounds CO 2 , H 2 O, and urea. This destruction 

 of the proteid molecule takes place in or under the influence of the living cells, 

 and it gives rise to a liberation of energy mainly in the form of heat. It is 

 impossible to follow the various ways in which this physiological oxidation 

 takes place. It is probable, however, that some of the proteid undergoes de- 

 struction without ever becoming a part, an organized part, of the living cells, 

 although its oxidation is effected through the agency of the cells. It has been 

 proposed by Voit l to designate the proteid which is oxidized in this way as 

 " the circulating albumin or proteid." According to Voit, a well-fed animal 

 has in its lymph and tissues always a certain excess of proteid which is to 

 undergo the fate of the circulating proteid, and this supposition is used to 

 explain the fact that for the first day or so a starving animal metabolizes 

 more proteid, as determined by the nitrogenous excreta, than in the subse- 

 quent days, after the supply of the circulating proteid has been destroyed. 

 A portion of the proteid food, however, before its final destruction is utilized 

 to replace the nitrogenous waste of the tissues ; it is built up into living proto- 

 plasm to supply the place of organized tissue which has undergone disassirni- 

 lation or to furnish new tissue in growing animals. To the proteid which is 

 built up into tissue Voit gives the name of " organeiweiss," the best translation 

 of which, perhaps, is " tissue-proteid." It should be stated that this division 

 of the proteid into circulating proteid and tissue-proteid has been severely 

 criticised by some physiologists, but it has the merit at least of furnishing 

 a simple explanation of some curious facts with regard to the use of proteid 

 in the body. To avoid misunderstanding, it is well to say that the sepa- 

 ration into circulating proteids and tissue-proteids does not mean that the 

 proteid which is absorbed from the alimentary canal is of two varieties. 

 The terms refer to the final fate of the proteid in the body: a certain 

 portion is utilized to replace protoplasmic tissue, and it then becomes "tis- 

 sue-proteid," while the balance is metabolized in various ways and con- 

 stitutes the "circulating proteid." Any given molecule of proteid, as far 

 as is known, may fulfil either function. With regard to the general nutri- 

 tive value of proteids, it has been demonstrated clearly that they are abso- 

 1 Hermann's Handbuch der Physiologic, 1881, vol. vi. p. 300. 



