862 PHYSIOLOGY 



amino-acids, provided only that it is supplied with the proper non- 

 nitrogenous acids. These latter need not be derived from proteins 

 at all, but, like lactic acid, be a result of carbohydrate metabolism. 

 Thus, if the fitting non- nitrogenous food be given (e.g. oxy-fatty acids, 

 or carbohydrates, from which these bodies may be formed), part of 

 the nitrogen set free by protein disintegration might be recombined 

 with the formation of amino-fatty acids without giving rise to urea 

 or appearing in any way in the nitrogen balance-sheet of the body. 

 This possibility enjoins the necessity of caution in interpreting the 

 results of metabolism experiments where the nitrogen excreted is 

 taken to represent the total protein metabolism of the body. The 

 fate of the nitrogen does not, however, matter much to the energy 

 balance-sheet of the body, since so far as regards energy the residue 

 of the protein molecule or the amino-acid molecule which is left 

 behind after the process of deamination has taken place, has lost only 

 from one-fifth to one-tenth of the total energy of the original molecule. 

 This is shown in the following Table of the heat equivalents of some of 

 the amino-acids and their corresponding fatty and oxy-acids : 



Calories 

 per grm. molecule 



Leucine ...... 855 



Isobutylacetic acid .... 837 



Alanine 389 



Propionic acid ..... 367 



Lactic acid 329 



Pyruvic acid ..... not determined 



Even in the case of the smallest molecule the loss of energy 

 attendant on simple deamination and conversion into the corresponding 

 oxy-acid only amounts to about 20 per cent. We thus come to the 

 conclusion that the urea output in the urine after a protein meal tells 

 us nothing whatever about the fate of that part of the protein which 

 contains 80 to 90 per cent, of the total energy of the protein food. So 

 far as concerns the output of energy, the exogenous protein metabolism 

 may be regarded as practically non-nitrogenous. The rise in the rate 

 of excretion of urea after a protein meal was regarded both by Voit 

 and Pfliiger as a sign that the cells of the body preferred to use proteins 

 for all their requirements if this substance were available. We see 

 now that the big output of urea after a protein meal affords no basis 

 to this view, but is rather a sign that the body has no need for all the 

 nitrogen contained in its food and that this must be got rid of before 

 the really valuable part, the energy-giving part of the protein molecule, 

 is admitted into the general circulation. 



The important problem in the energy metabolism of protein is thus, 

 not the origin of the urea, but the nature of the substances that are 



