CHAPTEK XI 

 THE HISTORY OF THE FOOD-STUFFS 



SECTION I 

 PROTEIN METABOLISM 



A PKOTEIN consists of the elements carbon, hydrogen, oxygen, 

 nitrogen, and sulphur. In the oxidation which these bodies, in 

 common with the other food-stuffs, undergo in the body, the carbon 

 and hydrogen are converted to carbon dioxide and water. A certain 

 proportion escapes this complete oxidation, being excreted by the 

 kidneys in combination with nitrogen as the essential constituents 

 of the urine (chiefly urea). When proteins are oxidised in the body 

 there is a definite relation between the carbon dioxide which is pro- 



duced and the oxygen consumed. This respiratory quotient 



C0 2 



2 



in the case of proteins equals 0-81. Since the respiratory quotient on 

 a pure consumption of fats is only 0-71 and on carbohydrates equals 1, 

 we may, by a study of the respiratory exchanges, arrive at some idea 

 of the extent to which protein metabolism is responsible for the 

 energy exchanges of the body as a whole. Such information by 

 itself will always be somewhat uncertain, since it is possible : by a 

 combination of fat and carbohydrate metabolism in proper propor- 

 tions, to produce a respiratory quotient identical with that obtaining 

 when the metabolism is purely of protein. On the other hand, the 

 specific constituents of proteins, namely, nitrogen and sulphur, are 

 excreted entirely by the urine (if we exclude the small traces which 

 may leave the body in the sweat, or as scales of epidermis, hair, nails, 

 &c.). It has therefore been customary to take the nitrogen output 

 in the urine as an index of the protein metabolism. This proceeding 

 is only justified if we remember that we are dealing in the urine 

 merely with the nitrogen and sulphur of the protein molecule, and 

 that a large proportion of the carbon and hydrogen moiety of this 

 molecule is being left unregarded. 



Since we cannot follow all the stages in the changes undergone 

 by proteins on their way through the body, it will be convenient to 

 take the nitrogenous end-products of protein metabolism such as 



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