CHAPTER XI 

 THE HISTORY OF THE FOOD-STUFFS 



SECTION I 

 PROTEIN METABOLISM 



A PROTEIN 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 



CO 



which is produced and the oxygen consumed. This respiratory quotient - 



U 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 proportions, 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 appear in the urine, and to deter- 

 mine, where possible, their precursors, and the conditions which determine 

 their formation in the body. The chief nitrogenous constituents of urine are 

 urea, ammonia, uric acid, creatinine, hippuric acid. There is a small residue 



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