CHAPTER XII 



THE METABOLISM OF THE CARBOHYDRATES 



CARBOHYDRATES constitute the greater proportion of our dietary. 

 The approximate composition of the diet of a man of average 

 weight (70-75 kg.) doing a moderate amount of muscular work is, 

 according to Ranke, proteid 100 grm., fat 100 grm., carbohydrate 

 240 grm., and when muscular work is undertaken the amount of 

 carbohydrate is usually much in excess of this, viz. 400-700 grm. 



The greater proportion of this carbohydrate is taken in the 

 form of starches (polysaccharides), some as cellulose, and the 

 remainder as sugar. During digestion all carbohydrates are re- 

 solved, by the various amylolytic and inversive ferments, into 

 simple sugars (monosaccharides), and it is as such that they are 

 absorbed into the portal blood. A study of their metabolism 

 starts from this point i.e. from the moment at which they enter 

 the circulation as simple sugar it then considers what role the 

 sugar plays in the production of energy and in the nutrition of the 

 tissues, and it ends with an account of what becomes of the used-up 

 sugar i.e. in what form or forms it is excreted from the system. 1 



We see, then, that a very large amount of carbohydrate is 

 taken each day in the food. If, however, an examination of 

 the various excreta urine and faeces be made, no trace of sugar 

 under normal conditions will be found in them. The ingested 

 sugar must, therefore, become converted in its passage through 

 the organism into something not sugar, and since it is plainly 

 impossible that any large amount, at least, can be built up into 

 the tissues else would the organism very soon assume enormous 

 dimensions we are driven to the conclusion that it is oxidised, 

 and thus converted into carbon dioxide and water. An examina- 

 tion of the expired air shortly after a carbohydrate-rich meal 



1 Space in this article will not permit of an account of the chemistry of the 

 carbohydrates nor of the chemical processes which they undergo during their 

 digestion. The necessary information on these points will be found in any 

 of the elementary text-books on Physiological Chemistry. 



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