SECTION IV 

 THE METABOLISM OF CARBOHYDRATES 



ALL the carbohydrates which are taken in with the food are ultimately 

 transformed in the alimentary tract, or in its walls, into the three mono- 

 saccharides, glucose, fructose, and galactose. These three, together with 

 mannose, are the only sugars which are directly fermentable and directly 

 assimilable by higher animals. A consideration of their structural formulae 

 shows that they are fairly easily interconvertible, galactose presenting the 

 greatest divergence from the general type. This conversion actually takes 

 place in watery solution. If a solution of any one be left for some months, 

 it will be found to contain all four at the end. 



Since these monosaccharides, for the greater part glucose, must enter 

 the blood in large quantities during the absorption of a heavy carbohydrate 

 meal, one would expect to find a greater proportion in the blood during 

 such a meal than during a period of starvation. The amount of reducing 

 sugar in the blood, however, is practically constant, and varies between 

 0-1 and 0-15 per cent. 



Searching for the origin of this constant proportion of reducing sugar, 

 Claude Bernard found that the blood of the hepatic vein in a fasting animal 

 contained more sugar than the blood taken at the same time from the portal 

 vein. Although the reliability of this experimental result has been put in 

 doubt by more recent investigators, it was important in that it attracted 

 Bernard's attention to the liver. If the liver be taken from an animal which 

 has been dead for some time, and extracted with water, the extract is found 

 to contain a large quantity of reducing sugar (glucose). If, however, it be 

 removed immediately the animal is dead, its vessels washed out with ice-cold 

 saline fluid, and be then cut up and thrown into boiling water, ground and 

 extracted, the extract, after separation of the coagulable proteins, contains 

 hardly a trace of sugar, and no more than is present in the blood. The 

 fluid is, however, opalescent ; and Bernard found that, this opalescence was 

 due to the presence of a substance at that time new to science, belonging to 

 the class of polysaccharides. This substance he called glycogen, i.e. the 

 sugar-former. 



After a carbohydrate meal, glycogen may be present in very large 

 amounts in the liver, up to 12 per cent, of the weight of the fresh liver. 

 From its solution in water it can be thrown down by the addition of alcohol 

 to 60 per cent When collected and dried, it forms a snow-white powder. 



