THE METABOLISM OF CARBOHYDRATES 913 



diabetes. The radical defect is the inability, relative or complete, of 

 the organism to assimilate carbohydrate. We may find all grades 

 between such cases and those in which there is still a considerable 

 power of assimilation. In order to determine the grade of the dis- 

 order it is usual to give a test diet with a certain proportion of carbo- 

 hydrate, e.g. 100 grm. of bread with meat, bacon, eggs, butter, green 

 vegetables, cheese, lettuce, coffee and wine. If the urine remains free 

 from sugar on this diet, the diabetes is mild in character. More bread 

 may then be added to the diet from time to time until sugar appears 

 in the urine and the limit of tolerance for carbohydrate has been 

 reached. In many cases the sugar will disappear from the urine on 

 the administration of a diet consisting entirely of proteins and fats. 

 When this has been effected carbohydrates may be added in small 

 proportions to the diet until the limit is found at which the assimilatory 

 powers of the patient are reached. It seems that administration of 

 any carbohydrate in excess of this limit is of disadvantage to the 

 patient and hastens the progress of his disorder. When the power of 

 assimilating carbohydrates is entirely abolished the prognosis is almost 

 absolutely fatal. This point may be determined in two ways. In 

 the first place, a patient with no power of carbohydrate assimilation 

 will continue to excrete sugar in the urine on a pure protein fat diet, 

 and the D : N ratio will be 2-8 or higher. Information may also be 

 obtained from a study of his respiratory quotient. The production 

 of dextrose from protein involves the absorption of oxygen. Oxygen 

 will therefore be taken in which will not reappear as carbon dioxide 

 in the expired air. In severe cases of diabetes therefore the respira- 

 tory quotient will fall below that representing fat metabolism, i.e. 

 below 0-7. In most cases of diabetes, where there is still some power 

 of assimilating carbohydrate and of storing up glycogen, the respiratory 

 quotient will be found approximately normal. A very low respiratory 

 quotient is a sign of the severity of the disorder. 



This study of the conditions of carbohydrate metabolism shows 

 how all three classes of food-stuffs co-operate in the maintenance of the 

 chemical processes which lie at the root of the existence and the 

 activities of living organisms. We see how fallacious were the ideas 

 that the proteins alone were necessary for life and that protoplasm 

 was simply living protein. Protoplasm, i.e. the material substrate of 

 life, must be regarded as a complex in which proteins, fats, carbo- 

 hydrates, nucleins, salts, and water all play a part and of which each 

 is an essential constituent. In the higher animals proteins are neces- 

 sary to furnish the proteins of the tissues, and the food must contain 

 jifst those amino-acids which are requisite for the building up of the 

 proteins characteristic of each separate tissue. Moreover certain 

 groups of the protein molecule appear to be destined to serve as 



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