THE METABOLISM OF CARBOHYDRATES 809 



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 respiratory 

 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, carbohydrates, nucleins, salts, and water all play 

 a part and of which each is an essential constituent. In the higher animals 

 proteins are necessary to furnish the proteins of the tissues, and the food 

 must contain just 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 mother- 

 substances of hormones and other chemical compounds which play a 

 dynamic rather than static part in the phenomena of life, and supply 

 conditions of activity rather than material for the production of energy. 

 The carbohydrates not only act as sources of energy, but are necessary to 

 the building up of the proteins into the protoplasmic complex. Without 

 them moreover this complex cannot properly utilise the fat contained in 

 itself or supplied in its food. On the other hand, the carbohydrates by 

 themselves are not available as food, but require some connecting link, 

 which may be protein or nitrogenous in character, to enable their associa- 

 tion with the active part of the protoplasm and their utilisation by oxidation. 

 At the same time there is a certain possibility of interconversion between 

 these different substances ; sugar may be formed from proteins, fats from 

 carbohydrates. On the other hand, the formation of fats from proteins is 

 apparently impossible in the cells of the higher animals, and the evidence 

 for the formation of sugar from fat is limited to the study of the respiratory 

 quotient in hibernating animals. With the exception of a few cases quoted 

 by Pniiger and von Noorden, no support for such a conversion is obtained 

 from the conditions observed in the glycosuria caused by the administration 

 of phloridzin or by extirpation of the pancreas. 



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