THE METABOLISM OF CARBOHYDRATES 



803 



The constant drain of sugar will in time involve a relative carbohydrate 

 starvation of the tissues, which will make good their energy requirements as 

 much as possible at the expense of protein and fat. The administration of 

 meat will diminish the fat metabolism to a certain extent, but since it does 

 not alter the D : N ratio it would appear that the latter does not depend in 

 any way on the quantity of fat undergoing oxidation. This is shown in the 

 following respiration experiment (Mandel and Lusk) on a dog with phloridzin 

 glycosuria, in which the metabolism during starvation and after ingestion of 

 meat was determined : 



The enormous waste of energy involved in such a constant loss of sugar 

 will be apparent if we consider that a D : N ration of 3-65 means that 52-5 

 per cent, of the energy in the protein taken as food or set free from the tissues 

 is lost to the organism in the form of glucose. According to Rubner 28-5 of 

 the energy of meat protein is not utilised in the body, but is liberated simply 

 as heat. This stimulating effect of protein on metabolism or on the processes 

 of oxidation in the body is described by Rubner as the * specific dynamic 

 action ' of proteins. If we accept this view and add this 28-5 per cent, lost 

 as heat to the 52-5 per cent, lost as sugar there would remain a balance of 

 only 19 per cent, actually available for the vital activities of the tissues. 

 It is not to be wondered at that the nitrogenous metabolism may be 

 increased three- to five-fold as a result of the artificial induction of the 

 diabetic condition. 



The carbohydrate starvation has other deleterious effects, since we have 

 evidence that a certain amount of carbohydrate food is a necessary con- 

 dition for both fat and protein metabolism. The necessity of carbohydrate 

 for the assimilation of protein is brought out in an experiment by Cathcart. 

 It has long been known that carbohydrate administration has a sparing effect 

 on protein metabolism. If an animal or man be starved, the nitrogenous 

 output sinks to a certain level and there remains practically stationary. If 

 now pure carbohydrate food be administered sufficient to meet the energy 

 requirements of the animal or man (about 35 calories per kilo), there is at 

 once a rapid drop in the output of nitrogen and therefore in the protein 

 waste of the tissues. Fat has a much slighter or no sparing effect on the 

 nitrogenous metabolism. Indeed in certain experiments by Cathcart the 

 administration of fat caused an actual rise in the nitrogenous output. 



The importance of carbohydrates is borne out by the results of feeding 

 animals with proteins which have been digested with pancreatic juice until 

 the biuret reaction has disappeared. After Loewi had shown that it was 

 possible to maintain nitrogenous equilibrium in dogs with such a digest. 



