Carbohydrate sparing mechanism

The biochemical mechanism by which carbohydrates spare proteins is not known. Carbohydrate breakdown yields ATP, NADH, and amino acid precursors—all compounds needed for amino acid or protein synthesis, and each of them could alone be responsible for the sparing effect. To assume that in the presence of excess carbohydrates the cell selects carbohydrates rather than protein as an energy source is obviously an oversimplified interpretation of the sparing effect. Yet, glucose and fructose administration reverses urea formation and glycine catabolism. 

It would seem that normally the oxidation of ketone bodies would proceed largely to completion in the liver by the ketolytic mechanism. Whenever the supply of carbohydrates here is sufficiently reduced, appreciable amounts of ketones then escape oxidation and pass into the blood. When the concentration of ketones becomes sufficiently elevated, a ketonuria occurs and also some ketones will be utilized by the tissues. Such a theory would largely limit the ketolysis mechanism to the liver. It would explain the specificity of the sugars in preventing ketonuria and the discrepancy between the amount of D-glucose required to prevent ketosis and the caloric value of the fat spared. It is further supported by the demonstration that the liver is capable of exhibiting ketolysis. 

On the other hand, the situation is now fairly clear with those substances which make an animal independent of dietary sources of at least several of the B. vitamins. The mechanism has been worked out best for starcih, sorbitol, and ascorbic acid, insofar as they spare thiamine, but there is no reason to suppose that the same mechanism does not apply also to the other carbohydrates and allied substances which produce the same effect and to the other vitamins which are spared. 

---