606 METABOLISM 



diminish the protein breakdown during starvation by giving carbo- 

 hydrates. By using protein sparers we can indeed reduce the daily nitro- 

 gen excretion to about one-third its amount in complete starvation. Re- 

 moval of carbohydrate from the diet is said to entail a failure of the 

 muscles to use again in their metabolism certain of the products (e. g., 

 creatine) which result from their disintegration. At any rate it has 

 been found that creatine is excreted in the urine under these conditions. 



As to the nature of the processes occurring in the body during protein 

 sparing, two possibilities have to be borne in mind: either the body pro- 

 tein is catabolized less rapidly or the protein sparer unites with certain 

 of the breakdown products of protein to form new protein. Recent work 

 by Davis, Hall and WTiipple 68 affords strong support to the latter view. 

 These workers investigated the rate of repair of the liver and the curve 

 of urinary nitrogen excretion in dogs after causing destruction of a large 

 part of the liver tissue by chloroform administration. In animals in 

 which about one half of each lobule had been destroyed, only about 50 

 per cent was found to be repaired in nine days, and the urinary nitrogen 

 was higher than the normal starvation level although no food was given ; 

 to other animals to which sugar was given the repair of the liver was 

 complete in nine days and the curve of nitrogen decidedly below that 

 of starvation. These observations open up a new field for the investiga- 

 tion of problems of the growth of new tissue in the adult animal and their 

 prosecution should afford aid in determining the influence of various 

 dietetic conditions on such growth. Davis and Whipple 69 have already 

 published some important observations in this direction and among other 

 things have found that a diet of bread and milk, or one of cooked liver 

 or kidney, causes more rapid repair than one of cooked skeletal muscle. 

 Fats do not accelerate the repair process. 



The Irreducible Protein Minimum. In the case of a man living on an 

 average diet, although the daily nitrogen excretion is about 15 grams, 

 it can be lowered to about 6 grams provided that in place of the protein 

 that has been removed from the diet enough carbohydrate is given to 

 bring the total calories up to the normal daily requirement. If an excess 

 of carbohydrate over the energy requirements is given, the protein may 

 be still further reduced without disturbing the equilibrium. It has been 

 found that it is not the amount of carbohydrate alone that determines 

 the ease with which the irreducible protein minimum can be reached ; the 

 kind of protein itself makes a very great difference. This has been very 

 clearly shown by one investigator, who first of all determined his nitro- 

 gen excretion while living exclusively on starch and sugar, and who then 

 proceeded to see how little of different kinds of protein he had to take 

 in order to bring himself into nitrogenous equilibrium. He. found that 



