572 METABOLISM 



diabetic patients, in whom the power to use carbohydrates is greatly 

 depressed). The marked protein-sparing action of carbohydrates is il- 

 lustrated in another way — namely, by the fact that we can greatly 

 diminish the protein breakdown during starvation by giving carbo- 

 hydrates, in this way we can indeed reduce the daily nitrogen excre- 

 tion to about one-third its amount in complete starvation. Carbohy- 

 drate starvation 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 when no carbohydrates are available. 



In the case of man living on an average diet, although the daily nitro- 

 gen excretion is about 1.5 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 with- 

 out 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 nitrogen 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 him- 

 self into nitrogenous equilibrium. He found that he had to take the 

 following amounts: 30 gm. meat protein, 31 gm. milk protein, 34 gm. 

 rice protein, 38 gm. potato protein, 54 gm. bean protein, 76 gm. bread 

 protein, and 102 gm. Indian-corn protein. The organism is evidently 

 able to satisfy its protein demands much more readily with meat than 

 with vegetable proteins. 



This variability in the food value of different proteins depends on their 

 ultimate structure — that is, on the proportion and manner of linkage 

 of the various amino acids that go to build up the molecule. In no two 

 proteins are these building stones, as they are called, present in exactly 

 the same proportions, some proteins having a preponderance of one or 

 more and an absence of others, just as in a row of houses there may be 

 no two that are exactly alike, although for all of them the same build- 

 ing materials were available. Albumin and globulin are the most im- 

 portant proteins of blood and tissues, so that the food must contain the 

 necessary units for their construction. M' il fails in this regard, even to 

 the extent of lacking only one (if the units, the organism will either be 

 unable to construct that protein, and will therefore suffer from partial 

 starvation, or it will have to construct for itself this missing unit. It 



