CH. XLI.] METABOLISM OF PROTEIN 623 



changes; both are concerned in the life of the organism; but one 

 more directly than the other. When we now speak of endogenous 

 protein metabolism we refer to that in the material highly endowed 

 with life; when we apply the term exogenous protein metabolism to 

 the changes by which the liver brings about the conversion of amino- 

 acids from the food into urea, we refer to its action on intercalated 

 material, and no longer use the phrase " circulating protein." 



We have already discovered in our study of the urine, that 

 exogenous protein katabolism is mainly represented in the urine by 

 urea and inorganic sulphates; while the final katabolites of endo- 

 genous metabolism are substances like creatinine and " neutral 

 sulphur"; but there is no doubt that some urea is formed also: this 

 is seen, for instance, during starvation. 



Let us consider a man taking the customary Voit dietary of 

 16 or 17 grammes of nitrogen in his daily food; probably only a 

 quarter or even less of this is destined for endogenous use, and the 

 protein sufficient to maintain this is indispensable. Would it be 

 possible to dispense entirely with the amount which is exogenously 

 metabolised, and reduce our protein intake to the low level of, say, 

 4 grammes of nitrogen per diem. The old observations on starving 

 animals we have just referred to shows that this would not be 

 possible; the minimum is not the optimum; and even Chittenden 

 (see p. 480) does not recommend a reduction lower than 7 or 8 daily 

 grammes of nitrogen. If an animal cell is presented only with 

 protein food, it takes and uses it eagerly, even although it may not 

 ultimately build much of it into protoplasm. If substances such as 

 fat, carbohydrate, or the incomplete protein we call gelatin, is' pre- 

 sented to it also, the amount of protein necessary is reduced, and 

 so we speak of such foods as being "protein -sparing." 



The important character of Chittenden's work has given the 

 faddists on matters of diet an important opportunity of being 

 listened to. There is, for instance, a group of these to whom 

 the very necessary act of chewing has assumed almost the nature 

 of a religious ceremony, and they have sought to convince mankind 

 of its superlative importance. These, however, need not concern 

 us, but there are some even in the scientific world who seem almost 

 to believe that the law of conservation of energy does not apply to 

 the chemical changes in a living animal. They cite instances of 

 people who do a large amount of work, and do it upon what most 

 would regard as an insufficient diet, without detriment or loss of 

 body-weight. If a man only receives food in the day of the energy 

 value say of 1500 large calories, and the heat he produces and the 

 work he does are equivalent to 2000 ; then the additional 500 

 must have come from his internal resources, and he must have used 

 up some of the material formerly stored in his body. This is as 



