THE SIGNIFICANCE OF THE FOOD-STUFFS 721 



be replaced by either of the other food-stuffs. With ivuanl to its 

 second fate, viz. the furnishing of <MI<TU\ to the body, protein stands 

 on the same level as carbohydrates or fats, its relative value as 

 compared with these two classes of substances being represented 

 by its physiological heat- value. Owing to the inability of the body 

 to store protein to any marked extent, any protein which is absorbed 

 in the alimentary canal in excess of the nutritional requirements of 

 the body is at once broken down and oxidised to satisfy the energy 

 requirements of the cells. The NH and NH 2 groups in the protein 

 molecule add little or nothing to its chemical or potential value. 

 The energy value of the protein depends on its carbon and hydrogen 

 content, and it seems probable that the greater part of the nitrogen 

 is split off as ammonia from the protein molecule during or shortly 

 after its absorption from the alimentary canal. It is on this account 

 that an increased excretion of urea is the almost immediate con- 

 sequence of the ingestion of protein. The point made by Pfluger 

 against Voit, viz. that no oxidation occurs in the lymph or blood, 

 is really beside the mark. The living cell is a complex system which 

 may include food in all degrees of oxidation or chemical change, 

 without this food material necessarily forming part of the living 

 framework. Every histologist distinguishes the paraplasma from 

 the more active and essential protoplasm, and we must assume 

 that it is in the ultra-microscopic interstices of the foam-like 

 protoplasm that the chief processes of chemical change and oxidation 

 occur. A proof therefore that oxidation depends on the condition of 

 the cells and not on that of the blood does not justify the conclusion 

 that the whole nitrogenous metabolism of the body is confined to 

 the living protoplasm itself. 



Folin has lately brought forward a number of facts which point 

 not only to a twofold origin of the nitrogen of the urine but also 

 to a qualitative difference in the two orders of protein metabolism. 

 Whereas in the urine of man on a normal diet the urea nitrogen 

 forms 85 per cent, or more of the total nitrogen, a reduction of the 

 protein ration to the minimum necessary to meet the nutritional 

 requirements of the body causes not only an absolute diminution 

 of the urea but a large relative diminution when compared with the 

 other constituents of the urine, such as creatinin. He concludes 

 therefore that the nitrogenous end-products of nutritional meta- 

 bolism are different from those of the energy metabolism. As 

 Speck has pointed out, there is also a difference in the time-relations 

 of the two orders of metabolism. Whereas the nitrogen, which 

 furnishes no energy to the body, is rapidly eliminated when protein 

 is being utilised for the supply of energy to the body, the occurrence 

 of increased tissue waste causes a rise of nitrogenous excretion, 



46 



