208 NUTRITION 



THE PROTEIN REQUIREMENT— NITROGEN BALANCE 



Protein cannot be considered merely as a source of energy 

 owing to the important part which it plays in maintaining 

 the body structure. The adequacy of the protein supply 

 can be tested by comparing the nitrogen ingested with the 

 nitrogen excreted — the nitrogen balance. This at once 

 shows that proteins have a more complex metabolic history 

 than carbohydrates or fats. In the first place, it is im- 

 possible in the healthy adult to induce a surplus of intake 

 over output of nitrogen merely by feeding with excess of 

 protein. There is no retention of nitrogen except during 

 growth or convalescence. In the second place, reduction 

 of nitrogen intake leads to an excess of output over intake, 

 even though there may be an adequate carbon and there- 

 fore calorie supply. This adverse nitrogen balance is seen 

 in its most extreme form in starvation, when the nitrogen 

 output falls to a low value (about 10 gms.), which is 

 constant day by day. When to the starving person is 

 given daily an amount of protein corresponding to the 

 amomit of nitrogen which he lost daily when starvation 

 was complete there is still an excess of output over intake. 

 It is not until the nitrogen intake is two and a half times 

 the starvation output that equiUbrium is attained. But 

 when in addition to the protein, carbohydrate or fat is 

 given, nitrogen equihbrium is reached with a lower protein 

 intake. This is the protein-sparing action of the non- 

 nitrogenous foods. 



These facts show that of the protein which is absorbed, 

 part, in virtue of the carbon which it contains, goes to 

 supplying energy — this is the part which can be replaced 

 by carbohydrates or fats. The other part has a fate other 

 than that of supplying energy — it becomes built up into 

 the hving cell. 



From a study of the nitrogen balance we therefore come 

 to the same conclusion regarding protein metabolism as 

 we did from a study of the efiect upon nitrogen ehmination 



