458 THE HUMAN BODY 



abundant ingestion of water and a speedy restoration of the lost 

 weight. 



Nitrogen Equilibrium. Those metabolic activities of living 

 tissues which result in tissue breakdown are particularly associ- 

 ated with the use of protein foods, since, as we have seen, their 

 repair can be accomplished only by proteins. The characteristic 

 constituent of protein is nitrogen; and the simplest way to esti- 

 mate the amount of protein contained in any food mass, or repre- 

 sented by any particular amount of excretion, is to determine the 

 nitrogen and multiply the weight of it present by 6.25, the fraction 

 of protein which is nitrogen. We shall learn in the chapter on 

 Excretion (Chap. XXXI), that in the healthy Body an accumula- 

 tion of nitrogen-containing excretory products never occurs; as 

 fast as wastes are formed they are gotten rid of. It follows, 

 then, that if there is less nitrogen being given off than taken 

 in, the living tissues of the Body must be increasing in amount, 

 and if more is given off than is obtained in the food the living 

 tissues must be wasting away. In the healthy adult Body, 

 neither of these conditions is at all usual; the intake and outgo 

 of nitrogen balance each other and the Body is in nitrogen equi- 

 librium. 



It has been chiefly through experimental studies of nitrogen 

 equilibrium that our ideas of the twofold function of protein, as 

 tissue-restorer and as fuel, have been gained. If an animal be fed 

 large enough quantities of protein he requires no other food, and if 

 healthy maintains nitrogen equilibrium upon this high level, the 

 large nitrogen intake being exactly balanced by an equally large 

 outgo. Now by substituting other foods, as carbohydrates or 

 fats, for part of the protein, the nitrogen intake and outgo are each 

 less in quantity, but they still balance; the animal is in nitrogen 

 equilibrium upon a lower level. If the substitution of other foods 

 for protein is increased a point is presently reached when the 

 nitrogen outgo exceeds its intake; the animal is not getting enough 

 protein for his needs, and so his own tissues are breaking down. 

 It was stated in an earlier paragraph (p. 452) that this breakdown 

 begins while the protein intake is still somewhat in excess of the 

 tissue-repair requirement, indicating that there is a fuel-protein 

 requirement which must be met. Two lines of evidence point to 

 this conclusion. The first is gained by starvation studies. If an 



