THE ROLE OF PROTEINS IN METABOLISM 435 



of body weight; in order to satisfy this waste it w r ould be necessary to ad- 

 minister i . 50 grams per kilo of meat; this at once increases the urea excreted 

 to about 0.75 gram per kilo of body weight, and nitrogenous equilibrium 

 is not attained until over three times, viz., 3 grams per kilo of body weight, 

 of meat is given. Foster gives even a larger figure. The effect, therefore, 

 of protein food is largely to increase the excretion of urea. 



Studies in nitrogenous equilibrium are based on the fact that when an 

 animal is given a diet with a constantly increasing amount of protein food 

 from day to day, after a fe\v days the total nitrogen found in the excreta 

 exactly balances that taken in the food. This condition of nitrogenous 

 equilibrium is established at different levels, varying sometimes according to 

 the individual and with the kind and quantity of other food principles taken 

 at the same time as the nitrogenous foods. 



Chittenden's latest metabolism experiments have shown that with free 

 choice, but moderate use, of accessory articles of diet, the human body can 

 maintain itself in nitrogenous equilibrium for at least several months on an 

 average of 6 to 10 grams of nitrogen per day, the equivalent of 37.5 to 62.5 

 grams of dry protein. 



The Role of Proteins in Metabolism. The proteins of food are 

 described by Voit as having two relations to the protein metabolism, also to 

 outgoing urea. The first part of the protein of the food goes to maintain the 

 ordinary and quiet metabolism of the tissues, for which purpose it is actually 

 built up into the living protoplasmic molecule. The second part being 

 more directly oxidized causes a more rapid formation of urea, but never 

 becomes a part of the actual protoplasmic molecule. The former proteins 

 are called mot -photic or tissue proteins, the latter circulating or floating pro- 

 teins. Normally more protein is eaten than is needed to supply protein 

 to the protoplasm for growth, as has just been stated. Pfliiger takes the 

 view, however, that the tissues must have an excess of protein to destroy in 

 order to perform their metabolic processes normally. This use of the 

 proteins to form heat by their oxidation, and not to produce tissue, was 

 looked upon by the older physiologists as a wasteful use of good material, 

 and was called luxus consumption. 



Folin has recently announced a theory of protein metabolism in which 

 he calls special attention to the relation of the nitrogenous excretion products 

 to the nitrogenous intake. He has presented evidence to show that the urea 

 contained in the urine varies almost directly w r ith the quantity of protein in 

 the food; that the ammonia also varies with the protein in the food; that the 

 uric acid decreases (and increases) with the protein in the food, but not in 

 direct ratio; while the creatinin excreted is "wholly independent of quan- 

 titative changes in the total amount of nitrogen eliminated." 



