DETERMINATION OF METABOLISM 149 



If having determined the daily nitrogen loss in an animal 

 during starvation it be fed with an amount of protein equivalent 

 to this loss, then nitrogen equilibrium would by no means be 

 established. The nitrogen excretion would be nearly double 

 the starvation excretion, and with a progressive increase of 

 food protein the excretion of nitrogen would become greater, 

 but at a diminishing rate, until finally nitrogenous equilibrium 

 is established with an excretion of nitrogen, say three times 

 as much as the starvation excretion. The amount of protein 

 food necessary for nitrogen equilibrium depends upon the 

 condition of the organism a muscular, well-nourished body 

 requiring more. Having established nitrogen equilibrium, 

 every increase in food protein leads to an increase of nitrogen 

 excretion, but also to some laying on of flesh until nitrogen 

 equilibrium is again established at some higher level. 



If, at any level of nitrogen equilibrium, fat be added to the 

 diet, it will be found possible to maintain equilibrium with a 

 much reduced amount of protein food. The fat economizes, 

 to a certain extent, the protein destroyed. On the other hand, 

 when protein in large quantities is given to a fat animal the 

 destruction of fat is accelerated. In the Banting cure for 

 corpulence the patient is put upon a diet containing much 

 protein, but little fat or carbohydrate. Carbohydrates are 

 even more effective than fats in economizing protein, and 

 carbohydrates economize fats as well. Albuminoids, like gelatin, 

 are still more effective as protein sparers, but will not act as 

 substitutes for protein. Their value is much like that of carbo- 

 hydrates, but, owing to their furnishing certain valuable " build- 

 ing stones," more effective. There has been much speculation 

 as to the reason for the inability of gelatin to maintain an 

 animal. It contains most of the amino-acids of protein, but 

 lacks tyrosin, tryptophan, and cystin. It has been claimed 

 that nitrogen equilibrium has been maintained upon a diet 

 of gelatin to which the above amino-acids, plus carbohydrates 

 and fats, were added. Nitrogen equilibrium has been main- 

 tained upon the products of pancreatic digestion of casein. 

 But when casein is hydrolyzed with hydrochloric acid this 

 is no longer possible, owing, perhaps, to the too thorough 

 breaking up of the polypeptids. 



Within those limits in which, in a normal adult, nitrogenous 



