1 68 HUMAN PHYSIOLOGY 



B. Metabolism during sufficient nutrition. The partak- 

 ing of food increases the metabolism as compared with that 

 during inanition. In this respect the animal body is not 

 like a furnace in which increase of consumption follows in- 

 crease of supply, for the body can store up a considerable 

 amount of material for combustion. Besides this, the increase 

 in metabolism is less dependent upon the absolute quantity 

 of material furnished than upon the composition of the 

 material. 



I. The effect of proteid upon metabolism. The effect of 

 proteid upon metabolism can best be investigated in the 

 carnivorous animals, which can maintain life if merely pro- 

 teids are given in the food in addition to salts and water. 

 Suppose a dog is fed with as much proteid as it consumes. 

 Its body will be in nitrogenous equilibrium, for the nitro- 

 genous income and outgo are balanced. If to such a dog 

 more proteids are fed, the larger part is used, while only a 

 small part is stored up in the body as flesh. 



By this laying up of flesh the demand for proteid is in- 

 creased, for the demand is proportional to the weight of the 

 body flesh. Nitrogenous equilibrium is again obtained, 

 when the proteids demanded by this newly laid up flesh 

 equal the increase in the quantity of the food proteids. But 

 the possibility of such storing up of flesh is limited, for the 

 digestive organs cannot cope with very large quantities of 

 proteids. 



If a dog, maintained in nitrogenous equilibrium by proteids 

 only, receive less proteids, the body loses some of its flesh 

 till the amount of body proteid has reached the point where 

 the demand upon proteid is equal to the proteids supplied in 

 the food ; nitrogenous equilibrium is then once more estab- 

 lished. At a certain low limit of proteid supply, nitrogenous 

 equilibrium is not regained, for then the body continually 

 uses more proteid than is supplied, and hence death by 

 starvation must result. 



The smallest amount of proteid with which an animal 

 living upon a pure proteid diet can maintain nitrogenous 



