54 Feeds and Feeding. 



75. Feeding protein only. — We have seen that during starvation 

 there is a small but steady waste of the protein tissues of the body, 

 both thru natural wear and the heavier loss due to the effort to 

 support life by using the energy these tissues furnish on being 

 oxidized. 



On the basis of the theory of circulating protein, it is assumed 

 that the protein of the body tissues, such as the muscles, is nor- 

 mally subject to only slow oxidation. On the other hand, the cir- 

 culating protein, which is relatively large in the well nourished 

 body, but which rapidly disappears during starvation, is easily 

 and quickly decomposed, and furnishes by far the larger part of 

 the nitrogen waste of the liberally fed animal. If a mature starv- 

 ing animal is fed a limited amount of practically pure protein sub- 

 stance, such as washed lean meat, the circulating protein in the 

 body will be increased, with a proportionately greater nitrogen 

 waste. Even tho the food consumed contains more protein than 

 balances the daily waste from the tissues, there will be no storage 

 of nitrogen during the feeding of this one-sided ration. 



When protein is fed far in excess of the waste of the starving 

 body, nitrogen equilibrium may be established ; that is, the amount 

 of nitrogen excreted will equal but not exceed that consumed in 

 the food. If the supply of protein given to a mature animal be 

 still further increased after nitrogen equilibrium is reached, the 

 excess of protein fed will not be stored in the body as protein, 

 thereby increasing the muscular tissues, but will still be decom- 

 posed, and the nitrogen excreted in the urine. Thus the nitrogen 

 waste keeps pace with the supply of nitrogen in the food. This 

 does not mean, however, that the food value of the protein so de- 

 composed has been entirely lost to the animal. After the splitting 

 off of the nitrogen from the protein molecules the non-nitrogenous 

 residue which remains may be converted into glucose and finally 

 into glycogen or fats. 



Supplying a heavy exclusive protein ration not only tends to 

 check the waste of the fat already stored in the body, but probabl.y 

 with flesh-eating animals at least, fat may be formed from the pro- 

 tein of the food and stored as body fat during such exclusive pro- 

 tein feeding. (83) It is probable, then, that the carnivora or flesh- 

 eating animals can live on pure protein food alone, providing it is 

 supplied in abundance. Whether the herbivora, or plant-eating 

 animals, can live on protein alone has not been settled, and altho 

 of scientific interest the subject has no practical importance. 



