254 NUTRITION OF FARM ANIMALS 



TABLE 33. TOTAL KATABOLISM OF FASTING DOG 



Obviously, we have here the reverse of what takes place in 

 the later days of fasting, viz., a gradual substitution of fat for 

 protein as the readily available supply of the latter in the body 

 is reduced. Doubtless the effect would have been found to 

 be still more marked on the first day of fasting, when the pro- 

 tein katabolism was equivalent to 28.1 grams of nitrogen. 



339. Physiological minimum of protein. The facts re- 

 corded in the previous paragraphs render it evident that the 

 lowest level of protein katabolism is not necessarily attained 

 during complete fasting. Although the protein katabolism of 

 a fasting animal soon reaches a comparatively low level which 

 changes but slowly, nevertheless its amount may be greatly 

 affected, on the one hand by the amount of protein previously 

 consumed, and on the other hand by the stock of non-nitrogenous 

 material (fat and glycogen) contained in the body. While 

 normally some 10 to 17 per cent of the energy metabolized 

 in complete fasting is derived from protein (335), the proportion 

 may rise to twice this amount on a day following heavy protein 

 feeding, or to almost 100 per cent in case of an animal whose 

 stock of body fat is exhausted. In such cases it is evident that 

 part of the protein is katabolized simply for the sake of supply- 

 ing energy, since the smaller amounts katabolized in what may 

 be called a normal or average condition of the fasting animal 

 are at least sufficient to maintain all the vital functions, the 

 latter proceeding for a considerable time in a substantially 

 normal manner. 



The level of protein katabolism being so dependent on the 

 amount of non-nitrogenous material available as a source of 

 energy, the question naturally arises whether by supplying an 



