EEL ATI ON TO ENERGY SUPPLY. 



87 



It has been shown that this effect is produced not only by the true 

 fats and by the soluble hexose carbohydrates, such as starch and the 

 sugars, but likewise, in the case of herbivorous animals, by those ill- 

 known ingredients of feeding stuffs, especially of the crude fiber and 

 the nitrogen-free extract, which disappear in the passage of the food 

 through the alimentary canal and which are commonly spoken of as 

 being digested. This statement covers also the organic acids, whether 

 resulting from the fermentation of the carbohydrates or contained 

 in the feed. 1 



We are not, however, to conceive of a sharp distinction in this 

 respect between an insufficiency and a sufficiency of nonnitrogenous 

 nutrients, but rather of a tendency on the part of the latter to 

 diminish the protein katabolism, a tendency more or less marked 

 according to their abundance in the ration. We are not to under- 

 stand that no nitrogenous material is katabolized for fuel purposes 

 as long as sufficient nonnitrogenous nutrients are present to supply 

 the demands for energy, nor that even the largest quantities of 

 the latter can prevent the katabolism of protein supplied in excess 

 of the possible constructive use by the body. We may believe that 

 the protein cleavage products, either derived from the feed or from 

 tissue katabolism, are always present in the blood and that more or 

 less deamidization is continually going on, resulting in a use of 

 protein material as fuel. On the other hand, nonnitrogenous sub- 

 stances, derived from the feed or the body fat, are also present and 

 take their share in supplying energy. We may probably conceive 

 of the quantitative character of the katabolism as being determined, 

 in a very broad sense, by the law of mass action. An increase of non- 

 nitrogenous materials in the blood or lymph tends to diminish the 

 deamidization and subsequent oxidation of the cleavage products 

 of protein and through this, secondarily, to diminish the breaking 

 down of body protein or to stimulate and prolong the limited storage 

 of protein possible in the mature animal. 



As regards the maintenance requirement, it is evident, then, that 

 the sufficiency of a given amount of protein depends not only upon 

 the plane of protein nutrition of the body, but also upon the amount 

 of nonnitrogenous nutrients supplied with the protein. With an 

 abundant supply of the former an amount of protein equal to the 

 fasting katabolism, or perhaps even less, appears to be a sufficient 

 minimum for maintenance. As the supply of nonnitrogenous ma- 

 terials is reduced a larger supply of feed protein seems to be required 

 to reach equilibrium because more and more of it is diverted for 

 use as fuel, so that in the total absence of nonnitrogenous nutrients 

 a large excess of protein must be fed before equilibrium between in- 

 come and outgo is reached. In interpreting experiments or formulat- 



1 Compare Armsby, Principles of Animal Nutrition, pp. 117-127. 



