Tf^Z THE PLACE OF PROTEIN IN NUTRITION. 



of this method will be of considerable value in the interpretation 

 of the results of feeding^ experiments. 



Feeding definite mixtures of amino-acids in place of pro- 

 tein is a line olf investigation which will perhaps throw more 

 light upon the nutritive value of the proteins of feeding-stuffs 

 than any other. It has been shown that the animal organism 

 can be maintained in equilibrium, and even normal growth 

 secured, when the nitrogenous requirements are covered by pre- 

 parations obtained by completely hydrolysing protein material^ 

 when presuma'bly only amino-acids (and small amoimts of 

 amides) are present. By completely removing- certain amino- 

 acids from the mixture of the products of protein hydrolysis 

 and determining the nutritive value of the residual mixture of 

 amino-acids it can be clearly demonstrated whether or not the 

 amino-acids removed are indispensable to maintenance or growth, 

 or both. 



Other workers have used as a sole source of nitrogen in 

 rations isolated and carefully purified proteins obtained from a 

 wide variety of sources, and have shown that different proteins 

 have different values in nutrition. Those differences have been 

 traced very successfully to differences in the amino-acid con- 

 tent. 



B}' these methods the significance of several of the amino- 

 acids ha\e been made evident. It is an unfortunate fact that 

 amino-acids are very expensive and otherwise difficult to obtain 

 in quantity, so that it is out of the question to attempt to feed 

 amino-acid mixtures to farm animals. 



Most df our present knowledge of the role of amino-acids 

 in nutrition has been obtained from the results of feeding experi- 

 ments with small animals, usually mice or rats, in which isolated 

 proteins or definite amino-acid mixtures were used as the sole 

 source of nitrogen. Such results are of fundamental impor- 

 tance in revealing the character of the chemical processes in- 

 volved in nutrition, but are not necessarily fully applicable to 

 farm animals. 



The third line of investigation suggested, namely, feeding 

 experiments to determine the nutritive value and relative effi- 

 ciency of the proteins of feeding-stuff's, will i)robably appeal to 

 the practical animal husbandry man more than the previous two. 

 Practical animal husbandry men must always deal with the 

 natural mixtures of proteins as they occur in ordinary feeding- 

 stuff's. Moreover, we can use farm animals for this work, as 

 there need be no special expense involved in the preparation of 

 the rations beyond the ordinary market cost of the feeds, and 

 no question olf the applicability of the results obtained to prac- 

 tical stock-feeding. Of course, this line of work is closely re- 

 lated to the previous two suggestions, and would yield a greater 

 measure of success, the greater the progress attained in the other 

 two directions, since the object would be to interpret the results 

 of the feeding experiments in terms of the amino-acid content 

 of the rations fed, if it be possible to do so. 



