4 FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 



important point for us is that we must deal with such possibilities ; 

 we must plan and interpret our nutrition experiments to-day in the 

 light of these newer ideas. 



If we accept the synthetic theory of metabolism, disregarding 

 the cooperation of bacteria, it becomes possible to understand how 

 the lack of some fundamental unit, like tyrosine, tryptophane, 

 lysine, or cystine may lead to malnutrition or death through the 

 inability of the organism to construct normal protein and normal 

 protoplasm, because of a deficit of essential structural complexes. 

 The errors of a one-sided diet present a new point of view. The im- 

 portance of these theories is enormous, not alone for the nutrition 

 of man but also for the welfare of our domestic animals. Rational 

 and economical feeding is based upon a correct interpretation. 



Considerations such as the foregoing certainly justify an exten- 

 sion of experimental work in nutrition along the lines suggested by 

 the rapidly accumulating data on the structure of the individual pro- 

 teins. The theories must be subjected to the rigorous test of experi- 

 ment. The relative and absolute value of individual proteins must 

 be determined by physiological trials as a preliminary to definite 

 and more permanent, rational dietary programs. We have attempted 

 the beginnings of such a study. 



At the outset we were confronted by the fact that there are on 

 record few, if any, successful experiments in feeding isolated food- 

 stuffs. Still fewer are those in which the protein compound was fed 

 isolated, in a reasonable degree of purity. By "successful" we mean 

 experiments in which the health and vigor of animals were main- 

 tained under adequately controlled conditions for sufficient periods of 

 time to permit of more than tentative conclusions. The total number 

 of feeding trials in which the role of the protein food is in some way 

 concerned is very large. We shall omit reference to all except those 

 which bear specifically on the problems under consideration, viz., 

 the significance of individual proteins in nutrition. 



It may not be amiss at the outset to point out the broader re- 

 quirements which any adequate dietetic regime involves and upon 

 which its nutritive success depends in good measure. In the first 

 place the nutrients must be presented in a form that is digestible and 

 thus available for physiological utilization. The physical texture as 

 well as digestibility per se plays an important part in this respect. 

 Again, the available parts of the diet must be adequate in amount 

 to cover the calorific needs of the organism to which it is supplied, i. e., 

 there must be sufficient metabolizable energy. One might be inclined 

 to omit reference to such apparently obvious facts had they not been 

 serious factors in previous experimental failures in feeding isolated 

 food-substances. In recent years much emphasis has further been 

 laid upon certain less evident considerations involving nutrition more 



