84 FEEDING EXPERIMENTS WITH ISOLATED FOOD-SUBSTANCES. 



the protein-free milk present in the same proportions as in our milk 

 food, but less than one-third of the protein constituents of the milk. 

 It is therefore evident that only a small proportion of the protein 

 constituents of the milk are required to produce normal growth, and 

 it may be assumed that the presence of a small quantity of milk 

 proteins in our protein-free milk powder would manifest itself by at 

 least some slight growth. 



DISCUSSION OF THE RESULTS AND THEIR BEARINGS. 



We have stated that by our plan a biological comparison of dif- 

 ferent proteins in respect to their role in growth can at length be 

 made. Our work in this direction must be regarded as barely begun. 

 Nevertheless it is of interest to speculate as to the indications already 

 gained and the outlook for future work. A comparison of the two 

 groups of proteins those adequate and those inadequate for growth 

 purposes at once reveals the fact that the latter category comprises 

 proteins (gliadin, hordein, zein) commonly spoken of as chemically 

 "incomplete." They lack one or more of the amino-acid complexes 

 which are obtainable from the so-called "complete " proteins. None 

 of them furnish glycocoll or lysine, and zein in addition is devoid of 

 tryptophane. By feeding relatively small quantities of proteins like 

 casein with gliadin growth begins at once. Here we can determine 

 the minimum of suitable protein to satisfy this growth requirement 

 a study already begun (cf. Charts CXX, CXXI, CXXII, and 

 CXXIII). The addition of amino-acids to "complete," as it were, 

 the inadequate proteins can now be studied amid controllable factors ; 

 the biological role of hydrolyzed proteins and the significance of 

 complete hydrolysis or digestion in nutrition can be examined anew. 



The experiences which have demonstrated the striking differ- 

 ences in value of the individual proteins and the small proportion of 

 casein which suffices to induce growth instead of standstill (cf . Charts 

 CXX, CXI, CXXII, and CXXIII, for example) emphasize the impor- 

 tance of the purity of the protein fed. We have devoted much labor 

 and incurred a very considerable expense to obtain proteins in a form 

 as uncontaminated as present methods will permit. The products 

 used were as pure as one would expect them to be if employed for 

 purposes of refined protein analysis. Had less perfect products been 

 employed it is quite conceivable and indeed likely that the admix- 

 tures would have sufficed to alter completely the outcome of many 

 experiments. For example, gliadin is prepared free from glutenin 

 only be very careful purification methods ; and although the nutritive 

 properties of these two companion proteins are extremely unlike, as 

 clearly indicated by our trials, a failure to effect a complete separa- 

 tion of a little glutenin from gliadin would have been sufficient to 

 prevent the deficiencies of the latter from exhibiting themselves. Or 



