10 THE PROTEIN ELEMENT IN NUTRITION 



protein, zein, experimented with by Hopkins and Willcock. 

 Zein, given in the food, cannot be recovered from the tissues, and 

 in all probability certain of its amino-acids are made use of to 

 build up tissue protein ; certain other units, not being of the 

 particular composition required, are rejected and eliminated as 

 waste. 



From the standpoint of the synthesis of tissue proteins by 

 selection of those particular amino-acids or units that are suitable, 

 it will be acknowledged that, in the proper nutrition and growth 

 of the nitrogenous tissue, a protein diet in excess of the actual 

 requirements will be necessary, in order that the body tissues 

 may have plenty of choice. This principle would hold good for 

 all proteins, but especially for protein of vegetable origin, for, if 

 the amino-acids formed during digestion are not present at all, 

 or not present in the necessary proportions, and this is all the 

 more likely to be the case with vegetable proteins, then, the 

 suitable groups being relatively scanty, much unsuitable protein 

 will require to be hydrolyzed in order that the proper supply of 

 the essential units may be obtained. In the light of the opinions 

 at present held regarding protein synthesis within the body, 

 these considerations would forcibly combat the view that has 

 been put forward that great benefits would accrue to the economy 

 from dietaries low in protein, unless it could be absolutely 

 insured that the low protein dietary supplied just those particular 

 amino-acids in sufficient quantities which the body required. 



The modern views held regarding the substance protein, the 

 changes that are brought about during digestion, its fate after 

 absorption, and the processes by which it is finally broken down 

 into waste products, may now be briefly summarized. 



By the united efforts of the three ferments, pepsin, trypsin, and 

 erepsin, the breaking down of the protein molecules during 

 digestion is a far-reaching process, the protein being disinte- 

 grated into a series of organic radicles, of which the amino-acids 

 are the most important. These amino-acids may be looked on 

 as the units or building-stones from which the protein molecule 

 is made, the numbers and combinations of these units differing 

 in different proteins, and being by no means the same in the 

 proteins of the food as in the body proteins. During this demo- 

 lition of the protein molecule into amino-acids, the protein loses 

 but little of its potential energy. 



These amino-acids are taken up by the epithelial cells of the 



