LIFE AND HABIT HI 



needs by means of synthetic amino-acids, these experiments have led 

 to investigations that have explained much that has been obscure in 

 the physiology of nutrition. Formerly it was believed that proteins when 

 ingested were digested by the enzymes of the intestinal tract and con- 

 verted into simpler substances in the main albumoses and peptones, 

 which were absorbed. These albumoses and peptones, while simpler 

 than most food proteins, are nevertheless, still very complex substances. 

 It \\ as believed that they are absorbed and then converted by the animal 

 into the protein characteristic of that particular animal. How that con- 

 version was accomplished was not understood. Now every species of 

 animal and plant has its own characteristic proteins. The proteins of 

 even closely related species are different. The proteins of the food supply 

 are quite different from those of the animal taking that food. Much work 

 was done to explain how the proteins of the food were converted into 

 proteins of the body and where this conversion took place. At first it 

 was believed to occur in the blcod. Later a difference of opinion arose 

 as to whether it took place in the tissues or in the intestinal wall. As 

 food proteins could be demonstrated in neither place, the matter remained 

 unsettled. We know to-day that neither hypothesis is tenable. Proteins 

 are not ordinarily absorbed as such. They are completely dismembered 

 within the intestinal canal into their component amino-acids and these 

 are absorbed. As long as it was not known that an animal can be main- 

 tained upon pure synthetic amino-acids, no one had any reason to believe 

 that proteins were completely digested before absorption. (Italics mine.) 

 These explanations should do away with the theory pro- 

 pounded in all seriousness by some writers that the most ideal 

 food is that obtainable through in-feeding, and that in the case 

 of man, for instance, the ideal diet would be human flesh. 

 It should do away with the more widely held absurdity that one 

 organism inevitably needs to kill, to absorb, and to " assimilate " 

 another in order to satisfy its real food needs. The fact that 

 proteins are completely digested, i.e., broken up before absorp- 

 tion, moreover, is not only interesting in connection with the fact 

 that an animal can be maintained upon pure synthetic amino- 

 acids. It also shows, in my opinion, that highly complex proteins 

 are not really wanted as food. True, it is part of the function 

 of the digestive tract to get rid of impurities and non-congenial 

 substances. But we must not abuse our digestive and elimina- 

 tive powers by food that is too rich, i.e., too complex. The best 

 materials that our diet can ever furnish are the " building stones " 

 coming originally from the plant, and these make no undue 

 claims on digestion and elimination. The really vital and 

 abiding union sought after in animal nutrition, is between the 

 amino-acids of the plant and the blood of the animal. It is in 

 conformity with the principle of reciprocity or reciprocal 

 differentiation that we want in our diet " quite different proteins " 



