alsbeeg: biochemical analysis of NUTRITION 



Whatever may be the ultimate practical significance of the 

 observations that animals can supply themselves with most 

 or all of their nitrogen needs by means of synthetic amino-acids. 

 th— xperiments 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 converted 

 into simpler substances, in the main albiunoses and peptones, 

 which were - rbed. These albumoses and peptones, while 

 simpler than most food proteins, are. nevertheless, still very 

 complex substances. It was believed that they are absorbed 

 and then converted by the animal into the protein characteristic 

 of that particular animal. How that conversion was accom- 

 plished - oot understood. Xow 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 the proteins of the 

 body and where this conversion took place. At first it was be- 

 lieved to occur in the blood. Later a difference of opinion arose 



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



s food proteins could be demonstrated in neither place, the 

 matter remained unsettled. We know today that neither hy- 

 pothesis is tenable. Proteins are not ordinarily absorbed 

 such. They -are completed 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 maintained 

 upon pure synthetic amino-acids. no one had any reason to 

 believe that proteins were completely digested before absorption. 

 w what happens to these amino-acids after they are ab- 

 sorbed? As ordinary diets may contain more nitrogenous 

 material than is needed by the organism, a part of the amino- 

 acids is changed within the walls of the intestinal canal by the 

 removal of the amino group to form ammonia. As this takes 

 place in the presence of carbonic acid, ammonium carbonate 

 and ammonium carbamate are formed. It has recently been 



