76 THE PHYSIOLOGY OF PROTEIN METABOLISM 



tein molecule is an instance. That such a formation may take place 

 his been suggested by Halle (163), who stated that after digesting 

 suprarenal pulp with tyrosine more adrenaline could be obtained than 

 from control experiments carried out without such addition. This 

 work, however, has been questioned by Ewins and Laidlaw (121) who 

 found no evidence of the conversion of tyrosine, or even of the more 

 closely related bases, parahydroxyphenylethylamine and dihydroxy- 

 phenylethylmethylamine, into adrenaline by ferment activity. 



From the above evidence, it is clear that apart from the caloric 

 intake and the protein, carbohydrate and fat content of the food, there 

 is some factor or factors which influence the utilization perhaps also the 

 amount of food required. This evidence practically points to the pres- 

 ence of some " minimal " substance or substances in normal food which 

 are absent in "pure" food 



The Psychic Influence. 



Still another factor, of which at present we know but little, is probably 

 of fundamental importance, namely the psychic factor. Owing to the 

 nature of the experimental diet it is frequently monotonous in flavour, 

 or may be even nauseous, and it causes a certain amount of reluctance 

 on the part of the animal to eat it. This distaste leads directly to a 

 failure in appetite which eventually results in faulty digestion and in- 

 complete absorption and utilization. In other words, in these pure 

 diets there is a lack of those stimuli which PavlofT has proved to be 

 so essential for gastric digestion at least, and which in all probability 

 will be found to be necessary for the other organs. McCollum (271) 

 investigated this question with a certain amount of success, and reached 

 the general conclusion that the palatability of a diet was the most im- 

 portant factor. He found that, even with many of the simple mixtures 

 of pure proteins, if sufficient care were taken to change the character 

 and the palatability of the food, by altering the flavour for example, it 

 was possible to induce an appreciable retention of nitrogen. Very 

 young animals, probably due to the fact that their metabolism is so 

 active and hunger so predominant a feature of their daily lives, are 

 found to adapt themselves more readily than adult animals to a ration 

 of a comparatively low degree of palatability. 



