584 THE ANIMAL BODY AS A MACHINE 



more serious source of waste, however, is the incomplete utilizability 

 of the small proportion of protein which the vegetable diet does con- 

 tain. We have seen from the researches of London (Chapter XI) that 

 the intestinal epithelium exerts a preliminary selective action upon the 

 amino-acids which are submitted to it for absorption, rejecting a 

 proportion of those which are present in unwonted excess. Now 

 the proportions of the various Amino-acids in the proteins of vegetable 

 origin differ very decidedly from those which obtain in proteins of 

 animal origin and therefore, on a purely vegetable diet, the arnino-acids 

 presented for absorption are in abnormal proportion to one another. 

 A portion of the amino-acids derived from' vegetable proteins by diges- 

 tion are therefore rejected and voided in the feces. The following 

 table shows the percentage of the nitrogen in various types of food- 

 stuffs which is actually assimilated : 



Percentage of nitrogen 

 Type of food. actually assimilated. 



Flesh 98 



Fish . . . . . . - <" . . . . 97 



Eggs . ' , " . . . -. 95 



Milk . ..... . , . 94 to 95 



Peas, Beans . . . . ,. . . . 85 



Corn 83 



Wheat-flour 81 



Rice ,......, ' ; 80 



Potatoes .,.,... . 78 



The following shows the relative proportion of wastage on a purely 

 vegetable diet, an average mixed diet and a high meat-diet (Atwater 

 and Langworth) : 



Nitrogen in grams per day Percentage 



Type of diet. , > ^ ^ of nitrogen 



In food. In urine. In feces. wasted. 



Vegetable diet ...... 13.8 13.9 3.9 28 



Mixed, average meat .... 19.4 15.6 2.4 13 



Mixed, large amount of meat .33.1 24.5 2.9 9 



Even the amino-acids which fail to undergo assimilation, however, 

 do not represent all the wastage which occurs on a purely vegetable 

 diet, for the process of selection and rejection which initiates in the 

 intestine continues in the tissues, and the rejected excess of unutilizable 

 radicals simply enters the exogenous metabolism and, while it is avail- 

 able for the production of heat, is useless for the maintenance of the 

 integrity and repair or synthesis of tissues. This fact is very well 

 illustrated by the experiments of K. Thomas, who, subsisting upon a 

 diet of starch and sugar, estimated the minimal daily loss of tissue- 

 protein and then added to his diet food materials of various types in 

 order to determine the relative power of the proteins which they con- 

 tained to save the body from loss of tissue-protein, or, as he terms 

 it, the Biological Values of the various proteins. The following were 

 some of his results: 



