1048 ABSORPTION 



ternal agent, the oxidation power of the tissues is interfered with, 

 thereby causing an excessive storage of this material. Secondly, 

 obesity may be due to an unusually high efficiency of those organs 

 which are directly concerned with fat metabolism, enabling them to 

 keep the body as a whole in a proper condition on an unusually low 

 supply of food. Obviously, this condition can only be remedied by 

 a lessened ingestion of food and a greater expenditure of energy. 

 The latter alone can do no good, because if the patient is then allowed 

 to control his intake in accordance with his appetite, he no doubt would 

 endeavor to balance the greater outgo by a greater intake. It should 

 be remembered, however, that a fat person actually needs a slightly 

 greater production of energy than a lean person, because his body sur- 

 face is larger, favoring heat dissipation. 



THE METABOLISM OF THE PROTEINS 



The Source of the Protein of the Body. — It will be remembered 

 that the proteins of the food are completely hydrolyzed into their 

 amino-acids, and are absorbed as such and passed into the portal 

 blood stream. Under ordinary conditions, the only slightly hydro- 

 lyzed products of protein digestion, such as peptone and proteose, 

 are not absorbed in significant amounts, because it is a well-known fact 

 that these substances, when injected directly into the blood stream, 

 produce symptoms of intoxication. This anaphylaxis, however, does 

 not follow if they are introduced into the intestinal canal. Whatever 

 proportion of them may find its way into the epithelial lining cells 

 must, therefore, be reduced and changed in its course through these 

 cells into inert proteins. This change may also be effected while they 

 circulate, because their concentration in the blood can never be in- 

 creased sufficiently to produce injurious effects. Such a result is 

 prevented ordinarily, because they are brought into contact with a 

 very large quantity of blood, and because the quantity of the still un- 

 reduced protein within the intestine is very small. At all events, the 

 evidence so far presented does not show that the intestinal lining cells 

 synthetize the amino-acids into the proteins of the blood, which in 

 turn would have to be changed into tissue proteins. Consequently, 

 it may be concluded that the body synthetizes its proteins from the 

 amino-acids directly, using in this case only those which are of^special 

 value to it. The others, as well as those transferred into the blood by 

 the cells as waste, are split into two portions, one of which represents 

 the ammonia and the other the remnant of the amino-acid molecule. 

 The urea is derived from the former, while the latter is immediately 

 oxidized to yield energy. For this reason, we commonly speak of the 

 so-called tissue-protein and circulating protein, the former being rep- 

 resented by that portion of it which enters the cells of the tissues to 

 become an intricate part of them, while the latter is broken down 

 immediately without having been converted into cellular protoplasm. 



