GENERAL METHODS HISTORY OF PROTEIN FOOD. 903 



the loss of this matter that is continually going on; second, 

 it furnishes a supply of energy for the heat of the body and for the 

 work done by the various cells, the contraction of the muscle, the 

 secretion of the gland, the discharges of the nerve-cells, etc. This 

 second function, the energy requirement, is met by any of the three 

 energy-yielding food-stuffs, carbohydrates, fats, or proteins, es- 

 pecially, as we shall find, by the carbohydrates. For the first 

 function protein (or its split-products) is absolutely needed, and 

 perhaps is alone needed. In any event, if the supply of non-pro- 

 tein is sufficiently large, then the amount of protein can be lowered 

 to a certain irreducible minimum, which is required for purposes 

 of genuine assimilation, that is, the construction of living material. 



The Nutritive History of the Protein Food. The digestive 

 changes undergone by protein and its subsequent absorption have 

 been described in the section on Digestion. It will be remembered 

 that the view adopted was that the proteins are completely hydro- 

 lyzed into their amino-acids and in this form are absorbed into the 

 blood and distributed to the tissues. The blood from the intes- 

 tines passes first through the liver before reaching the general cir- 

 culation, and a question to be determined is whether the amino- 

 acids are acted upon in any specific way by the liver. On this 

 point opinions have differed greatly. After a meal the amino- 

 acids in the blood and in the tissues generally are increased in 

 amount, and this fact would indicate that these substances pass 

 through the liver without undergoing any intermediate metabolism. 

 They are disseminated over the body at large and undergo their 

 subsequent metabolic changes in the several tissues; but it is 

 probable, as is explained below, that one stage in the metabolism of 

 some of these amino-acids may be carried out especially in the liver. 



Concerning the further history of the amino-acids or the nature 

 of the processes of protein metabolism in the tissues there have 

 been many different theories and points of view.* It would be 

 confusing to give summaries of these diverging theories, and it may 

 suffice to indicate briefly the conception that seems to be most 

 in accord with recent work. It may be accepted as a necessary 

 conclusion that some of these amino-acids are recombined by syn- 

 thetic processes to form organized protein, of the kind character- 

 izing the particular tissue, for only in this way can we understand 

 how the wear and tear of the tissue is replaced or how new protein 

 is made in the growing animal. Whether this synthetic combina- 

 tion is effected by endo-enzymes or under the influence of the living 

 protein of the cells cannot be stated, but we may assume that the 

 several amino-acids which enter into the structure of that particular 



* For a review of these theories consult Mendel, "Ergebnisse der Phys- 

 iologie," 11, 418, 1911. 



