GENERAL METHODS HISTORY OF PROTEIN FOOD. 897 



liver, but this view has been brought into question lately.* 

 That the process takes place in the body is beyond doubt, and 

 it is likely that the liver shares in this activity even if it is not 

 the chief tissue concerned. That the body is able to build up 

 its own protein from a mixture of amino-acids, such as is pro- 

 duced in a complete digestive hydrolysis of protein, has been 

 demonstrated beyond doubt by feeding experiments in which 

 the nitrogenous food was all supplied in the form of such a mix- 

 ture. The first experiments of this kind were made by Loewi.f 

 He fed dogs on a diet consisting of fats, carbohydrates, and protein 

 which had been submitted previously to a prolonged pancreatic 

 digestion until it was completely hydrolyzed. On this diet the 

 animal was maintained in nitrogen-equilibrium. This experiment 

 has been verified and extended by others on man as well as upon 

 dogs, and indeed Abderhalden and Rona report that they have 

 been able to keep a dog not only in nitrogen-equilibrium but with 

 a plus balance of nitrogen when fed on the split products of meat 

 alone, without addition of fats or carbohydrates. This last 

 experiment would indicate that the amino-bodies not only give to 

 the body material from which it can reconstruct its own protein, 

 but they furnish also a usable source of energy for the body 

 needs, a result which we can understand on the hypothesis men- 

 tioned above, according to which the amino-acids, after removal 

 of the NH 2 group, furnish an organic acid residue capable of further 

 oxidation or of synthesis to fats or carbohydrates. 



We may accept as a clear result of modern investigation that the body is 

 capable of building up its protein from such relatively simple substances as 

 the amino-acids, in fact the evidence goes to show that normally it is from these 

 amino-acids that the nitrogenous material of the body protein is derived. 

 This striking result has led investigators to test whether the body may utilize 

 sources of nitrogen of even simpler construction, such, for example, as the 

 organic or inorganic salts of ammonia. It has long been known that the plant 

 organism utilizes inorganic forms of nitrogen, such as the ammonia salts or the 

 nitrates, in building up its protein, and it has long been believed that the 

 animal protoplasm is not able to utilize these salts in the same way, that, in 

 other words, its processes of synthesis so far as the protein material is concerned 

 are more limited than in the plants. It has been taught universally that ani- 

 mals for the construction of new tissue must have their nitrogenous food in the 

 form of protein, or, a recent addition, in the form of the split-products of pro- 

 tein. Several observers report (see Grafe, "Zeitschrift f. physiolog. chemie.," 

 78, 485, 1912) that when animals are fed upon an abundance of carbohydrate 

 food together with some ammonia salt, such as ammonium citrate, acetate, or 

 carbonate, they may maintain a positive nitrogen balance for long periods. 

 In the metabolism of the amino-acids in the body it is recognized that by 

 diaminization and oxidation an oxyacid or ketonic acid may be produced, 

 and it is suggested that this process may be reversed, that an oxyacid, lactic 

 acid formed from sugar, may combine with ammonia, furnished by an am- 



1908, 



57 



