THE PEOTEINS AND THEIR METABOLISM 121 



get along perfectly well, grow to maturity, maintain his body weight and 

 nitrogenous equilibrium on protein levels exactly one-half that of Voit's 

 (that is, 0.130 gram per kg. of body weight) provided, of course, that he 

 has a plentiful supply of dynamogenetic substances in the form of carbohy- 

 drates and fats to cover all of the body requirements. 



From the mere fact that the hardest possible physical work is not 

 associated with any increase in protein metabolism we may justly con- 

 clude that protein was not intended for dynamogenetic purposes. Its main 

 function is to supply the "wear and tear" quota, "growth" quota with a 

 reasonable surplus to allow for reserve and "factors of safety." 



Sufficient data seem to have been gathered to date to show that 0.130 

 gram of nitrogen per kg. of body weight per twenty-four hours covers all 

 of these requirements. 



The Function of Protein in the Diet 



Incomplete Proteins 



The object of all food is to supply fuel, which, in the process of its 

 catabolism, will yield energy to the cells. The use of protein serves a 

 double function. While it may be used for dynamogenetic purposes, of 

 far greater importance is its use in supplying the building stones of the 

 protein to the body, i. e., the amino acids. 



Originally it was believed that the peptones in the digested protein 

 were the products that were resorbed and used for protein regeneration, 

 and that the protein derived from the same species were utilized to 

 greater advantage than proteins derived from foreign species (Michaud, 

 1909). It was further believed that, in those peptones were nuclei of 

 linked amino acids, which corresponded to those of the animals experi- 

 mented upon, which made it possible for that animal to maintain equilib- 

 rium with a smaller amount of nitrogen derived from protein that was 

 similar to its own protein. This conception, however, cannot stand, in 

 view of the results obtained by Loewi (1902 (a.) ). He was the first to keep 

 an animal on a diet consisting of carbohydrates and fats, with all the 

 nitrogen that it required, supplied in the form of digested protein, that 

 gave no biuret reaction, i. e., digested to the amino acid stage; proving 

 that the animal body is capable of synthesizing its own protein from 

 the elementary amino acids. These experiments have been repeated by 

 Abderhalden and corroborated in a very convincing way. He not only 

 cleared up the problem as to the possibility of synthesizing protein from 

 the simple amino acids, but also introduced a new method for studying 

 whether certain amino acids were dispensable or indispensable in the ani- 

 mal economy, and whether the body has the power of producing them 



