504 , THE HUMAN BODY 



proteins of wheat. This protein lacks the amino acid lysin. The 

 conclusion drawn from this observation is that the lysin which is 

 one of the constituents of living protein is not involved in the 

 processes of tissue break-down. After the tissue is once formed, 

 therefore, it does not require continual supplies of this substance. 

 No new tissue can be made, however, unless lysin is provided. A 

 curious incidental discovery in connection with the experiments 

 by which this was established was that the rats which were used 

 as subjects could be maintained in health with the weight and 

 bodily dimensions of young animals for months after they would 

 have become full grown on an ordinary diet. If then they were 

 changed from maintenance proteins to proteins that were ade- 

 quate for growth they promptly began growing and presently 

 attained full size. The significance of this is that the ability to 

 grow is not restricted to the early periods of life and does not come 

 to an end with the attainment of a certain age. 



Fuel Protein. We have learned that proteins are absorbed from 

 the digestive tract into the blood as amino acids. Of these a 

 portion are destined to provide for tissue repair and growth. The 

 excess is used as fuel. The nitrogenous portion is virtually devoid 

 of value as a source of energy. To fit the remainder to serve as 

 fuel the nitrogen-containing radicals are dissociated from the 

 molecules, leaving non-nitrogenous residues of high energy value. 

 This process of setting aside the nitrogenous radicals is known as 

 deaminization. It was formerly believed to occur during the pas- 

 sage of the amino acids through the intestinal walls in the process 

 of absorption, but recent investigations have shown that the 

 amino acids are absorbed as such and that deaminization is prob- 

 ably carried on by the tissues generally. The further history of 

 the nitrogen-containing radicals will be considered in a later chap- 

 ter (p. 517). The non-nitrogen residues join themselves with the 

 other energy-yielding food stuffs and will be discussed together 

 with them (p. 507). 



Should the Diet Include Much or Little Protein? We have seen 

 that it is possible to maintain the tissues adequately upon a diet 

 containing only a fraction of the amount of protein ordinarily 

 taken. Are we to conclude from this that the human race eats 

 too much protein? To this question no final answer can be given 

 at present. Eminent dietitians have argued on both sides of it. 



