THE PROTEIN REQUIREMENT 209 



of variations in the diet (p. 165). The fate of the protein 

 is twofold : either exogenous or endogenous. 



If the place of the exogenous portion of the protein 

 absorbed can be taken by carbohydrate or fat it should 

 be possible to reduce the protein intake very considerably — 

 down to endogenous requirements, provided that non- 

 nitrogenous food is given in abundance. This considera- 

 tion led to the Chittenden experiments, in which different 

 classes of people were fed on a very low protein diet for 

 a period of several months. Chittenden claimed that 

 health and working capacity were improved owing to 

 diminished strain upon the kidneys and diminished intes- 

 tinal putrefaction. If Chittenden's results are correct they 

 constitute a severe indictment of human instinct, for man 

 in almost every race takes a far more hberal protein 

 supply. The experiments have been subjected to con- 

 siderable criticism. The period over which they were 

 performed, long as it was, was not long enough to allow 

 conclusions to be drawn. Again, nations which for any 

 reason subsist on a low protein diet are distinguished 

 by a low degree of virihty and increased susceptibility to 

 bacterial infection. 



The Need for Individual Amino-acids 



A more serious criticism of Chittenden's theory is that 

 for the purpose of maintaining the structure of the tissues 

 it is the form of the protein that matters. The capacity 

 of the animal body to manufacture amino-acids is, as we 

 have seen, limited to very few of these. The majority 

 have to be obtained from the food. Since the amino-acids 

 are present in varying quantities in different proteins, and 

 since in some proteins certain amino-acids may be present 

 only in minute quantity, it follows that the adequacy of 

 any given protein for tissue-building depends upon its 

 content of the amino-acid present in least amount, and 

 that any protein deficient in an amino-acid which the 

 animal cannot svnthesise is inadequate even though it may 

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