CHAPTER IX 



MAINTENANCE (Continued) THE REQUIREMENTS OF 

 MATTER 



As was pointed out in the introduction to the previous Chap- 

 ter (361), the maintenance requirement is a twofold one, calling 

 for the presence in the feed of adequate amounts of certain 

 specific substances as well as for an adequate supply of energy. 

 The former phase of maintenance in some of its broader aspects 

 forms the subject of the present Chapter. These specific sub- 

 stances may be grouped for the purpose of this discussion as 

 proteins or their constituents, ash ingredients and accessory 

 constituents. 



i. THE PROTEIN REQUIREMENTS FOR MAINTENANCE 



398. Nature of protein requirement. As was shown in 

 Chapter VII (340), the loss of protein which the fasting body 

 suffers may be interpreted in two ways. First, it may be re- 

 garded as due to the complete breaking down of a certain amount 

 of protein as , the necessary accompaniment of cell activity 

 (Rubner's " wear and tear " quota). Second, it may be sup- 

 posed that certain atomic groupings contained in the protein 

 molecule may be indispensable for the normal functioning of 

 the body, so that, if they are not contained in the feed, body 

 protein may be katabolized for the sake of obtaining them. 



In either case, it is clear that what the feed must supply in 

 order to maintain the body in nitrogen equilibrium is not, 

 strictly speaking, protein as such, but materials whose digestive 

 cleavage will yield certain amounts and proportions of the con- 

 stituent amino acids. On the first hypothesis, the requirements 

 for the different " building stones " would be determined sub- 

 stantially by the quantities of each existing in the body tissues 



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