CHAPTER XI 

 METABOLISM, NUTRITION, AND DIET. 



THE term metabolism means, literally, an exchange of material. In its 

 broadest physiological sense it includes the study of the exchange of material 

 between the living tissues of the body and their surrounding media. This 

 includes the study of the income and outgo of material; the storing of energy- 

 yielding materials in the body; the transfer of this potential energy into kinetic 

 energy; and the nutritional processes within the various tissues. The build- 

 ing up of absorbed food material into the protoplasm of the cell or of simpler 

 compounds into more complex ones, which may be stored in the cell, is known 

 as anabolism, and the compounds themselves as anabolites. The breaking 

 down of these substances into simpler forms, whereby the potential energy 

 of the anabolites is transformed into kinetic energy, is known as katabolism, 

 and its products as katabolites. 



In order to form an estimate of these processes going on in the body, the 

 amount and nature of the ingested material must be known, as well as the 

 amount of refuse or unused material that passes out of the alimentary canal 

 as feces, and the amount of excreted material from the various excretory 

 organs. It is also necessary to know the potential energy of the ingested 

 materials, and the possible potential energy must be checked against the ac- 

 tual energy liberated. 



The food is intended to supply the place of the material which has been 

 utilized by the body, and, in a simpler form, eliminated in the excretions. 

 But in the choice of a diet this is not enough; the food should be sufficient 

 to supply such need without waste and without unduly increasing the output 

 of excreta, while at the same time the body should be maintained in health, 

 without increase or loss of weight. The food must also supply the energy 

 liberated without undue waste of the tissues themselves. 



These requisites of a diet scale then allow for wide alterations in the 

 amount of different kinds of foods under different circumstances. Numer- 

 ous and most valuable experiments have been performed in recent years to 

 determine just what each article of the common food materials contributes to 

 the growth of the tissues and to the kinetic energy liberated by the tissues. 

 The potential energy of the food can also be checked against the kinetic 

 energy liberated. A single illustration of this class will serve. In an experi- 

 ment with mixed food lasting through four days, on a man with body weight 

 of 64 kilograms, and doing a minimum amount of work, Atwater made the 

 following determinations: 



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