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 building 

 up of absorbed food material into the protoplasm of the cell or of simpler com- 

 pounds 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 catabolism, 

 and its products as catabolites. 



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. Numerous 

 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 experiment 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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