CHAPTER XVII. 



METABOLISM WITH VARIOUS FOODS, AND THEIR NECESSITY 



TO MAN. 



I. GENERAL DISCUSSION AND METHODS USED IN THE STUDY OF MATTER 

 AND FORCE METABOLISM. 



THE conversion of chemical energy into heat and mechanical work 

 which characterizes animal life, leads to the formation of relatively 

 simple compounds carbon dioxide, urea, etc. which leave the organism, 

 and which, moreover, being very poor in energy, are for this reason of 

 little or po value to the body. It is therefore absolutely necessary for 

 the continuance of life and the normal course of the functions of the body 

 that the organism and its different tissues should be supplied with new 

 material to replace tjiat which has been exhausted. This is accom- 

 plished by means of food. Those bodies are designated as food which 

 have no injurious action upon the organism and which serve as a source 

 of energy and can replace those constituents of the body that have been 

 consumed in metabolism or that can prevent or diminish the consumption 

 of such constituents. 



Among the numerous dissimilar substances which man and animals 

 take with the food all cannot be equally necessary or have the same value. 

 Some perhaps are unnecessary, while others may be indispensable. We 

 have learned by direct observation and a wide experience that besides the 

 oxygen, which is necessary for oxidation, the essential foods for animals 

 in general, and for man especially, are water, mineral bodies, proteins, 

 carbohydrates, and fats. 



It is also apparent that the various groups of food-stuffs necessary for 

 the tissues and organs must be of varying importance; thus, for instance, 

 water and the mineral bodies have another value than the organic foods, 

 and these again must differ in importance among themselves. The 

 knowledge of the action of various nutritive bodies on the exchange of 

 material from a qualitative as well as a quantitative point of view must 

 be of fundamental importance in determining the value of different 

 nutritive substances relative to the demands of the body for food under 

 various conditions, and also in deciding many other questions for instance, 

 the proper nutrition for an individual in health and in disease. 



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