CHAPTER XVIII. 



METABOLISM WITH VARIOUS FOODS, AND THEIR NECESSITY 



TO MAN. 



I. EXCHANGE OF MATTER AND FORCE IN GENERAL WITH METHODS OF STUDY. 



THE conversion of chemical energy into heat and mechanical work 

 which characterizes animal life, leads, as previously stated in Chapter I, 

 to the formation of relatively simple compounds carbon dioxide, urea, 

 tc. which leave the organism, and which, moreover, being very poor in 

 energy, are for this reason of little or no value to the body. It is there- 

 fore 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 that which has been 

 exhausted. This is accomplished 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 w r hich 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 foodstuffs 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. 



Such knowledge can be attained only by a series of systematic and 

 thorough observations, in wrhich the quantity of nutritive material, rela- 



822 



