CHAPTER XL. 



GENERAL CONSIDERATIONS UPON THE COMPOSITION 

 OF THE FOOD AND THE ACTION OF ENZYMES. 



Foods and Foodstuffs. The term food when used in a popular 

 sense includes everything that we eat for the purpose of nourishing 

 the body. From this point of view the food of mankind is of a most 

 varied character, comprising a great variety of products of the 

 animal and vegetable kingdoms. Chemical analysis of the animal 

 and vegetable foods shows, however, that they all contain one or 

 more of five different classes of substances which are usually 

 designated as the foodstuffs (older names, alimentary or proximate 

 principles) on the belief that they form the useful constituent of our 

 foods. The classification of foodstuffs usually given is as follows: 



f Water. 



Inorganic salts. 

 Foodstuffs < Proteins. 



Carbohydrates. 

 I^Fats. 



From the scientific point of view, a foodstuff or food may be defined 

 as a substance absolutely necessary to the normal composition of 

 the body, as in the case of water and salts, or as a substance which 

 can be acted upon by the tissues of the body in such a way as to 

 yield energy (heat, for example) or to furnish material for the pro- 

 duction or repair of living tissue. Moreover, to be a food in the 

 physiological sense, the substance must not directly or indirectly 

 affect injuriously the normal nutritive processes of the tissues. 

 The five substances named above are all foods in this sense. The 

 water and certain salts of sodium, potassium, calcium, mag- 

 nesium, iron, and perhaps other elements are absolutely necessary 

 to maintain the normal composition of the tissue. Complete 

 withdrawal of any one of these constituents would cause the death 

 of the organism. Proteins, fats, and carbohydrates, on the other 

 hand, are substances whose molecules have a more or less com- 

 plex structure. When eaten and digested they enter the body 

 liquids, and are employed either in the synthesis of the more com- 

 plex living matter or they undergo various chemical changes, spoken 

 of in general as metabolism, which result finally in the breaking up 

 of their complex molecules with a liberation of some of their internal 



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