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 or six 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: 



/ Water. 



I Inorganic salts. 

 ) Proteids. 



Foodstuffs ( Albuminoids, a group of bodies resembling proteids, but hav- 

 } ing in some respects a different nutritive value. 

 I Carbohydrates. 

 \ 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 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 or six 

 substances named above are all foods in this sense. The water and 

 certain salts of sodium, potassium, calcium, magnesium, 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. 

 Proteids, fats, and carbohydrates, on the other hand, are substances 

 whose molecules have a more or less complex structure. When 

 eaten and digested they enter the body liquids and are employed 

 either in the synthesis of the more complex living matter, or they 

 undergo various chemical changes, spoken of in general as metab- 

 olism, which result finally in the breaking up of their complex 



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