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 inchides 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 varietj^ 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 six different classes of substances which are designated 

 as the foodstuffs (older names, alimentary or proximate prin- 

 ciples) on the belief that they form the useful constituents of our 

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



Foodstuffs 



Water. 



Inorganic salts. 



Proteins. 



Carbohydrates. 



Fats. 



Vitamins. 



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

 as a sulistance 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 produc- 

 tion 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 six substances named above are all foods in this sense. It 

 has been demonstrated by feeding experiments that animals can 

 be maintained and can grow normally when supplied with purified 

 proteins, fats, and carbohydrates, together with water, the neces- 

 sary inorganic salts, and the vitamins. The water and certain 

 salts of sochum, potassium, calcium, magnesium, iron, and per- 

 haps other elements are necessary to maintain the normal com- 

 position of the tissue. Complete withdrawal of any one of these 

 constituents would cause the death of the organism. The vitamins 

 form a group of organic substances found in varying amounts in 

 the ordinary foods which are not useful as sources of energy, but 

 are essential in some as yet unknown way to normal metabolism. 



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