PART III. HUMAN PHYSIOLOGY 



XXVII. FOODS 



Why we need Food. — We have already defined food as anything 

 that forms material for the growth or repair of the body of a plant or 

 animal, or that furnishes energy for it. Food, then, not only fur- 

 nishes our body with material to grow, but also gives us the 

 energy we expend in the acts of walking, running, breathing, 

 and even in thinking. 



Nutrients. — Certain nutrient materials form the basis of food of 

 both plants and animals. These have been stated to be carbohy- 

 drates (starches, sugars, gums, etc.), hydrocarbons (fats and oils, 

 both animal and vegetable), proteids (such as lean meat, eggs, the 

 gluten of bread), and mineral matter and water. Oxygen, although 

 not a nutrient, ought to be considered as food because it enters 

 into the composition of the body, and without it no energ}^ could 

 be released. Let us apply this general knowledge with reference 

 to the human body in order to determine the uses made of food 

 taken into the body; for parts of the human body, be they muscle, 

 blood, nerve, bone, or gristle, are built up from the nutrients in our 

 food. 



The Body a Machine. — The body has been likened to a machine 

 in that it turns over the latent or potential energy stored up in 

 food into kinetic energy (mechanical work and heat) which is 

 manifested when we perform work. One great difference exists 

 between an engine and the human body. The engine uses fuel 

 unlike the substance out of which it is made. The human bodv, 

 on the other hand, uses for fuel the same substances out of which 

 it is formed; it may, indeed, use part of its own substance for food. 



Let us now consider more in detail the nutrients used by man 

 as food, and determine the use of each to him. 



Proteids. — As we know, proteids, in some manner unknown 

 to us, are manufactured in the leaves of plants. Proteid sub- 



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