Feedi 



16 



The animal must receive in its food all nutrients necessary for the 

 construction and repair of metabolizing tissues and, in addition, suffi- 

 cient oxidizable substances to furnish the energy required. Although 

 the animal body is often compared with an internal-combustion en- 

 gine or similar machine, there is at least one essential difference. 

 When fuel is burned in an internal-combustion engine, heat and 

 mechanical energy are produced from the chemical energy of the fuel. 

 When food is eaten by the animal, the same phenomenon occurs but 

 with the essential difference that the food also contributes to the 

 manufacture of new tissues and to the repair of old ones, whereas fuel 

 cannot contribute to new cylinders and pistons in the engine. There- 

 fore the engine wears out and deteriorates, whereas the living machine 

 is continuously being remade with new materials. 



A complete food might be defined as one that contributes to every 

 need of the body when this food is the sole article of diet for an ap- 

 preciable period of time. It is impossible to find a single foodstuff 

 that meets the exacting requirements of a healthy body. Perhaps 

 milk most nearly approaches this ideal, but we shall find later that 

 even milk may be lacking in a number of essential factors. 



Most of the foodstuffs that we eat are deficient in many respects. It 

 is for this reason that man has found by experience that dietary 

 diversity encourages health and growth, whereas both suffer if one- 

 sided diets are ingested for appreciable periods of time. Later chap- 

 ters discuss the needs for the various food constituents and consider 

 the fate of these substances during metabolism. 



In this chapter we shall devote our attention primarily to the 



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