CHAPTER XV 



THE METABOLISM OF FATS AND CARBOHY- 

 DRATES 



IT was pointed out early in our treatment of the subject 

 that foods serve a twofold purpose : to some extent they 

 are built into the body as relatively permanent parts of its 

 structure, while in a much larger degree they are steadily 

 oxidized, yielding their energy to maintain its activities. 

 The proportion between the two divisions of the supply 

 cannot be constant. Clearly, the fraction of the diet 

 devoted to construction must be larger in childhood than 

 in adult life. There may be other periods during which the 

 constructive work is notably prominent, for example, re- 

 covery from illness or from fasting, pregnancy, and perhaps 

 athletic training. Apart from these times we must assume 

 that the actual building of tissue is a very small item. In 

 other words, the living matter of the body is comparatively 

 stable and needs only slight though perfectly definite 

 contributions to insure its up-keep from day to day. 



The food-stuffs entering the circulation may be destined 

 for immediate destruction or for storage. Throughout 

 long terms of our lives a fair balance is preserved between 

 the income and the consumption of these compounds. 

 If we receive in one day certain quantities of proteins, 

 fats, and carbohydrates, and there is evidence of an exactly 

 equal decomposition of the three classes of material, we 

 cannot say with precision whether the oxidation affected 

 the particular food eaten or corresponding matter stored 

 previously, but in either case the condition of the system 

 at the beginning and at the end of the twenty-four hours 

 is the same. We must now proceed to discuss the possibil- 

 ities of transformation and retention of the different food- 



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