SECTION I 



METHODS EMPLOYED IN DETERMINING THE 

 TOTAL EXCHANGES OF THE BODY 



THE determination of the material exchanges of the body involves 

 an accurate comparison of its income and output. The income 

 consists of the food-stuffs and oxygen. The food-stuffs may be 

 divided into two classes, namely, (1) the organic food-stuffs, which 

 on oxidation may serve as sources of energy, and (2) the inorganic 

 food-stuffs, such as salts and water. 



The latter class neither add to nor subtract from the total energy 

 of the organism, but their presence is a necessary condition of all 

 vital processes, and as they are contained in the various excreta 

 a corresponding amount must be present in the food in order to make 

 good this loss. 



In spite of the bewildering complexity of the nature of the foods 

 taken by man, their essential constituents can always be confined 

 , to the three classes, proteins, fats, and carbohydrates, and any 

 analysis of the food must give the relative amounts present of these 

 three classes of substances. The approximate analysis of the 

 food-stuffs presents little difficulty. The nitrogen is determined 

 by KjeldahPs method. The figure thus obtained is multiplied by 

 the factor 6'25, and the resulting figure is taken to represent the total 

 protein in the food. Of course such a valuation may give too high 

 a value when the food-stuff is one that is rich in nitrogenous extrac- 

 tives. The total fat is determined by extracting the food in a 

 Soxhlet apparatus with ether. It is advisable to precede this 

 extraction by an extraction with boiling alcohol. The total ethereal 

 and alcoholic extract obtained is reckoned as fat. Th.e amount of 

 water is determined by drying the food-stuff at 110 C., and the 

 amount of inorganic constituents by ashing the dried remainder. 

 Carbohydrates may be determined directly by boiling the food 

 with dilute acids in order to convert all its disaccharides and 

 polysaccharides into hexoses, which are then reckoned as glucose, 

 and estimated by their copper-reducing power. In most cases, 

 however, the total protein, fat, and ash are subtracted from the dried 

 weight of the food and the remainder is taken as carbohydrate. 



Although the methods for the analysis of food-stuffs are by no 



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