SECTION I 



METHODS EMPLOYED IN DETERMINING THE 

 TOTAL EXCHANGES OF THE BODY 



THE determination of the material exchanges of the body involves an ac- 

 curate 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 Kjeldahl's 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 extractives. 

 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. The 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 means diffi- 

 cult, the total analysis of the food during a metabolism experiment may 

 become extremely tedious on account of the very large number of analyses 

 which have to be performed. The labour is lightened by the fact that nearly 

 all the ordinary food-stuffs have been subjected to analysis and their average 



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