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 

 foodstuffs and oxygen. The foodstuffs may be divided into two classes, 

 namely, (1) the organic foodstuffs, which on oxidation may serve as sources 

 of energy, and (2) the inorganic foodstuffs, 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 assigned 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 foodstuffs 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 foodstuff 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 foodstuffs at 



110 C., and the amount of inorganic constituents by ashing the dried 



remainder. Carbohydrates may be determined directly by boiling the food 



\\ ith 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 foodstuffs are by no means difficult, 

 the total analysis of the food during a metabolism experiment may become extremely 

 tedious mi account of the very large number of analyses which have to be performed. 

 The labour is. lightened l>y the fact tliaf nearly all the ordinary foodstuffs have been 

 subjected to analysis and their average composition published by the Agricultural 

 Board of the United States. Since however the foods vary in composition, especially 

 in water content, from time to time, a calculation of the total income of proteins, 



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