84 METABOLISM AND NUTRITION 



nitrogen-containing substances other than proteid occur in both animal and 

 vegetable foods, the quantity as calculated by this method is too high. Espe- 

 cially with a low percentage of proteid a considerable error might thus arise. 

 Moreover, the kind of proteid eaten is not a matter of indifference, since one 

 may well imagine that different proteid bodies behave differently in metabolism. 

 The little we know on this subject will be summarized later. (2) As fat, is 

 reckoned the total extract with ether, although this contains also other sub- 

 stances soluble in ether. (3) The dry substance and (4) the ash constituents 

 are determined by desiccation at 100 C. and by incineration respectively. 

 (5) The carbohydrates are determined usually by subtracting from the total 

 dry substance the proteid -f- the fat -f- the ash. 



Finally, under the ingesta is to be reckoned the oxygen, methods for the 

 determination of which will be given under B. 



B. DETERMINATION OF THE EXCRETA 



The products of metabolism are eliminated by the lungs, skin, intestine 

 and kidneys. 



The excreta resulting from the combustion of organic foods, to which we 

 shall confine ourselves for the present, contain the following elements : N, S, 

 P, C, H, and 0. Nitrogen, sulphur and phosphorus are derived from pro- 

 teids; carbon, hydrogen and oxygen are contained in all the organic foods. 

 In estimating the excreta we have therefore to determine quantitatively the 

 amount of N, S, P, C, and H eliminated within a certain time. 



The determinations can be simplified considerably, however. Ordinarily, 

 in order to find out how much proteid has been metabolized in the body, it 

 is sufficient to determine the amount of nitrogen eliminated. One need not 

 consider the sulphur and phosphorus unless the investigation is especially 

 concerned with the behavior of the phosphorus-containing proteids. The anal- 

 ysis for a complete metabolism experiment therefore can be restricted to 

 N, C, and H. Commonly the excretion of hydrogen also is neglected. 



The amount of proteid destroyed in the body is obtained by multiplying 

 the amount of nitrogen eliminated as a product of metabolism by 6.25. 



Analysis for the elements found in the excreta is by no means always suffi- 

 cient; in many cases it is necessary either for the purpose of gaining a deeper 

 insight into the mode of the metabolic processes, or in order to estimate the 

 percentage of combustible materials in the faeces, to determine the separate 

 compounds quantitatively. In the latter case the analysis is carried out in 

 precisely the same way as in making similar determinations for the food to 

 be ingested. 



If the metabolism experiment is to be of any use whatever, it is necessary 

 to collect every trace of the excreta for exactly the period covered by the 

 experiment. The urine and the faeces offer no particular difficulties in this 

 respect, although some remarks with regard to the latter seem called for. 



It is apparent at once that analysis of the faBces can only be of importance 

 for the study of metabolism, if they can be identified as belonging to a definite 



