90 METABOLISM AND NUTRITION 



to about 15-16 g., but exhibits wide variations, depending primarily upon the 

 quantity of nitrogen ingested in the food. 



The quantity of carbon in the urine as compared with that in the expired 

 air is very small. Only when very great exactness is desired does it need to be 

 determined directly, for it can generally be calculated without any considerable 

 error from the nitrogen of the urine. The ratio N : C in the urine exhibits but 

 very slight variations; according to Atwater, it has for a mixed diet a mean 

 value of about 1:0.72 (0.64-0.79). 



(4) Faces. The faeces are composed partly of unabsorbed residues of the 

 food, partly of residues of the digestive fluids, and partly of worn-out epithelial 

 cells and excretory products from the alimentary tract. In a fasting man 

 these residues make up a faecal mass, which contains from 0.11 to 0.32 g. of 

 nitrogen per twenty-four hours. When a nonnitrogenous diet, or one very 

 poor in nitrogen, is given, from 0.5 to 0.4 g. of N appear in the fasces per day 

 (Rubner, Rieder). This quantity of N must evidently have its origin in the 

 intestine itself. We can say, therefore, that the intestine has a very large share 

 in the formation of the fasces, and that in round numbers one gram of the N 

 eliminated as a product of metabolism is to be found in the intestinal evacu- 

 ations. The nitrogen contained in the bacteria of the faeces is also included 

 in this figure. 



Since experiments from Pawlow's laboratory (Chapter VII) show that the se- 

 cretion of the digestive juices and their N-content present considerable variations 

 with different diets, one might be tempted to look upon the total quantity of 

 nitrogen in the faeces as a pure product of metabolism. But this is not true, for 

 many observations have shown that with certain articles of diet a considerable 

 part of this faecal nitrogen actually represents a residue of the food. 



In any given case therefore it is quite impossible to decide how much of 

 the faecal nitrogen comes from the one source and how much from the other. 

 For this reason it has become customary to regard the total nitrogen in the 

 faeces as a residue of the food. Although it must be admitted that such an 

 assumption is quite incorrect from a purely theoretical point of view, it makes 

 no difference in the calculation of results of metabolism experiments. For if 

 we suppose that the faeces are exclusively a product of metabolism, the implica- 

 tion is that all the food was absorbed without loss; and vice versa, if we regard 

 the faeces as only a residue of the food, then the quantity utilized must be 

 diminished by the mass of the faeces. In both cases we reach exactly the same 

 result with regard to the amount of metabolism actually taking place. In this 

 presentation of the subject of metabolism, therefore, we shall reckon the faeces 

 as a residue of the food. 



Respecting the nonnitrogenous substances given off in the faeces, we may 

 merely mention here the fact that fat occurs in appreciable quantity both on 

 a fat-free diet and in fasting. In the latter case 0.6-1.4 g. per day have been 

 found in the fasces, and on a fat-free diet, 3-7 g. per day. If, therefore, on a 

 fat diet the faeces do not contain more than 7 g. of fat per day, we can say 

 that the fat in the food has been almost entirely absorbed in the intestine. 



In the faeces the ratio of N:C for a mixed diet is about 1:9.2 (6.8-13.8). 

 Inasmuch as the quantity of nitrogen in the faeces ordinarily does not amount 

 to more than 2 g. per day, in most cases it is sufficient to calculate the carbon 

 from the nitrogen. 



