GENERAL METABOLISM AND DIETETICS 329 



usually chosen. This occurs mainly in the urine, to a small extent in 

 the faeces and sweat. The total nitrogen of the urine is usually deter- 

 mined by Kjeldahl's process (see p. 455), and to this 1 gramme of 

 nitrogen is added that is, the average amount excreted in the faeces. 

 The nitrogen of the faeces is derived in part from unabsorbed food, 

 and in part from the various secretions of the alimentary tract. The 

 above average has been ascertained by experiments upon animals 

 placed on a nitrogen-free diet. It has to be borne in mind that several 

 days are required for the elimination of all the nitrogen taken in in 

 the case of some proteins. The nitrogen is eliminated at varying 

 rates when different types of proteins are ingested. 



The proportion of sulphur in urine to nitrogen is 1 : 5-2 about 

 the same as in protein. The determination of the sulphur excretion 

 is therefore a guide as to the breaking down of protein and a control 

 of the nitrogen determinations. The determination of the protein 

 metabolized from the sulphur excretion has the advantage that in 

 general the sulphur of the protein is more quickly e % xcreted than the 

 nitrogen, but has the disadvantage that, being small in amount, the 

 experimental error is likely to be greater. 



Protein yields a certain amount of carbon combined with the 

 nitrogen of the urinary excreta. It has been determined that for 

 every gramme of nitrogen excreted 0-67 gramme of carbon is excreted. 

 This ratio is constant, so that it is not necessary to estimate the 

 proportion of carbon in the nitrogenous bodies of the urine. 



Protein on its oxidation also yields C0 2 and water. The C0 2 

 output is more easy to estimate than the water output of the body. 



Of the carbon dioxide excreted a small part comes from protein, 

 the remainder from fat and carbohydrate. The amount of carbon 

 coming from protein is found by multiplying the amount of nitrogen 

 excreted by 3-3, since the proportion of carbon to nitrogen in protein 

 is 3-3 : 1. Since the tissues contain far more fat than carbohydrate, 

 any carbon retained in the body is usually reckoned as fat. Each 

 gramme of carbon represents 1-3 grammes of fat, the proportion of 

 carbon to the total weight of fat. A small amount of carbon is lost 

 as fat in the faeces. 



Let us suppose that with the above intake the output was 



Carbon. Nitrogen. 



In urine 11(16-5x0-67) 16-5 



In faeces . . . . . . 5 1*0 



In breath . . . . . . 254 



270 17-5 



There is a retention in the body of 30 grammes of carbon and 

 2-5 grammes of nitrogen. This nitrogen = 2-5 x 6-25 = 15-625 grammes 

 of protein. In this protein there is 2-5 x 3-3 = 8-25 grammes of carbon; 

 so that 30-8-25 = 21-75 grammes of carbon is represented as fat. 

 To estimate this as fat we must multiply the carbon by 1-3 (fat 

 contains 76 per cent, of carbon), 21-75 x 1-3=28-275. Therefore on 



