METABOLISM 545 



amounts of nitrogen and carbon excreted and to calculate from the 

 known composition of protein how much protein must have undergone 

 metabolism. We then compute how much carbon this quantity of pro- 

 tein would account for, and we deduct this from the total carbon excre- 

 tion. The remainder of carbon must have come from the metabolism of 

 fats and carbohydrates, and although we can not tell exactly which, yet 

 we can arrive at a close approximation by observing the respiratory quo- 

 tient (R. Q.), which is the ratio of the volume of carbon dioxide exhaled 



CO 



to that of oxygen retained by the body in a given time, i. e., . 2 . By ob- 



u 2 



serving this quotient, therefore, we can approximately determine the 

 source from which the nonprotein carbon-excretion is derived. 



Having in the above manner computed how much of each of the proxi- 

 mate principles has undergone metabolism, we next proceed to compare 

 intake and output with a view to finding whether there is an equilibrium 

 between the two, or whether retention or loss is occurring. 



It may serve to make clear the methods by which these calculations are 

 made to study the following example : 



Example of a Metabolism Investigation. It is desired to know whether a diet con- 

 taining 125 grams protein, 50 grams fat, and 500 grams carbohydrate is sufficient for a 

 man doing a moderate amount of work. 



INTAKE 



Carbon Nitrogen Calories 



Protein, 62 gm. 20 gm. 512.5 



Carbohydrate, 200 2050.0 



Fat, 38 465.0 



Total, 300 gm. 20 gm. 3027.5 



OUTPUT 



Carbon Nitrogen 



In urine, 11 gm. (16.5 x 0.67) 16.5 gm. 



In feces, 5 1.0 



In the breath, 254 



Total, 270 gm. 17.5 gm. 



Eetained in Body. 30 gm. carbon and 2.5 gm. nitrogen. This amount of nitrogen repre- 

 sents 2.5 x 6.25 = 15.6 gm. protein or 75 gm. muscle. Now, this amount of protein will 

 account for 8.25 gm. carbon; so that 30- 8.25 ^=21.75 gm. carbon represents 21.75 x 

 1.3=: 28.3 gm. fat. On this diet, therefore, the subject retains in his tissues 15.6 gm. 

 protein and 28.3 gm. fat per diem. 



Furnished with these data we may now proceed to compute how much 

 energy must have been liberated in the body. 



To express the above result in terms of energy liberated, we know that 

 3027.5 C. were supplied and that all these have been used except 15. 6 X 

 4.1=64 retained as protein, and 28.3x9.3=263.2 retained as fat; or in 

 toto 327.2 C. We find, therefore, that 3027.5 327.2 = 2,700 C. have been 

 required. 



