NUTRITION 141 



fore the amount of nitrogen multiplied by 6-25 (16 X6-25 =100) 

 is used as the measure of the amount of protein. By deducting 

 the amount of nitrogen in the faeces from that in the food the 

 amount of nitrogen which has been absorbed is obtained. 

 The amount of nitrogen in the urine gives the amount of 

 nitrogen excreted. As the animal does not store appreciable 

 quantities of nitrogen* the normal condition is that of nitro- 

 genous equilibrium in which the nitrogen excreted equals the 

 nitrogen absorbed. 



From the nitrogen figures it is possible to calculate the 

 amount of carbon and hydrogen which has been used in the 

 form of protein ; by deducting these from the total figures 

 for the carbon and hydrogen of the food, faeces and urine 

 respectively the amount of carbon and hydrogen used in the 

 form of carbohydrate and fat is obtained. 



RESPIRATORY QUOTIENT 



We can gain further information from the measurement of 

 the respiratory gas exchange. 



When carbohydrate is oxidised, owing to there being 

 sufficient oxygen in the molecule to combine with the hydrogen 

 to form water, the volume of carbon dioxide formed is exactly 

 equal to the volume of oxygen used up. Thus C 6 H 12 O 6 -f 6O 2 

 = 6CO 2 +6H 2 O. 



In the oxidation of fat and protein some of the oxygen is 

 required to combine with hydrogen to form water, therefore 

 the volume of oxygen taken in is greater than the volume of 

 carbon dioxide given off. We express this as a ratio of 

 volume of carbon_dioxi^ given off called ^ irat 



volume of oxygen absorbed 

 quotient (R.Q.)- 



As pointed out above the respiratory quotient for carbo- 

 hydrate is i, but for fat and protein it is less than i, namely 

 071 and 0-80 respectively. 



The amount of nitrogen excreted being known the amount 

 of carbon dioxide and oxygen corresponding to the oxidation 

 of the equivalent amount of protein can be calculated. On 

 subtracting these amounts from the total values for carbon 

 dioxide and oxygen the corrected values give the amount of 

 carbon dioxide and oxygen corresponding to the carbohydrate 

 and fat metabolised. 



The corrected respiratory quotient having been thus 

 obtained, a simple sum in proportion gives the relative amount 

 * Plants store nitrogen as aleurone grains. 



