THE METABOLISM DURING STARVATION 



703 



may form a considerable fraction of the total metabolism of the 

 body, and can hardly be excluded without introducing serious errors. 

 The relative parts played by protein, carbohydrate, and fat respec- 

 tively in the chemical exchanges of a starving animal may be deter- 

 mined in the following way : The amount of protein consumed is 

 given by estimating the total nitrogen of the excreta by Kjeldahl's 

 method and multiplying the result by the factor 6-25. The loss of 

 weight of the body minus the protein consumed may be roughly 

 taken as equivalent to the fat plus carbohydrate consumption. But 

 this is only a rough method, since the quantity of water in the tissues 

 may undergo considerable variations, and so affect the total weight 

 of the body. For any accurate results the respiratory exchanges 

 must be measured, including both oxygen intake and carbon dioxide 

 output. After deducting the carbon dioxide due to the combustion 

 of the carbon of the proteins which does not appear in the urine in 

 combination with nitrogen, the remainder of the carbon dioxide 

 is derived entirely from carbohydrate and fat metabolism. Knowing 

 the respiratory quotient and the total amount of carbohydrate and fat 

 metabolism, it is possible to come to a conclusion as to how much 

 of the carbon dioxide is derived from oxidation of glycogen and 

 how much from the oxidation of fat. A very efficient check on this 

 calculation is furnished if the individual or animal can be placed 

 at the same time in an accurate calorimeter, as in Benedict's experi- 

 ments, owing to the fact that a gramme of fat when converted into 

 carbon dioxide and water produces more than double the amount 

 of heat which would be evolved by the complete oxidation of glycogen. 

 In Benedict's experiments the heat-value of the metabolism cal- 

 culated by the above method agreed with the heat as actually measured 

 by the calorimeter within 0-5 per cent., whereas if the total carbon 

 of the first day had been reckoned as fat, the discrepancy would 

 have been as high as 5 per cent, in many cases. The influence of 

 glycogen metabolism on that of protein during the first and second 

 days of fasting is shown in the following experiments (Benedict) : 



