METABOLISM DURING STARVATION 



673 



THE METABOLISM OF CARBOHYDRATE, FAT, AND 

 PROTEIN DURING STARVATION 



Since during starvation no energy is supplied to the body from without 

 in the shape of foodstuffs, we can regard the whole expenditure of the 

 animal during its period of starvation as occurring at the expense of its 

 capital. The amount of carbohydrate which can be stored up as glycogen 

 or other forms is strictly limited. In many experiments the glycogen meta- 

 bolism has therefore been entirely disregarded, and it has been estimated 

 that the chemical capital of the body consisted entirely of proteins and fats. 

 The glycogen metabolism however, during the first day of a period of starva- 

 tion, 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 respectively in the 

 chemical exchanges of a starving animal may be determined 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 equiva- 

 lent 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 ex- 

 changes must be measured, including both oxygen intake and carbon dioxide output. 

 After deducting the carfeon 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. Know- 

 ing 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 experiments, 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 calculated 

 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) : 



The total metabolism per kilo body weight very soon attains a constant 

 level. The relative part taken in the production of the total energy by fats 

 43 



