

INFLUENCE OF FOOD ON METABOLISM 105 



In the first the destruction of fat increases slightly with the amount of fat 

 fed. In the second we see that the addition of 30-150 g. fat to 1,500 g. meat 

 (51 g. N) increases neither the fat destruction nor the total metabolism. 

 Metabolism, therefore, is not influenced by feeding fat to anything like the 

 same extent that it is by feeding proteid. 



D. METABOLISM AFTER INGESTION OF CARBOHYDRATES 



In order to decide to what extent carbohydrates fed have participated in 

 the total metabolism it is not sufficient to determine merely the N and C in 

 the excreta; for we have in these data alone no means of telling how much 

 C belonging to the nonnitrogenous substances came from carbohydrates and 

 how much from fat. But if the 2 absorbed in the same period be determined 

 also, it is possible to decide whether the nonnitrogenous metabolism has been 

 mainly from carbohydrates or mainly from fat. 



When carbohydrates alone are burned the ratio between the volumes of 

 carbon dioxide excreted and oxygen absorbed i. e., the respiratory quotient 



~~i s J ug t equal to 1; when fat alone is burned, it is only 0.71. The 



amount of C0 2 -elimination and 2 -absorption corresponding to the proteid 

 destroyed can be calculated without any difficulty from the N-excretion. But 

 a certain quantity of carbon dioxide eliminated and of oxygen absorbed re- 

 mains to be accounted for by the oxidation of fat or carbohydrate or both. 

 Now if the respiratory quotient is high (near 1) we know that carbohydrates 

 have participated to a great extent, but if it is low (near 0.75) we know that 

 fat has entered largely into the metabolism. 



So far only brief experiments performed by the use of the face mask have 

 been made along this line. From these it may be gathered: that on a carbo- 

 hydrate diet the respiratory quotient increases over that found in fasting; 

 that the carbohydrates, therefore, are attacked immediately after their ab- 

 sorption from the intestine; and that in part at least they take the place of 

 body fat in the general metabolism. 



'From these experimental facts it has been concluded further, that the 

 carbohydrates fed are all destroyed before the body fat, and also before the 

 fat in the food; and the results of all experiments in which carbohydrates 

 are ingested by the body are calculated on this basis. This conclusion, how- 

 ever, is not fully warranted by the facts, and in opposition thereto it might 

 be urged, among other things, that even in fasting the body protects its 

 glycogen; and hence this carbohydrate at least is not all destroyed before 

 the body fat. 



The matter can only be settled positively by experiments covering a long 

 period of time, in which either the amount of oxygen consumed or the amount 

 of heat lost from the body shall be determined directly. Although unfor- 

 tunately we have no experiments of this kind, still in the calorimetric studies 

 of Atwater there are some very valuable results which permit us to take 

 perfectly definite ground with regard to this question (cf. page 94). As 

 mentioned before, the total transformation of energy in these experiments was 

 both calculated indirectly from the heat of combustion of the food and ex- 



