EFFECT OF MUSCULAR WORK ON METABOLISM 641 



/'i/ \ 



Material Respiratory quotient 2 



Starch 1-0 



Cane sugar . . . . . .1-0 



Glucose 1-0 



Animal fat 0-711 



Protein 0-809 



The respiratory quotient in an animal at any given time is therefore 

 determined by the nature of the substances which are undergoing oxidation 

 in its body. If the performance of muscular work involved special chemical 

 processes, a metabolism of one of the main constituents of the body in 

 preference to either of the others, this sudden change in the quality of the 

 metabolism should show itself in the respiratory quotient. 



According to Speck and Lowy, moderate muscular work which is not 

 associated with dyspnoea, although attended by a large increase in the 

 carbon dioxide output and the oxygen intake of the body, does not alter the 

 respiratory quotient. This is probably correct if the respiratory changes 

 are taken over a sufficient length of time, when the respiratory quotient 

 must depend on the food which is furnished to the body however the energy 

 of this body is expended. When however, as in Benedict's and Cathcart's 

 experiments, the observations are of short duration, muscular exercise is 

 almost always found to be associated with a rise in the respiratory quotient, 

 which lasts too long to be accounted for by a mere washing out of the carbon 

 dioxide from the blood and the body tissues. After the cessation of the 

 exercise the respiratory quotient sinks below normal. The respiratory 

 quotient however rarely rises even during exercise to 1 as it would if the 

 muscular work were performed solely at the expense of carbohydrates. 



These results suggest that while muscular work can be performed at the 

 expense of any of the food- stuffs or of the three classes of constituents of 

 the body, carbohydrate is the immediate or the most readily available source 

 of muscular energy. When the body passes suddenly from a resting to an 

 active condition the first call is therefore on the carbohydrates of the blood 

 and those stored up in the muscles and liver, whereas after exercise these 

 carbohydrate stores are slowly replenished probably at the expense of the 

 proteins. The fact that the body can draw on its fat stores for the perform- 

 ance of muscular work suggests that this substance may also serve as a source 

 of muscular energy, but there is no evidence that the body is able to convert 

 fats under any circumstances into carbohydrates, so that we must assume 

 that under certain conditions the fats or their decomposition products may 

 be directly utilised by the muscles. 



21 



