THE EFFECT OF MUSCULAR WORK ON METABOLISM 685 



In the experiment already quoted the oxidation of carbon was increased 

 more than threefold during a six-hour period of work. This carbon might 

 form part of the molecule of protein, fat or carbohydrate ; but if we com- 

 pare the protein metabolism of the same individual during these experiments 

 no corresponding alteration is observed. During rest and starvation the 

 average output of nitrogen per day corresponded to the destruction of 

 82 grm. of protein. During rest and with an approximately sufficient amount 

 of food the average daily consumption of protein ^as 98-8 grm. During a 

 work day, in which the individual received the same amount of protein in 

 the food and a somewhat insufficient quantity of carbohydrates and fats, 

 the consumption of protein was 1094 grm. Thus there was a three- to 

 fourfold increase of the carbon metabolism of the body, but only a 10 per 

 cent, increase in the protein metabolism. 



There is another method by which we can arrive at some idea of the 

 nature of the material which is furnishing by its oxidation the necessary 

 energy for the performance of muscular work. It is evident, if we compare 

 the formulae of a carbohydrate and a fat respectively, that it will require a 

 relatively larger amount of oxygen to oxidise the fat than is necessary in the 

 case of the carbohydrate. In the latter there is sufficient oxygen to combine 

 with all the hydrogen present and form water. The whole of the oxygen 

 therefore which is taken in is employed in the oxidation of the carbon, 

 and one volume of oxygen will produce one volume of carbon dioxide. Thus : 

 C 6 H 12 6 + 60 2 = 6H 2 + 6C0 2 . If the whole of the animal's energy 

 requirements were furnished by the oxidation of carboydrates, the output 

 of carbon dioxide expired would be exactly equal in volume to the oxygen 



. C0 2 expired 

 inspired, and the respiratory quotient of the animal, namely, -~ A - - 



would be equal to unity. In fats the amount of oxygen is only sufficient to 

 combine with four atoms of the hydrogen of the molecule. When fats 

 undergo oxidation, of the oxygen used only a portion is devoted to the 

 formation of carbon dioxide, the rest being employed in the oxidation of 

 hydrogen to water. In an animal using only fats the carbon dioxide output 

 of the body would be considerably less than the oxygen intake and its 

 respiratory quotient would be less than unity. The respiratory quotients 

 for protein, fats, and carbohydrates are given in the following Table. 



Material Respiratory quotient ^^ 



Carbohydrates . . . . . . .1-0 



Animal fat ....... 0-707 



Protein . 0-81 



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. 



