SECTION IV 



THE EFFECT OF MUSCULAR WORK ON 

 METABOLISM 



WHEN an animal performs muscular work its total output of energy 

 must be increased, and this must take place at the expense either 

 of an increased intake of food or an increased destruction of the 

 tissues of the body. For many years a theory put forward by Liebig 

 received universal acceptance among physiologists. According to 

 this view the food-stuffs could be divided into two classes, namely, 

 the proteins, which were the plastic food substances and were con- 

 cerned in the building up of tissues, and, secondly, the fats and carbo- 

 hydrates, which took no part in the building of the tissues, but were 

 oxidised for supplying heat to the body. Muscular work was sup- 

 posed to be attended with a breakdown of the muscular tissue, 

 and therefore to be performed at the expense of an increased protein 

 metabolism, which had to be made good by adding to the protein 

 intake in the food. If this view were correct one would expect to 

 find a great increase in the nitrogenous metabolism of the body with 

 every increase in muscular work. Such an increase has, in fact, been 

 found by Pfliiger in dogs which were nourished on a purely protein 

 diet. In these animals the sole source of energy to the organism was 

 protein, and therefore any additional call on the energies of the body 

 must be associated with an increased intake of food or an increased 

 loss of material from the body in the shape of protein. In an animal 

 which is enjoying a normal mixed diet, or has a store of carbonaceous 

 material in its tissues in the shape of fat, there is no increase of nitro- 

 genous metabolism during muscular work which would correspond in 

 any way to the amount of work done. This was shown long ago by 

 Voit in experiments on the dog. The following Table represents the 

 nitrogenous metabolism, i.e. the amount of muscular material in the 

 shape of protein metabolised during the day, under varying conditions 

 of rest and activity, during starvation and after food : 



DOG I 



Food Flesh metabolism Condition of <lo^ 



Grm. 



.... 164 . . Rest 



.... 167 .. Work 



.... 149 .. Best 



713 



