190 NUTRITIONAL PHYSIOLOGY 



mind that the large quantities of protein which they con- 

 sume are a source of sugar within the body. Gram for 

 gram, it must be remembered, sugar and fat are not equiv- 

 alent. One gram of fat has the energy of approximately 

 2i grams of carbohydrate. Quantities of the two food- 

 stuffs which are in this proportion (and hence equal in 

 fuel value) are said to be isodynamic. The principle may 

 be illustrated as f ollows : 50 grams of fat is withdrawn from 

 a day's ration and 113 grams of starch substituted. A 

 simple computation will show that the heat value of the 

 diet has neither been increased nor diminished. The 

 calories lost through the removal of the fat are 465, while 

 those introduced with the starch are practically the same. 

 Of course, the possibility of such substitutions is much 

 limited by considerations of palatability and individual 

 variations of digestive power. 



The fact that carbohydrate and fat are freely used to 

 yield energy for muscular movement does not exclude pro- 

 tein from such service. A carnivorous animal can be kept 

 for a long time strong and active upon a diet composed 

 almost wholly of protein. Yet it remains possible that 

 carbohydrate is the chosen fuel during feeding of this 

 kind. When enough protein is given to secure nitrogen 

 equilibrium and to furnish energy to the full extent nqeded 

 for the work done, the uncombined amino-acids must give 

 rise to abundant sugar, and it may well be that this isfne 

 chief factor in the muscular metabolism. An engine is 

 made of iron and steel, but its regular fuel is coal ; a muscle 

 is made essentially of protein, but protein is not its usual 

 fuel. Protein food must be needed in definite amounts for 

 the original development of muscles and also for the in- 

 crease in their substance under conditions of training. It 

 is not clear that any considerable supply is needed for their 

 best working when the desired level of development has 

 been reached. This distinction between growth and 

 operation has important bearings upon theories of hygiene. 



Mental States and Metabolism. After hearing of the 

 striking correspondence between the degree of muscular 



