INTERMEDIATE METABOLISM OF CARBOHYDRATES 397 



energy liberated by the body (work plus heat) corresponds to 9.33 

 small calories 1 for every kilogram-meter of work performed, while if the 

 carbohydrate be replaced by fat, the total liberation of energy is 10.37 

 calories for the same amount and kind of work. Now 2.35 small 

 calories are equivalent to one kilogram-meter of mechanical work, so 

 that on a carbohydrate diet 25 per cent, of the excess of energy-dissipa- 

 tion due to work was actually converted into mechanical work, and on a 

 fat diet 22.7 per cent. There is thus little difference of efficiency 

 whether fats or carbohydrates furnish the source of energy. Now if 

 fats had first of all to be converted into carbohydrates, before they 

 could be utilized for work, a great deal of oxygen would have to be 

 introduced into the molecule, since the fats contain a much higher pro- 

 portion of hydrogen to oxygen than the carbohydrates. If all this 

 preliminary oxidation were unavailable for the production of muscular 

 energy, not less than 29 per cent, of the energy of the fat would be 

 wasted, and we would expect the performance of mechanical work on a 

 fat diet to be only two-thirds as efficient as upon a carbohydrate diet. 

 It is highly probable; therefore, that fats undergo but little preliminary 

 modification before they are available for muscular work. They are 

 not the first choice of the muscles, however, if all dietary materials are 

 available, carbohydrates are used first. Fats are pressed into the 

 service when carbohydrates begin to fail, and proteins form a last 

 resource. 



The performance of muscular work involves a considerable increase 

 of oxygen-intake, and carbon-dioxide output. The final products 

 of muscular exertion are therefore carbon dioxide and water. The 

 oxidation of glycogen or its hydrolytic cleavage-product, glucose, is not 

 accomplished in a single step, however. Intermediate products are 

 transiently formed, and of the nature of many of these we can only 

 form conjectures which, however, are gradually becoming more and 

 more clearly defined as persistent research reveals, one after another, 

 the various substances which may arise from the oxidations of glucose 

 in the animal body. One of the first of these intermediate products 

 to be clearly recognized was, however, Lactic Acid. 



The lactic acid which is found in muscular tissue is not the ordinary 

 racemic acid which may be obtained by synthesis in laboratory-glass- 

 ware. It is the dextrorotatory acid, or Sarcolactic Acid: 



CH 3 



I 



CHOH 

 I 

 COOH 



which, when pure, forms a viscous, acid syrup, forming crystalline salts 

 with a variety of bases. The zinc salt is the one usually employed for 

 the isolation and estimation of lactic acid in muscular tissues. 



1 The small calorie is the heat required to raise the temperature of 1 gram of water 

 one degree. The large calorie is the heat required to raise the temperature of a kilogram 

 of water one degree. 



