338 LECTURE XV. 



furthermore, made this dog do heavy work for considerable lengths of 

 time. The dog, therefore, had to perform all of its muscular work at the 

 expense of protein. This proves that protein can also serve as a source of 

 muscular energy. Under normal conditions, i.e., with a mixed diet the 

 muscle cells will first make use of the carbohydrates as a source of energy 

 and, if this is exhausted, then attack the protein. 



A much discussed question is the value of the fats as a source of muscular 

 energy. Chauveau 1 and others took the stand that fat, as such, can in 

 no case be utilized as a source of energy by the muscle cells for the per- 

 formance of work. The fat in every case must first be transformed into 

 sugar before it can be used by the muscle cells. According to this assump- 

 tion, the value of fats for the production of muscular energy could not be 

 larger than that corresponding to the quantity of sugar which this amount 

 of fat can form. Now 1 gram of fat is isodynamic with 2.56 grams of 

 dextrose when the heat units of both are taken into consideration. 



If we assume that the fats, before they can be utilized, must be changed 

 into carbohydrates, it follows that 1 gram of fat would correspond to 1 . 6 

 grams of carbohydrate, if the fat is oxidized directly to sugar. We might 

 expect to be able to determine by direct experiment how much of the 

 potential energy in fats the body can transform into muscular force. This 

 has not yet been satisfactorily accomplished. If we feed an animal with 

 a mixture of albumin, fat, and carbohydrate, the calorific value of which 

 we know exactly, we are. unable to decide by which part of the energy the 

 animal organism performs its different functions. We do not know whether 

 such a selection actually does take place in the case of a mixed diet, or if 

 it is not more probable that the organism takes all of the energy presented 

 as such, and uses it for all of its functions. Atwater 2 justly calls attention 

 to the fact that we have no means of differentiating internal from external 

 muscular work. We must also remember that in every case only a part of 

 the energy used for accomplishing work is shown by the work performed. 

 A large part of this energy is transformed into heat. We can obtain an 

 idea indirectly of the value of fats as a source of muscular work, if we regu- 

 late the conditions of the experiment so that an economical utilization 

 of the available energy is guaranteed. If in the food only barely enough 

 energy is supplied to the body to meet its requirements, or even less than 

 enough, we would expect it to utilize all of the available energy in the 

 most economical manner. Atwater has carried out such experiments. 

 Particular stress is laid on the fact that the albumin in the food must be 

 limited as much as possible in order to compare the fats and carbohydrates. 

 Atwater, therefore, used only about as much protein in severe muscular 

 work as he found necessary to maintain the nitrogen equilibrium in a 



1 A. Chauveau: Cf. Compt. rend. 121, 26, 91 (1895); 122, 429, 504, 1098, 1163, 1169, 

 1244, 1303 (1896); 123, 151, 283 (1897). 



2 Atwater: Ergeb. Physiol III, Abt. 1, p. 497 (1904). 



