158 THE BODY AT WORK 



is very great, involving the digestive organs, the liver, and 

 the kidneys in an excessive amount of work. On the other 

 hand, it is possible to reduce the consumption of proteins to 

 a minimum by substituting for them fats and carbohydrates. 

 But, again, after the proper balance is disturbed, the substitu- 

 tion ceases to be a simple problem in arithmetic. The carbon- 

 food has to be increased out of all proportion to the protein 

 which it replaces. If a dog which is being fed on a diet natural 

 to it chiefly meat is in a condition of nitrogenous equilibrium, 

 carbohydrate may be substituted for some of the meat. But 

 from the very beginning it is found that, if nitrogenous equili- 

 brium is not to be disturbed (if the dog is not to be induced to 

 consume its own tissues), a weight of carbohydrate must be 

 given considerably greater than the weight of the protein 

 withdrawn. The disproportion increases as the experiment 

 proceeds, until perhaps 12 to 15 grammes of carbohydrate 

 have to be substituted for every gramme of protein. The 

 proteid food has now come down to 1-5 gramme per kilo- 

 gramme of the animal's weight. Owing to the increase of 

 carbohydrate, the caloric value of the total food, nitrogenous 

 and non-nitrogenous, is several times as great as the animal 

 requires. The surplus is oxidized without any equivalent in 

 work. At about this point the experiment is brought to an 

 end, owing to the failure of the digestive organs to deal with 

 so large a mass of food. 



The value of gelatin as an article of diet is of interest in this 

 connection. Gelatin is not, strictly speaking, a protein, and 

 it cannot be built up into the tissues. It does not prevent, 

 nor even delay, starvation. Yet up to a certain point it can 

 be used as a substitute for proteid food. In the observation 

 just referred to, protein might be withdrawn at any stage, 

 without disturbing nitrogenous equilibrium, by substituting* 

 about 2 grammes of gelatin for every gramme of protein 

 withdrawn. It spares protein, although it does not take its 

 place. It is said that the minimum of protein necessary for 

 the maintenance of nitrogenous equilibrium may be reduced 

 to about one- half by the substitution of gelatin. This has 

 been interpreted as indicating that when we have reduced the 

 oxidation of nitrogenous substance to its smallest amount 

 the nitrogen comes from two sources in about equal propor- 



