THE FACTORS WHICH MODIFY THE METABOLISM 193 



facts with reference to the varying properties of the differ- 

 ent food-stuffs. 



Suppose that a man remains for two days in a calori- 

 meter, the first being a day of fasting and the second a day 

 of feeding. His occupation on the two days is made as 

 nearly identical as may be. We will assume that the 

 metabolism of the first day is found to be 2000 Calories. 

 The food value of the ration allowed for the second day may 

 be also 2000 Calories. It can be predicted that the metabo- 

 lism will rise somewhat in consequence of the taking of 

 food, but the increase will not be striking. The new total 

 will perhaps be something like 2300 Calories. A far greater 

 increment would have resulted from the prescribing of 

 moderate muscular work. The precise extent of the ad- 

 vance will be conditioned largely upon the make-up of the 

 diet. If protein is given quite freely the stimulation of 

 the metabolism will be decidedly more evident than if the 

 food furnished is almost wholly non-nitrogenous. Protein 

 is said to exert a specific dynamic effect upon the decom- 

 position processes of the body. 



The divergence between protein and other types of food 

 in the matter of increasing the metabolism has been care- 

 fully estimated by Rubner. He has given us some helpful 

 figures. If the fasting heat production of an animal is 

 represented by the number 100, the requirement will not 

 be exactly met by a supply of any food having this value, 

 but must be met by larger supplies. ^Carbohydrate is the 

 most economical of the three kinds; a fasting" metabolism 

 of 100 Calories may be compensated and equilibrium of 

 income and outgo established by giving 106 Calories in the 

 form of starch or sugar. (The quantity must be that ac- 

 tually assimilated, not merely what is swallowed.) In 

 other words, feeding an animal for one day on carbohydrate 

 exclusively, and liberally enough to make the energy given 

 equal that evolved, may be expected to raise its meta- 

 bolism some 6 per cent, above the previous fasting level. 



With fat there is a slightly more positive effect in the 

 same direction. If fat is given to an animal after an in- 



13 



