THE ENERGY OF THE METABOLISM 177 



a gram of hydrogen gas (11 liters) is oxidized to. water. 

 One gram of carbon oxidized to carbon dioxid gives nearly 

 8 Cal. These two illustrations do not bear directly on our 

 physiologic inquiry, for the body does not use the free 

 elements for oxidation, but their compounds. It is, there- 

 fore, of more interest to turn to the fuel values of carbo- 

 lydrates, fats, alcohol, and proteins, since these are the 

 actual sources of heat and kindred energy. The calorific 

 value of a compound is not precisely that of the carbon and 

 the hydrogen contained in it and not yet bonded to oxygen, 

 although some early work of a useful kind was based upon 

 that assumption. It is to be noted that the oxygen in the 

 physiologic compounds reduces their potency; the less they 

 contain the more largely they will consist of elements sub- 

 ject to oxidation. This is the main reason why fats have 

 fuel values greatly in excess of those of carbohydrates. 

 A gram of fat contains nearly twice as much carbon as a 

 gram of sugar. It also contains much hydrogen with un- 

 satisfied affinities for oxygen. 



The actual heat production observed when a gram of 

 starch is burned is a trifle more than 4 Cal. Sugars, which 

 are slightly richer in oxygen than starch is, have a little 

 lower fuel value. The figure (4 Cal.) is fairly representa- 

 tive of carbohydrates as a class. The oxidation of 1 

 gram of fat liberates about 9.3 Cal., or 2J times as much as 

 starch. A gram of grain alcohol fully oxidized gives an 

 intermediate quantity, about 7 Cal. All these non-nitrog- 

 enous compounds are made to yield the same simple prod- 

 ucts, namely, carbon dioxid and water, whether they are 

 destroyed by literal burning outside the body or by the 

 metabolic processes. We shall see shortly that the energy 

 which is found to be set free in their oxidation can be 

 proved to be equal in the two cases. 



Protein stands somewhat apart in its behavior. A gram 

 of dried protein burned in oxygen gives nearly 6 Cal. 

 But some of the products generated in such a laboratory 

 test are not those which the body forms from protein and 

 excretes. Urea, for example, is not found after the actual 



