THE ENERGY BALANCE. 85 



called, it has been found that the number of calories liberated 

 by burning one gramme of each of the proximate principles of 

 food is as follows : 



( Starch ... .4.1 



Carbohydrates ] . 



( Sugar 4.0 



Protein 5.0 



Fat 9.3 



The same number of calories will be liberated at whatever rate 

 the combustion proceeds, provided it results in the same end 

 products. When a substance, such as sugar or fat, is burned in 

 the presence of oxygen, it yields carbon dioxide and water, which 

 are also the end products of the metabolism of these foodstuffs 

 in the animal body ; therefore, when a gramme of sugar or fat 

 is quickly burned in a calorimeter, it releases the same amount 

 of energy as when it is slowly oxidized in the animal body. But 

 the case is different for proteins, because these yield less com- 

 pletely oxidized end products in the animal body than they yield 

 when burned in oxygen; so that, to ascertain the physiological 

 energy value of protein, we must deduct from its physical heat 

 value (calories) the physical heat value of the incompletely ox- 

 idized end products of its metabolism. It is obvious that we can 

 compute the total available energy of our diet by multiplying 

 the quantity of each foodstuff by its calorie value. 



In order to measure the energy which is actually liberated in 

 the animal body, we must also use a calorimeter, but of some- 

 what different construction from that used by the chemist, for 

 we have to provide for long continued observations and for an 

 uninterrupted supply of oxygen to the animal. Animal calor- 

 imeters are also usually provided with means for the measure- 

 ment of the amounts of carbon dioxide (and water) discharged 

 and of oxygen absorbed by the animal during the observation. 

 Such respiration calorimeters have been made for all sorts of ani- 

 mals, the most perfect for use on man having been constructed 

 in America (see Fig. 8). As illustrating the extreme accuracy 

 of even the largest of these, it is interesting to note that the act- 

 ual heat given out when a definite amount of alcohol or ether is 



