METABOLISM 571 



with accuracy in both intake and output. From balance sheets of intake 

 and output of carbon and nitrogen and from information obtained by ob- 

 serving the ratio between the amounts of oxygen consumed by the animal 

 and of carbonic acid excreted, we can draw far-reaching conclusions re- 

 garding the relative amounts of protein, fat and carbohydrate that have 

 been involved in the metabolism. 



As has already been stated, the essential nature of the metabolic proc- 

 ess in animals is one of oxidation that is, one by which large unstable 

 molecules are broken down to those that are simple and stable. Dur- 

 ing this process of catabolism, as it is called, the potential energy locked 

 away in the large molecules becomes liberated as actual or kinetic energy 

 that is, as movement and heat. It therefore becomes of importance to 

 compare the actual energy which an animal expends in a given time with 

 the energy which has meanwhile been rendered available by metabolism. 

 We shall first of all consider this so-called energy balance and then pro- 

 ceed to examine somewhat more in detail the material balance of the body. 



ENERGY BALANCE 



The unit of energy is the large calorie (written 0.), which is the amount 

 of heat required to raise the temperature of one kilogram of water through 

 one degree (Centigrade) of temperature.* We can determine the calorie 

 value by allowing a measured quantity of a substance to burn in com- 

 pressed oxygen in a steel bomb placed in a known volume of water at a 

 certain temperature. Whenever combustion is completed, we find out 

 through how many degrees the temperature of the water has become 

 raised and multiply this by the volume of water in liters. Measured 

 in such a calorimeter, as this apparatus is called, it has been found that 

 the number of calories liberated by burning one gram of each of the proxi- 

 mate principles of food is as follows : 



Carbohydrates j ^ ardl ' " ^ 



| Sugar 4.0 



Protein 5.0 



Fat 9.3 



The same number of calories will be liberated at whatever rate the com- 

 bustion 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 gram 

 of sugar or fat is quickly burned in a calorimeter, it releases the same 



*The distinction between a calorie and a degree of temperature must be clearly understood. The 

 former expresses quantity of actual heat energy; the latter merely tells us the intensity at which the 

 lieat energy is being given out. 



