HUMAN STRUCTURE AND LIFE-ACTIVITIES 495 



engine. Food for the human engine and fuel for a steam- 

 engine are both necessary because they contain stored 

 energy. It is interesting to recall ( 116) that in both cases 

 the original source of the stored energy was sunlight. 



418. Foods for Energy. The energy value of the three 

 important nutrients (proteins, carbohydrates, and fats) is 

 easily computed by chemists. Samples of such nutrients are 

 dried and then quickly burned inside an apparatus known as 

 a calorimeter (heat-measurer), and the heat thus generated 

 is measured in terms of the amount of heat necessary to raise 

 1000 grams of water one degree Centigrade. This amount 

 of heat is a calorie, the standard unit for heat measurements ;* 

 and since heat may be converted into other forms of energy 

 ( 416), it is convenient for measuring the total energy of 

 foods. 



The heat value of a gram of food measured in this way is 

 as follows : for dry protein, about 5.6 calories (i.e., 5600 grams 

 of water 1) ; for carbohydrates, about 4.1 ; and for fats, 

 about 9.3. It has been shown by a method described in the 

 next paragraph that a man or a higher animal obtains 

 9 calories from fats, and 4 from carbohydrate foods; but 

 protein is less completely oxidized in the living body, so that 

 one gram of protein gives only about 4 calories of the 5.6 it 

 contains. Obviously proteins are not economical as foods for 

 heat or muscular energy, while carbohydrates and fats give 

 almost as much energy in the body as when they are com- 

 pletely burned in a chemist's calorimeter. The chemical 

 result for fats and carbohydrates is the same in both cases, 

 for these foods produce carbon dioxide (CO 2 ) and water 

 (H 2 O) in both the living body and the calorimeter. 



Many substances which will produce heat energy in a 



* Some authors use the "small calorie," which is the amount of heat 

 necessary to raise one gram of water one degree C. It has an advantage in 

 measuring small quantities of heat less than enough to raise 1000 grams of 

 water one degree. 



