60 



POODS AST) NUTEITION 



fails to replace completely the worn out parts, the body 

 loses in power, and finally its action ceases altogether, and 

 what we call death follows. To furnish new material for 

 anabolism is what is meant by the use of nutrients as 

 building materials. 



If we recall that tissues are composed of cells, and cells 

 of protoplasm, we shall see that this function of nutrients 

 reaUy resolves itself into the building up and repair of 

 broken down protoplasm. The particular nutrients that 

 are best suited to use as building material are proteids 

 or the nitrogen compounds. The fact that proteid and 

 protoplasm are nearly identical in composition has a 

 new significance in the light of the above facts. How 

 far the other forms of nutrients share in this use as build- 

 ing material, is at present uncertain. In fact, experiment 

 has shown that food free from inorganic salts will not 

 support life, thus demonstrating that these substances 

 are an essential part of the tissue structure. 



Fuel. — Whether carbohydrates and fats are ever used 

 as building material or not, this is not their principal 

 function in the body. Their main purpose is to serve as 

 fuel, and thereby to produce what we call energy, or the 

 power to do work. 



Meaning of energy. — Whenever our muscles move 

 parts of the body, as in running, leaping, walking, and eat- 

 ing, we have examples of this power to do work, or energy. 

 Whence comes this power? An example may help us to 

 understand better the nature and production of energy. 

 An energy producing machine with which we are all 

 familiar is the steam engine. Such an engine derives its 

 power to do things from the coal which we put in the fire- 

 box. Here this fuel burns (that is, oxidizes, see p. 18), 



