BUILDING MATERIAL 59 



converted into animal proteid, and carbohydrate. If 

 now we note what happens when animal bodies and plant 

 bodies die, we find that in the process called decay these 

 nutrients are again broken down into mineral salts and 

 gases and thus complete a cycle. The plant world there- 

 fore stands between the mineral and the animal world as a 

 manufacturer of nutrients. 



Uses of nutrients. — Very careful experiments have 

 been performed in the past and are being performed at 

 the present time to determine the exact use of each kind of 

 nutrient to the body. What is already known in this 

 direction may be summarized as follows : 



Nutrients serve two purposes in the body. First, they 

 furnish the material out of which our tissues are built and 

 repaired. Second, they may be oxidized in the body, and 

 when this action occurs the result is the production of 

 either action (movement) or heat; both action and heat 

 being forms of energy. These two uses are usually ex- 

 pressed by saying that nutrients furnish the body with 

 either building material or fuel. 



Building material. — All the organs and tissues of the 

 body are continually tmdergoing two processes, a building 

 up and a tearing down. These two processes are spoken 

 of collectively as metabolism. The building up is called 

 anabolism, and the tearing down katabolism. Anabolism 

 and katabolism vary in amount at different periods of our 

 life. In childhood the anabolism is greater than kata- 

 bolism and the body increases in size and weight, that is, 

 it grows. In middle age the two processes are nearly 

 equal in amount so that the new material merely serves 

 to repair or replace the worn out parts. Finally, in old 

 age, katabolism becomes the greater, the new material 



