ii 4 GENERAL SCIENCE 



to do their best work and the whole body may begin to 

 feel the results coming from the reduced activity of the 

 cells. This condition of the body is usually known as 

 sickness. 



The cells of the body are very active; some organs 

 never cease their activity except for fractions of a second; 

 and new cells are continually being made in the body 

 to replace the old ones which have been used up or oxi- 

 dized. For these reasons a sufficient amount of food must 

 be taken into the body to supply the fuel and the building 

 material necessary to keep up all these vital activities. 

 There are some foods which we eat that are used only for 

 the production of heat, thus enabling the body to move 

 about freely. There are other foods which are used for 

 the repair of old cells that have been torn down and for 

 building up new ones when the body is growing. All 

 the foods that we eat may be divided into two classes, 

 which are (i) nutrients and (2) inorganic foods like water 

 and salt. 



79. Nutrients. -- The three nutrients are carbohy- 

 drates, fats, and proteids or protein. These are also 

 called organic foods or organized foods. They all come 

 from plants and animals. Since the body needs food 

 both for the production of heat and for building material, 

 it is necessary that we eat some foods which are easily 

 oxidized and produce much heat, and also eat foods 

 which are easily transformed into flesh and bone, so that 

 the body may retain its weight. The body also will be 

 kept more healthy if just a sufficient amount of each class 

 of food is eaten to produce the proper amount of fuel 

 and building material. In order to understand which 

 foods are for fuel and which build cells, it will be neces- 

 sary to study the three nutrients carefully. 



