The Digestive System 145 



Carbohydrates. — Carbohydrates are the chief source of energy for 

 the body. They enter the blood stream from the small intestine as 

 simple sugars, and are taken for temporary storage to the liver. Here 

 they are transformed into the polysaccharide, glycogen, and are changed 

 again to the soluble sugars as they are needed by the body. Excess 

 sugars can be changed into fat for more permanent storage — a well- 

 known phenomenon. When oxidized, sugars burn completely to pro- 

 duce water and carbon dioxide. 



Fats. — Fats enter the blood stream from the small intestine as 

 tiny droplets. They are utilized for energy by the body, producing 

 carbon dioxide and water when completely burned. More important is 

 their storage as a reserve food supply. They are stored under the skin 

 and in various other spots. Inasmuch as fat is a poor conductor of 

 heat, it is an excellent insulating material. The fat stores are utilized 

 only after prolonged fasting. 



Proteins. — The proteins are broken down into amino acids which 

 then are able to pass into the blood stream from the small intestine. 

 They can be utilized for energy production, but more important is their 

 use for repair and rebuilding of tissues. The cells of the body utilize 

 these amino acids as "building blocks" for constructing their own 

 unique proteins. 



For the repair and building of tissue, certain amino acids are abso- 

 lutely essential. Some of these can be synthesized by the body, but 

 most must be present in the food. In general, animal proteins are more 

 concentrated and useable than are plant proteins. It is true that all 

 the amino acids can be found in plants, but an immense variety of 

 plant material must be eaten if one is to be sure to get all of them. 



Water. — Water furnishes no energy, but is absolutely essential for 

 all cellular activity, and forms a large percentage of the total body 

 composition. 



Inorganic Salts. — Salts are continuously added to and removed 

 from the body tissues. Some form an essential part of various struc- 

 tures : calcium and phosphorus in bone and teeth ; iodine in the thyroid ; 

 and iron in hemoglobin. Further they play an important role in main- 

 taining cell permeability. 



Vitamins. — Until men started suffering from a lack of vitamins, 

 their existence was unknown. Now it is a well-known fact that vitamins 

 are essential to good health. They do not produce energy, but appear 



