THE CARBOHYDRATES AND THEIR METABOLISM 249 



a soluble starch, then going through various stages of dextrins, 

 until it finally reaches the stage of the perfectly soluble disaccharide, 

 maltose. 



It is impossible, at present to sharply separate the different inter- 

 mediary products in starch digestion. The different stages, however, can 

 be recognized by means of the iodin reaction. The native starches give 

 a blue coloration with iodin, and as digestion progresses dextrins are 

 formed which give at first a violet, red, then brown red, and finally no 

 color reaction at all with iodin. These dextrins arc known respectively as- 

 erythrodextrins and achroodextrins. 



In the salivary secretion we find another enzyme which acts on maltose 

 and is known as maltase. It acts on the maltose molecule, making it un- 

 dergo hydrolysis, and converting it into two molecules of glucose. 



Gastric Digestion of Carbohydrates. In the gastric secretion there 

 are no enzymes which attack carbohydrates. As long as the acidity of the 

 gastric contents is low the ptyalin and maltase, which are swallowed with 

 the saliva, may continue their activity. When the gastric acidity in- 

 creases in concentration it may help in hydrolyzing the disaccharides, but 

 this takes place only to an insignificant extent. 



Intestinal Digestion of Carbohydrates. In the pancreatic secretions 

 we find an amylolytic enzyme which has all the properties of ptyalin, but 

 which has the power of acting at a much greater velocity. The intestinal 

 juices also contain three enzymes: sucrase, which has the power of split- 

 ting sucrose into glucose and levulose; maltase, which splits maltose into 

 two molecules of glucose, and lactase, which splits lactose into glucose 

 and galactose. All the carbohydrates, therefore, are brought down in the 

 intestinal canal to the stage of monosaccharides. Separate enzymes are 

 present there for all types of carbohydrates that the human individual 

 ingests, except cellulose, which is left entirely untouched, and is eliminated 

 as such. 



Absorption of Carbohydrates 



The products of carbohydrate digestion are very soluble and easily dif- 

 fusible. The amount that is absorbed by the stomach is very small and 

 of no practical consequence. Practically all of the digested carbohydrates 

 are absorbed in the small intestines and very little is left in the material 

 that reaches the ileocecal valve. 



All the absorbed carbohydrates are carried away by the blood stream 

 into the portal vein, thence to the liver. It is remarkable that in spite 

 <>f the easy solubility of sucrose and lactose, none of it is absorbed under 

 ordinary circumstances. The intestinal wall is almost impermeable to 

 them, whereas maltose may be absorbed to a slight extent. The body 

 cells have the power of utilizing maltose, probably because of the pres- 

 ence of a maltase in the blood stream, but cannot utilize sucrose or lactose; 



