Digestion of carbohydrates in human body

  • The food that we consume contains three types of carbohydrates namely; polysaccharides, oligosaccharides and disaccharides which need digestion.
  • The polysaccharides include starch, glycogen and cellulose.
  • The oligosaccharides include dextrins.
  • The disaccharides are sucrose (cane sugar or table sugar) an lactose (milk sugar).
  • All these carbohydrates are digested by carbohydrases to monosaccharaides like glucose, fructose and galactose.
  • Carbohydrates are digested in the following way:

1. Buccal cavity (mouth):

  • The food receives saliva in the buccal cavity or mouth.
  • Saliva contains enzymes salivary amylase (ptyalin) and lysozyme.
  • Ptyalin is activated by chloride ions present in the saliva itself.
  • Ptyalin splits starch and glycogen first into dextrins and then into double sugars maltose and isomaltose and small dextrins called ‘limit dextrins’.
  • Mastication mixes the food with saliva and breaks it into small particles with larger total surface area which facilitates the action of ptyalin.
  • Water and mucin of saliva help in the teeth in masticating by moistening the food.
  • Salivary digestion in oral cavity is restricted because of the shorter time the food is retained there.
  • About 30% of the starch in the food is hydrolyzed in the oral cavity.
  • Lysozyme kills the microbes present in food.

Starch and Glycoogen ————> Maltose + Isomaltose + ‘Limit’ Dextrins

(by salivary amylase)

2. Stomach:

  • Salivary digestion of carbohydrates continues in the stomach till ptyalin is destroyed by HCl of the gastric juice.
  • This takes about half an hour and the gastric juice itself has no carbohydrases.

3. Small intestine:

  • The food meets two juices in the small intestine; pancreatic juice and the intestinal juice.

Pancreatic juice:

  • It contains a carbohydrase named pancreatic amylase which hydrolyses more starch and glycogen into dextrins and the latter to double sugars maltose and isomaltose, and ‘limit’ dextrins.

Starch and Glycoogen ————> Maltose + Isomaltose + ‘Limit’ Dextrins                                        (by pancreatic amylase)

Intestinal juice:

  • It contains six carbohydrases: Intestinal amylase (traces only), maltase, isomaltase, ‘limit’ dextrinase, sucrase and lactase.
  • Intestinal amylase hydrolyses the remaining starch and glycogen into dextrins and the latter to double sugars maltose and isomaltose, and ‘limit’ dextrins.
  • Maltase splits double sugar maltose into single sugar glucose.

Maltose ———-> Glucose (by maltase)

  • Isomaltase splits double sugar isomaltose into single sugar gluose.

Isomaltose ———-> Glucose (by isomaltase)

  • ‘Limit’ dextrinase converts ‘limit’ dextrins into single sugar glucose.

‘Limit’ dextrins ———-> Glucose (by ‘limit’ dextrinase)

  • Sucrase breaks the double sugar sucrose into single sugars glucose and fructose.

Sucrose ———-> Glucose + Fructose (by sucrase)

  • Lactase hydrolyses the double sugar lactose into single sugars glucose and galactose.

Lactose ———-> Glucose + Galactose (by lactase)

  • Cellulose in not digested in humans. It is digested by symbiotic microorganisms (bacteria and ciliates) living in the gut of herbivorous mammals.

Thus, the entire carbohydrate part of the ingested food is changed into glucose, fructose and galactose which are called the end products of carbohydrate digestion and are absorbable from the intestinal mucosa.

Digestion of carbohydrates in human body