Digestion and Absorption of Carbohydrate


Carbohydrate in diet 

The principal dietary carbohydrates are polysaccharides, disaccharides, and monosaccharides.

  • Polysaccharides are present in approximately all non-animal foods, especially grains (starch).
  • Disaccharides - sucrose (cane sugar).
  • Disaccharides - lactose (disaccharides- lactose.
  • Other carbohydrates: Amylase, glycogen, alcohol, lactic acid, pyruvic acid, pectin dextrin, etc.
  • Daily requirement: The average dietary intake of carbohydrates is 380-800 gm/day which represents 50-60% of the diet.


Digestion of carbohydrate

Mouth and stomach

During mastication (chewing), food is mixed with saliva which contains an enzyme known as 'ptyalin' (α-amylase) secreted by the parotid gland. This enzyme hydrolyzed starch into disaccharides (maltose) and other small polymers of glucose-containing 3-9 glucose molecules.


Starch   α-amylase       maltotriose and α-limit dextrin.
            -----------------

In the mouth, only 3-5% of all the starches eaten can be hydrolyzed because food remains in the mouth for a very short period. After the food is swallowed, digestion of starch by α-amylase continues in the stomach for as long as 1-3hours, after which the activity of α-amylase is blocked by the acidic gastric juices. The optimum pH for the activity of this enzyme is 6-7 but becomes inactive at a pH of 4.0.


Small intestine

In the small intestine, the potent pancreatic -amylase acts on ingested polysaccharides. It is similar in function to salivary α-amylase, but is more powerful 15-30 minutes after chyme enters the duodenum and mixes with the pancreatic juices, all the starches digested.


Hydrolysis by intestinal epithelial enzymes

The intestinal epithelial enzyme hydrolyzed disaccharides and small glucose polymers into monosaccharides. These enzymes, which include lactase, sucrase, maltase, and dextrinase are located in the membrane of the microvilli of the brush border of the lumen of the small intestine.


Absorption of carbohydrates

Carbohydrates are mostly absorbed in the form of monosaccharides and only a small percentage is absorbed in disaccharide form. Glucose and galactose are combined with the active transport of sodium. Carrier proteins have receptor sites for glucose, galactose, and sodium. Fructose is transferred by facilitated diffusion. The transported monosaccharide then move out of the epithelial cells by facilitated diffusion and enter the capillaries of the will from finally the general circulation.