DIGESTION OF CARBOHYDRATES

 

INTRODUCTION: In the diet carbohydrates are present as complex polysaccharides in form of starch, glycogen, and to a minor extent, as disaccharides such as, sucrose and lactose. They are hydrolysed to monosaccharide units in the gastrointestinal tract. Cooking makes the digestion process easier.

DIGESTION IN THE MOUTH: This process of digestion starts in mouth by the salivary alpha-amylase. However, the time available for digestion in the mouth is limited, because the gastric hydrochloric acid will inhibit the action of salivary amylase.

DIGESTION IN THE DUODENUM: In the pancreatic juice another alpha-amylase is available which will hydrolyse the alpha-1,4 glycosidic linkages randomly, so as to produce smaller subunits like maltose, isomaltose, dextrins and branched or unbranched oligosaccharides.

DIGESTION IN SMALL INTESTINES: The cells of brush border of intestine contain the enzymes, sucrase, maltase, isomaltase and lactase. They hydrolyse the corresponding disaccharides into component monosaccharides which are then absorbed.

Clinical Application; Lactose Intolerance is a medical condition that is characterised by inability to digest lactose. Lactase hydrolyses lactose to glucose and galactose. Lactase is present in the brush border of enterocytes. Deficiency of lactase leads to lactose this condition, where lactose accumulates in the gut. Irritant diarrhea and flatulence are seen. Lactase is an inducible enzyme. If milk is withdrawn temporarily, the diarrhoea will be limited. Curd is also an effective treatment, because the lactobacilli present in curd contains the enzyme lactase. Lactase is abundantly seen in yeast, which could also be used in treatment.

ABSORPTION OF CARBOHYDRATES: Only monosaccharides are absorbed by the intestine. Absorption rate is maximum for galactose; moderate for glucose; and minimum for fructose.

Absorption of Glucose: Glucose has specific transporters, which are transmembrane proteins. 1) Co-transport from Lumen to Intestinal Cell. This process is mediated by Sodium Dependent Glucose Transporter-1 (SgluT-1). 2) A membrane bound carrier protein is involved, which carries glucose, along with sodium. This sodium is later expelled by the sodium pump with utilization of energy. So energy is needed indirectly. The transporter in intestine is named as SGluT1 and the transporter in the kidney is called SGluT-2. The first one is involved in glucose-galactose malabsorption. The SGluT-2 is defective in congenital renal glycosuria.

CLINICAL APPLICATION: Common treatment for diarrhea is oral rehydration fluid. It contains glucose and sodium. Presence of glucose in oral rehydration fluid allows uptake of sodium to replenish body sodium chloride. Another Uniport System Releases Glucose into Blood. The same intestinal epithelial cells have a different transport mechanism on the membrane facing capillaries.

RELATED;

1. LACTOSE INTOLERANCE

2. METABOLIC PROFILE OF ORGANS

3.  DIGESTION OF LIPIDS

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