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Carbohydrate Digestion and Absorption

  • The human gastrointestinal system can only absorb a few types of carbohydrates, and three contribute the most number of calories in a normal diet. The most important carbohydrate is starch which is composed of large, branched chains of sugars. The human alimentary tract can also absorb sucrose, a disaccharide composed of glucose and fructose, as well as Lactose, a disaccharide composed of glucose and galactose. Although the normal human diet contains a substantial amount of cellulose, this carbohydrate cannot be digested and is thus not considered a dietary carbohydrate. The small intestine mucosa can only absorb monosaccharides and so digestion requires processing of these dietary carbohydrates into their individual component sugars.
  • Overview
    • Digestion of starch begins within the small intestinal lumen whereas digestion of short sugar chains occurs directly on the luminal surface of the small intestine epithelium just prior to absorption.
  • Luminal Digestion
    • As mentioned, only starches are digested within the luminal fluid of the small intestine where they are broken down into monosaccharide and disaccharide units. Digestion begins in the oral cavity with salivary amylase and continues in the small intestine with pancreatic amylase.
  • Epithelial Digestion
    • The brush border of the small intestine epithelium possesses membrane-bound enzymes which further digest disaccharides into absorbable monosaccharides. The major enzymes include lactase, sucrase, and maltase (Maltose is a disaccharide released from starch which is composed of two molecules of glucose).
  • End Result
    • The end result of carbohydrate digestion is the production of a small selection of monosaccharides (Glucose, Galactose, or Fructose) which can be transported past the small intestine epithelium.
  • Overview
    • Because individual monosaccharides are small molecules, they can be transported through the membrane of small intestine epithelium via membrane porters. Specialized porters are used for different monosaccharides and both active and passive transport processes are employed.
  • Glucose and Galactose
    • Glucose and Galactose are transported by secondary active transport via Na+-Glucose or Na+-Galactose symporters. The energy for this resorption is provided by an NaK ATPase on the basolateral membrane of small intestine enterocytes which reduces the cytosolic concentration of Na+ inside the cells and thus drives luminal resorption of sodium along with monosaccharides.
  • Fructose
    • Fructose is transported past the luminal membrane via passive diffusion down its electrochemical gradient. Consequently, fructose transport is much less efficient than that of gluocse and galactose.