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Updated: 03/11/08 02:32 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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P. Carbohydrate Malabsorption
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| SGLT transports glucose and galactose, but not fructose |
Certain children are born without the ability to transport glucose and galactose actively. Presumably, they lack the active coupled-carrier transport system. If these children do not have glucose and galactose removed from their diets, they will die of dehydration secondary to an osmotic diarrhea.
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Lactose intolerance is a common cause of diarrhea |
In normal adult mammals, lactase gradually disappears from the membranes of the intestinal microvillous. Most normal human beings begin losing their brush border lactase at 7 years of age. There is a mutation which is most common in populations of Northern and Central European origin (many U. S. inhabitants) which retains lactase into adult life. In the absence of lactase, the disaccharide lactose passes unabsorbed into the large intestine, taking considerable water with it. The lactose is broken down to glucose and galactose by bacteria within the colon. These hexoses cannot be absorbed by the colon, but they are fermented by bacteria into absorbable short-chain fatty acids and "gas" (CO2 and H2). The remaining unfermented hexoses increase the osmotic load causing diarrhea. Lactose malabsorption is a common cause of "gas" and diarrhea in adults who drink more than one glass of milk daily.
Rarer disaccharidase deficiencies are congenital lactase or combined sucrase-isomaltase deficiency which are present from birth. In the United States where the mutation governing persistence of lactase in adult life is common, temporary lactase deficiency is frequent after any injury to the intestinal mucosa (viral, bacterial, etc.). Severe injury to the small bowel with loss of villi causes loss of all three disaccharidases. Disaccharidase activity will be diminished in mucosal injury such as in patients with untreated celiac sprue. Carbohydrate malabsorption disappears as the small intestine becomes normal on a gluten-free diet. In the Colon Chapter, there are simplified overviews of the absorption of fat, carbohydrate, and protein. |
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Next Section (Q): Normal Absorption of Divalent Cations » |
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