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Updated: 12/26/06 02:17 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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T. Malabsorption of Vitamins
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| Vitamin D malabsorption leads to malabsorption of calcium |
If mixed bile salt micelle formation is defective for any reason (low luminal pH, bile salts below the critical micellar concentration, inadequate triglyceride hydrolysis), then, the absorption of fat-soluble vitamins will be impaired. Both impaired vitamin D absorption and dietary deficiency of vitamin D cause calcium malabsorption, resulting in the bone diseases rickets in growing children and osteomalacia in adults. Vitamins K and A are also poorly absorbed, with resultant clotting abnormalities and night blindness (nyctalopia).
Malabsorption of water-soluble vitamins is not usually a problem except for vitamin B12 and folic acid, both of which are essential for the integrity of the hematopoietic system. |
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| Vitamin B12 malabsorbtion in ileal disease, in loss of intrinsic factor secretion by gastric parietal cells, and in intestinal bacterial overgrowth |
Vitamin B12 absorption may be impaired in surgical excision or Crohn's disease of the ileum. It may be observed rarely in severe celiac sprue involving the whole length of the small bowel. Diseases which abolish gastric production of intrinsic factor will also impair vitamin B12 absorption. An example is pernicious anemia, a disease in which parietal cells are absent from the stomach so that no intrinsic factor can be synthesized. Abolition of gastric intrinsic factor production is also invariable after total gastrectomy and occasionally follows partial gastrectomy if postoperative gastritis has caused complete gastric mucosal atrophy with loss of parietal cells. Disease states associated with excessive bacterial growth in the small bowel impair vitamin B12 absorption either by binding or bacterial utilization of the vitamin. Fish tapeworms also can compete for vitamin B12 and produce a factor splitting B12-IF complex. In a rare familial disorder, vitamin B12 is malabsorbed because of lack of the specific ileal transport protein. Pancreatic insufficiency also impairs the efficiency of B12 absorption because food proteins bound to vitamin B12 must be removed from the vitamin by pancreatic enzymes if the vitamin is to be coupled to gastric intrinsic factor preparatory to absorption.
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Folic acid malabsorption in jejunal disease
Subtle causes of folate deficiency |
Folic acid malabsorption occurs in diseases of the jejunal mucosa, such as celiac sprue. It has been shown that certain anticonvulsant and birth control drugs can interfere with the intestinal conjugases and therefore prevent polyglutamate splitting to absorbable folic monoglutamate. The present evidence indicates that alcohol can inhibit folate uptake by the gut as well as folate release from the liver.
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Next Section (U): Diagnosis of Small Intestinal Diseases » |
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