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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
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Download Chapter ♦ Download Lecture Slides & Notes
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Q. Normal Absorption of Divalent Cations
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Ca++ balance is precarious |
The absorption of divalent cations is very slow, many times slower than monovalent ions, i.e., Ca++ is absorbed 50 times more slowly than Na+. The maintenance of a normal level of blood calcium is essential for health. The average U.S. diet contains 1,000 mg/day of calcium. There is an obligatory loss of 600 mg/day which is secreted into the intestinal lumen. The net daily absorption usually is 100 mg/day; it is an active but inefficient process. Absorption occurs throughout the small bowel, but it is highest in the duodenum and terminal ileum. The absorption of calcium increases when the physiological need increases. To stay in balance one must replenish losses in the stools and urine, as well as the added Ca++ utilized in growing children or in pregnant and lactating women. Thus, maintenance of calcium balance is precarious at best.
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| 1, 25 (OH)2 D3 also enhances transcellular Ca++ transport |
Calcium absorption requires vitamin D which is itself absorbed from the diet or is manufactured in the skin by the action of ultraviolet light on 7-dehydrocholesterol. Vitamin D must be activated by sequential hydroxylation in the liver and then the kidney, in order to stimulate the microvillous membrane of the enterocyte to synthesize the calcium-binding carrier protein necessary for active Ca++ absorption.
Mg++ is actively absorbed, but the mechanism is not yet established. About half of the 20 meq of Mg in the daily diet is absorbed - presumably along the whole length of the small bowel. |
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Pharmaceutical iron is ferrous (Fe++) |
Dietary iron consists of heme iron (about 30% of the iron in an average American diet) and non-heme iron (plant foods). From the 10-20 mg of total ingested iron, about 10% is absorbed daily (60% of which is absorbed as heme-iron). Non-heme iron is ferric (Fe+++) which is insoluble above pH3. Vitamin C not only forms a soluble chelate with iron, it reduces Fe+++ to the more soluble Fe++.
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| Iron is mainly absorbed in the duodenum |
Bile contains iron chelators and substances which reduce Fe+++ to Fe++, with which bile salts form soluble complexes that enhance iron absorption.
Dietary inhibitors of iron absorption include phytates (wheat), and oxalates (leafy vegetables, chocolate, tea, beer) so that iron absorption from different diets varies greatly. |
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| Transferrin is the serum transport protein |
Ferric iron can also be reduced to Fe++ by a brush border enzyme. The Fe++ crosses the apical cell membrane via a divalent metal transporter, and it is then released into the circulation. Hepatocytes retrieve free iron or transferrin-bound iron from the blood. The amount of iron in hepatocytes determines the expression of hepcidin, an iron regulatory hormone. Increases in circulating hepcidin down-regulate the release of iron from enterocytes so that iron is cast into the intestinal lumen as enterocytes die.
In hereditary hemochromatosis, tissue iron stores are greatly increased due to excessive iron absorption that is not constrained by normal downregulatory feedback mechanisms. |
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Next Section (R): Malabsorption of Divalent Cations » |
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