A. Fat soluble vitamins (i.e. A, D, E and K)
Along with the lipids, these are absorbed as a part of micelles (see above) in the upper small intestine.
B. Water soluble vitamins
Most are absorbed in the upper small intestine; vitamin B12 is absorbed in the ileum.
Vitamin B12 and folate absorption is sodium-independent ; other water soluble vitamins
are absorbed by a sodium-dependent mechanism.
The gastric parietal cells secrete a vitamin B12- binding protein called intrinsic factor (IF). The IF-vitamin B12 complex binds to a receptor on ileal enterocyte and the complex is absorbed.
Calcium absorption in the small intestine is regulated to maintain calcium balance. For example, absorption is increased in calcium deficiency and decreased in calcium excess. This regulation is mediated by 1,25 DHCC (this is the active derivative of vitamin D). Normally, the dietary intake of calcium is about 1000 mg; normally, about 25 to 80 % of this is absorbed. Main site is proximal intestine (jejunum).
Calcium absorption occurs via a membrane-bound carrier; the carrier is activated by 1, 25 DHCC
Factors affecting calcium absorption
- 1, 25 DHCC :
- 1, 25 DHCC enters the enterocyte, where it inserts the calcium carrier in the luminal membrane of the enterocyte.
- calcium (and also magnesium) absorption is increased by protein
- calcium absorption is inhibited by phosphates and oxalates (because these form insoluble salts with calcium)
Site of absorption :
Almost all of iron absorption occurs in the duodenum.
Forms in which iron is absorbed :
Iron can be absorbed as :
- Haeme (as present in meat)
- Free ion
Ferrous ion (Fe++) is absorbed much more efficiently than ferric ion (Fe3+). Most of the dietary iron is in the ferric form. Thus, it needs to be converted into ferrous form for absorption.
1. Conversion of ferric to ferrous : This occurs in
- stomach : The stomach acid tends to break the insoluble iron complexes within the chyme and thus releases the iron from the complexes. Once the iron is free, it is converted from ferric to ferrous in the presence of ascorbic acid (vitamin C)
- intestine : In the enterocyte brush border, the transporter for iron (called DMT 1) has ferric reductase activity.
2. Mechanism of iron absorption from lumen to cell :
- Haeme is absorbed by a haeme transporter called HT
- Ferrous ion is absorbed by a iron transporter called DMT 1 transporter.
- Inside the cell :
- Haeme oxidase acts on haeme to release ferrous ion and porphyrin ferrous ion
- Some ferrous ion is converted to ferric ion; the ferric ion â€˜combines withâ€™ an iron-binding protein called apoferritin to form ferritin (see below).
a. from cell to interstitium :
The ferrous ion which is not converted into ferric ion is transported across the basolateral membrane by a transporter called ferroportin 1; a protein called hephaestin (or Hp) helps ferroportin 1 in the basolateral transport of ferrous ion.
b. In the plasma :
Here, ferrous ion is converted into ferric ion and is bound to the transport protein called transferrin; transferrin is a beta-1 globulin.
Ferrous form :
Only this form can get across the cell membrane ; the iron in haeme is in ferrous form
Ferric form :
This forms most of the dietary protein, present in ferritin, hemosiderin, transferrin.
F. Regulation of iron absorption
1. Normal absorption :
Iron absorption is necessary for maintaining normal iron balance. Very little (0.75 mg in males and 1.5 mg in females) of the 15-25 mg of iron ingested each day is actively absorbed (approximately 3 to 6% of the ingested iron is absorbed).
2. Iron absorption from intestine is regulated :
In iron deficiency, more iron is absorbed; in iron excess, less is absorbed. There is a close association between iron levels and the amount of ferritin and transferrin.as shown below:
a. High iron levels cause ferritin to increase and transferrin to decrease.
b. Low iron levels cause ferritin to decrease and transferrin to increase.
3. Dietary factors affecting iron absorption :
Phytic acid (which is found in cereals), phosphates and oxalates form insoluble compounds with iron in the intestine; thus, they decrease iron absorption.
4. Ferritin micelle :
Ferritin is the tissue storage form of iron; it is present in enterocytes and other cells.
The ferric ion and the protein apoferritin together form ferritin micelles; ferritin micelles consist of ferric ion (in the form of ferric hydroxyphosphate) in the center surrounded by 24 subunits of apoferritin. Each ferritin micelle contains some 3000 to 4500 ferric atoms.
5. Significance of ferritin
- Normally, there is very little ferritin in the plasma. However, in patients with excess iron, the amount of ferritin in the plasma increases. The amount of ferritin in the plasma can be used as an index of body iron stores.
- Ferritin can be easily seen under electron microscope; thus, it can also be used as a marker for phagocytosis etc.
6. Hemosiderin molecule
This consists of aggregated deposits of partly degraded ferritin molecules in lysosomal membranes. It contains much more iron than ferritin molecule.
Ferritin molecule contains 23% iron whereas the haemosiderin molecule may contain 50% iron.