ABSORPTION OF IRON

ABSORPTION OF IRON

• Absorbed from upper small intestine mainly duodenum.

• In diet iron occurs in two forms –

– Heme iron – Better absorbed than inorganic iron.

– Inorganic (non-heme) iron – Major fraction in diet.

• Mostly in ferric form.

– Reduction to ferrous form required.

– Because iron is absorbed in ferrous form.

• After absorption, Iron transported into enterocytes.

– Occurs via divalent metal transporter – 1 (DMT-1). 

• Inside enterocytes,Ferrous form → Oxidized to ferric form.

– Fraction of absorbed iron is rapidly delivered to plasma transferrin.

• Most iron deposited in enterocytes as “Ferritin”. 

• Excretion of iron occurs predominately via feces.

TRANSPORT OF IRON:

Transferrin – 

• Transport glycoprotein for iron in blood.

• Transported into cells through transferrin attachment to specific membrane-bound receptors.

STORAGE:

• Stored as ferritin & hemosiderin.

• Major storage form.

• Complex of iron & apoferritin.

– (Iron + apoferritin = Ferritin).

• Storage of iron inside ferritin is in ferric form.

• Most of body iron is contained in hemoglobin and not in ferritin.

• Mainly stored in reticuloendothelial cells monocytes/macrophages of liver, spleen & bone marrow.

– Also in Hepatocytes (parenchymal cells of liver).

– Myocytes of skeletal muscles.

REGULATION OF IRON ABSORPTION:

• Regulated according to demand –

• E.g., During iron deficiency → Increased absorption.

• Regulation mediated by “Hepcidin” – 

– Iron metabolism regulatory hormone inhibiting iron absorption.

• On iron deficiency → Reduced hepcidin concentration → Increased iron absorption & Decreased release of iron from storage sites.

FACTORS AFFECTING IRON ABSORPTION:

1. Factors decreasing iron absorption – 

• By 2 mechanisms:

1a. By complexing (forming chelate) with iron – 

• Phytate, Phosphate, EDTA, Tetracycline & Milk.

1b. By opposing reduction of ferric to ferrous forms –

• Antacids, alkalies & pancreatic secretions.

2. Factors increasing iron absorption – 

• By enhancing reduction of ferric to ferrous form –

• Ascorbic acid, HCl, gastric secretions & citric acid.

CONSERVATION OF IRON:

• Mechanisms conserving & preventing iron loss –

1. On RBC lysis → Hemoglobin enters circulation.

• Being small molecular weight substance, urinary loss is easy.

• Prevented by immediate HB binding to haptoglobin → Hence, preventing loss.

2. On removal of globin part from Hb → Heme produced & released into circulation.

• Heme is bound by hemopexin → Hence, preventing urinary loss.

IRON RECYCLING MECHANISM:

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