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.

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:

ABSORPTION OF IRON

• Iron absorbed from upper small intestine mainly duodenum.
• Iron is absorbed in ferrous form.
• Excretion of iron occurs predominately via feces.
• Transferrin – Transport glycoprotein for iron.
• Ferritin – Major storage form.
• Storage of iron inside ferritin is in ferric form.
• Mainly stored in reticuloendothelial cells monocytes/macrophages of liver, spleen & bone marrow.
• Factors decreasing iron absorption –

By complexing (forming chelate) with iron – 

• Phytate, Phosphate, EDTA, Tetracycline & Milk.
• By opposing reduction of ferric to ferrous forms –
• Antacids, alkalies & pancreatic secretions.
• Factors increasing iron absorption – 
• By enhancing reduction of ferric to ferrous form –
• Ascorbic acid, HCl, gastric secretions & citric acid.
• Conservation of iron by Haptoglobin & Hemopexin.