PROCESS OF IRON ABSORPTION – ITS MECHANISM AND FATE

Iron is a trace element which is required for a number of vital functions in the body. Iron absorption occurs in the small intestine. Iron from plant based diet is in the ferric form ( Fe³⁺) called non – heme iron. Iron from animal sources contains both ferrous (Fe²⁺) and ferric (Fe³⁺) form and is called as heme iron. Ferric iron must be reduced to ferrous form for its uptake by intestinal cell whereas ferrous iron can be directly transported into enterocyte.

Vitamin C enhances iron absorption whereas tannins, phytates and oxalates decrease iron absorption.

Average intake of iron per day is about 20 – 25 mg out of which only 3 – 6% is absorbed. This is because if absorption exceeds excretion, then iron overload will develop.

SITE OF IRON ABSORPTION

Iron is absorbed in the duodenum and upper part of jejunum of the small intestine.

MECHANISM OF IRON ABSORPTION

• Within the intestinal lumen, iron is present in both ferrous and ferric form.
• An enzyme, ferric reductase converts ferric iron to ferrous form in the lumen of intestine.
• Divalent metal transporter, present on the apical side of intestinal cell/enterocyte transports ferrous iron from lumen of intestine to enterocyte.
• Inside the enterocyte, an iron binding protein called apoferritin binds to ferrous iron and converts it into ferritin which has iron in ferric form. (The ferro-oxidase activity in ferritin converts Fe²⁺ to Fe³⁺.[*Ferrous iron is toxic for cell in its free form as it can form free radicals.]
• The remaining Fe²⁺ ions are transported out of the enterocyte into blood circulation by ferroportin that is present on the basolateral side of enterocyte.
• With the help of a protein called hephaestin, Fe²⁺ is converted into Fe³⁺. This has to be done since, Fe²⁺ is not the transportable form of iron.
• Inside the blood vessel, apotransferrin binds to Fe³⁺. The resulting complex is called iron – transferrin complex. Transferrin is the transportable form of iron. It is then transported to other tissues and cells. It is transported to bone marrow for erythropoeisis or to liver for storage. Transferrin is 35% saturated with iron.

STORAGE AND RECYCLING OF IRON

Transferrin transports iron to the bone marrow where it binds to transferrin receptor present on pro-erythroblast. Here iron is utilized for erythropoeisis, that is formation of red blood cells/ erythrocytes.

Transferrin is also transported to liver. In the liver, iron is stored as ferritin. Few amounts are also stored as hemosiderin.

Malformed erythrocytes release hemoglobin-iron complex which binds to haptoglobin and heme-iron complex which binds to hemopexin.

All malformed red blood cells, old red blood cells and iron containing complexes are taken up by macrophages in bone marrow, liver and spleen and are broken down into free iron ions. Iron is then either stored as hemosiderin or recycled back into the blood circulation.

REGULATION OF IRON ABSORPTION

Hepcidin, a peptide which is released by liver is the primary regulator of iron absorption in the body.

Hepcidin causes internalization and destruction of ferroportin, thus decreasing iron absorption.

When iron levels are high or when there is inflammation, the levels of hepcidin increase, therefore decreasing iron absorption.

When iron levels are low or during increased erythropoeisis, the levels of hepcidin decrease thereby increasing iron absorption.

DAILY REQUIREMENT OF IRON

• Pre menopausal women – 18 mg/day
• Men – 8 mg/day
• Post menopausal women – 8 mg/day
• Pregnant women – 27 mg/day

FUNCTIONS OF IRON

• Iron forms structure of hemoglobin in RBCs and of myoglobin in muscle cells. These are concerned with oxygen delivery to tissues.
• It is required to produce energy in form of ATP through the electron transport chain.
• Iron is essential for fetal development during pregnancy.
• Iron is required maintenance of immune function of body.
• Iron supports development of brain in children and proper cognitive function in adults.
• Iron plays role in DNA replication, DNA repair and production of certain hormones.

EFFECTS OF IRON DEFICIENCY

Low iron levels can result in iron deficiency anemia which is the most common type of anemia worldwide.

Iron deficiency can lead to frequent infections due to compromised immune function.