41. Iron metabolism. Absorption of haem and non-haem iron in the small intestines. Transferrin receptors. Iron-binding and iron-storage proteins. Hepcidin. Post-transcriptional control of iron homeostasis. Disorders of iron homeostasis.

Introduction

Iron is needed to carry oxygen, to transfer electrons and to make many enzymes, but free iron catalyses the formation of damaging reactive oxygen species. The body therefore absorbs iron carefully, carries it bound to transferrin, stores it inside ferritin, and adjusts the whole system through the hormone hepcidin. An adult body contains about 3 to 4 g of iron, most of it in haemoglobin. Daily losses are small, so iron balance depends mainly on how much is absorbed.

Iron distribution and balance

• About two thirds of body iron is in the haemoglobin of circulating erythrocytes.
• Most of the rest is stored, mainly as ferritin and haemosiderin in the liver, spleen and bone marrow. Myoglobin and iron-containing enzymes hold the remainder.
• Daily loss is about 1 to 2 mg in men and postmenopausal women and is higher in menstruating women. The body has no efficient way to excrete excess iron, so control is exercised almost entirely through absorption.