Iron in the body
Iron is an essential nutrient required for many processes within the human body. It is primarily required for the production of haemoglobin, which carries oxygen within the erythrocytes, also known as red blood cells (RBCs), from the lungs to the rest of the body. Iron is also used in many enzymatic reactions, including energy metabolism and DNA synthesis, and is required for a healthy immune system.
Iron homeostasis involves a number of important processes, including:
- Regulation of intestinal iron absorption
- Transport of iron to the cells
- Storage of iron
- Incorporation of iron into proteins
- Recycling of iron after RBC degradation
Iron deficiency and iron deficiency anaemia (IDA)
Iron is vital for a wide range of biological processes in the body. A reduction in the levels of absorbed iron can impact upon the production of haemoglobin and consequently can leave the body’s organs and tissues lacking in oxygen.
Conditions in which there are no mobile iron stores with noted signs of compromised supply of iron to tissues.
- Absolute iron deficiency – Total body iron depletion
- Functional iron deficiency – Inability to access iron stores to support red blood cell production
Iron deficiency anaemia:
Absolute or functional iron deficiency leading to low haemoglobin.
How is iron absorbed and utilised?
Iron absorption is a finely balanced process that underpins the utilisation of iron in the human body. Watch this short video to understand how iron is absorbed and utilised, and the key role played by hepcidin in regulating the homeostasis of iron.
Schematic illustration of iron turnover in the body
As there is no active iron excretion mechanism in the body, maintaining levels of iron is strictly controlled by intestinal absorption.
The average iron content in the body is about 3–4 g and is distributed between RBCs, macrophages of the reticulo-endothelial system, liver, bone marrow, muscles and other tissues. A dynamic equilibrium is maintained by iron circulating between the different compartments. Excess iron is mainly stored in the liver as ferritin but can also be stored in other tissues. Almost all of the iron released by the breakdown of haemoglobin from RBCs is re-used. Only 1–2 mg of iron is lost per day, which must be replaced by dietary absorption.
RES – reticuloendothelial system; BM – bone marrow; RBC – red blood cell
The balance of iron within the body is regulated by the intake of iron-containing foods and by the tight control of dietary iron absorption in the duodenum. Iron excretion is not regulated by the body. In healthy individuals, iron is only lost through the shedding of mucosal or skin cells or through bleeding.
Iron is transported in the blood plasma via the iron transport protein transferrin, to sites of use. In the bone marrow erythropoiesis is stimulated by the hormone erythropoietin which is released from the kidneys. Iron is first incorporated into a haem group which is capable of binding oxygen, and then combines with other protein components to form haemoglobin within the red blood cells.
At the end of their lifespan, red blood cells are engulfed and digested by the macrophages of the liver and spleen, and iron is released to be recycled or stored within the hepatocytes of the liver as ferritin.4
IDA can develop as a consequence of poor dietary iron intake, increased demand for iron by the body, blood loss, systemic inflammation or malabsorption of iron by the gastrointestinal tract.
Main causes of IDA
Signs and symptoms of iron deficiency and IDA
Iron deficiency and IDA have multiple symptoms that may be observed on their own, or in combination with other symptoms. They can have a direct impact on a patient’s health and this can make it difficult to carry out normal, everyday activities, such as work or family life, exercise, and social activities.
Common symptoms include:1,2
- Mayo Clinic. Iron deficiency anemia. https://www.mayoclinic.org/diseases-conditions/iron-deficiency-anemia/symptoms-causes/syc-20355034 (Last accessed January 2019).
- NHS. Iron deficiency anaemia. http://www.nhs.uk/conditions/Anaemia-iron-deficiency-/Pages/Introduction.aspx (Last accessed January 2019).
- DignassAU et al. J Crohns Colitis. 2015;9(3): 211-222
Adapted from Figure 3 in Hentze et al, Balancing Acts: Molecular Control of Mammalian Iron Metabolism, Cell, Vol. 117, 285–297, April 30, 2004