Iron is a mineral with several roles in the human body. It is a component of hemoglobin, transporting oxygen in red blood cells, and myoglobin, storing oxygen in muscle cells. Iron also supports energy production, immune function, and hormone synthesis.
The Primary Site of Absorption
The duodenum, the first part of the small intestine, is the main site for iron absorption. Its acidic environment, influenced by stomach contents, helps keep iron soluble.
The duodenal lining contains specialized enterocytes with microvilli, which increase the surface area for absorption. These cells have transport proteins that move iron from the intestine into the body. While some absorption can occur elsewhere, the duodenum’s unique environment and cellular machinery make it the primary site for efficient iron uptake.
How Iron Enters the Body
Dietary iron comes in two main forms: heme iron and non-heme iron, each absorbed differently. Heme iron, found in animal products like meat, is absorbed more efficiently. It enters enterocytes via the heme carrier protein 1 (HCP1). Inside the cell, heme is broken down by heme oxygenase, releasing inorganic iron that joins the intracellular iron pool.
Non-heme iron, found in plant and animal sources, follows a different route. Before absorption, ferric iron (Fe3+) must be reduced to its ferrous form (Fe2+) by duodenal cytochrome B (DcytB) on the enterocyte’s brush border. Ferrous iron is then transported into the enterocyte by divalent metal transporter 1 (DMT1).
Inside the enterocyte, iron can be stored in ferritin or exported into the bloodstream. For export, ferrous iron moves through ferroportin. As it exits, it is re-oxidized to ferric iron by hephaestin, allowing it to bind to transferrin for transport in the blood.
Influences on Iron Uptake
Several factors influence iron absorption. Vitamin C (ascorbic acid) improves non-heme iron absorption by forming a soluble complex, preventing its precipitation. The “meat, fish, and poultry factor,” found in animal protein, also enhances non-heme iron absorption.
Conversely, various substances inhibit iron absorption. Phytates, in grains, legumes, and nuts, bind to non-heme iron. Tannins, found in tea and coffee, also reduce iron uptake. Calcium can inhibit the absorption of both heme and non-heme iron.
Certain medications can decrease iron absorption by altering the duodenal environment. The body’s iron status also plays a role, with absorption increasing when stores are low and decreasing when sufficient.
Maintaining Iron Balance
The body maintains iron balance mainly by regulating absorption from the diet. There is no major mechanism for excreting excess iron, so absorption control is key. After iron is absorbed into the bloodstream, it binds to transferrin, a protein that transports iron to tissues, including bone marrow for red blood cell production, and to storage sites.
Excess iron is stored within cells, primarily in the liver, spleen, and bone marrow, as ferritin. Ferritin stores iron safely, preventing cellular damage. The liver-produced hormone hepcidin regulates iron balance. It controls iron release into the bloodstream by binding to ferroportin, which reduces iron export from cells. This regulation ensures the body receives adequate iron while preventing harmful accumulation.