Als '''Eisenstoffwechsel''' wird die Gesamtheit jener Prozesse bezeichnet, die zur Aufnahme von Eisen in den Organismus, zur Eisenverteilung im Organismus sowie zur Ausscheidung von Eisen aus dem Organismus führen.

Das Element Zellen benötigt. Es wird für die Funktion von mitochondriale freier Radikale beteiligt sein.

== Kreislauf des Eisens ==
===Aufnahme des Eisens im Darm===
Mitteleuropäische Nahrung enthält ca. 6mg Eisen / 1000kcal, welches teilweise als Nonhämeisen und teilweise als Hämeisen vorliegt. 

Hämeisen wird besser aufgenommen, es wird von den Darmzellen endozytiert, die genaueren Vorgänge sind dabei noch unklar. 

Das in der Nahrung zumeist vorliegende dreiwertige Nonhämeisen muss zu Fe<sup>2+</sup> reduziert werden, dies geschieht entweder durch Vitamin C oder durch eine Ferrireduktase (membranständiges, duodenales Cytochrom b), die sich in der Darmzellmembran befindet. Dann wird es durch den DMT1-Transporter (gradientabhängiger Kotransport mit H+) in die Darmzellen aufgenommen. 

Dort wird es entweder in Ferritin gespeichert oder an den letzten beiden Lebenstagen der Darmzellen durch Ferroportin an das Blut abgegeben. Dieser Vorgang wird durch Hepcidin gehemmt, wodurch im Wesentlichen die Eisenaufnahme reguliert wird. Falls das Eisen in den Darmzellen nicht an das Blut abgegeben werden kann (z.B. weil der Transport in das Blut durch Hepcidin gehemmt ist), geht es mit der Abschilferung der Darmzellen wieder dem Organismus verloren. 

* täglicher Austausch von Eisen zwischen den Geweben: 40 mg;
* Aufteilung: Transport ('''Transferrin'''), Lagerung ('''Ferritin'''), Katalyse und Transport.
**   Jedes '''Transferrin'''molekül bindet 2 Atome Eisen in der dreiwertigen Form [Fe(III)]; jeder Retikulozyt nimmt 1 Mio. Eisenatome je Minute auf;
** '''Ferritin''' besteht aus 24 identischen Untereinheiten, die sich um eine zentrale Höhlung mit 80 Å Durchmesser gruppieren; diese  Höhlung nimmt bis zu 4500 Fe(III) Atome auf. Mobilisierung des Eisens erfordert Reduktion zur  zweiwertigen Stufe, Fe(II);
** im Verlauf der '''Häm-Synthese''' wird Fe(II) durch Ferrochelatase, ein Enzym der Mitochondrien-Innenmembran, eingelagert. Die für die Reduktion notwendigen Elektronen werden durch die Atmungskette bereitgestellt.

== Regulation des Eisenspiegels im Blut ==

Der Eisenspiegel auf Ebene des Organismus wird wahrscheinlich vor allem durch das erst kürzlich entdeckte Peptid '''Hepcidin''' reguliert. 

Ist der Eisenspiegel zu niedrig, bremst die Leber ihre Hepcidinproduktion. Als Folge wird die enterale Eisenresorption gesteigert.

Ist der Eisenspiegel zu hoch, dann steigert die Leber ihre Hepcidinproduktion und die enterale Eisenresorption sinkt.

frame|right|Human beings use 20 mg of iron each day for the production of new red blood cells, much of which is recycled from old red blood cells.

'''Human iron metabolism''' is the set of chemical reactions maintaining human homeostasis of iron. Iron is an essential element for most life on Earth, including human beings. The control of this necessary but potentially toxic substance is an important part of many aspects of human health and disease. Hematologists have been especially interested in the system of iron metabolism because iron is essential to red blood cells. Most of the human body's iron is contained in red blood cells' hemoglobin, and iron deficiency is the most common cause of anemia. 

Understanding this system is also important for understanding diseases of iron overload. 

Recent discoveries in the field have shed new light on how humans control the level of iron in their bodies and created new understanding of the mechanisms of several diseases.

==Importance of iron regulation==
thumb|right|200px|Structure of [[Heme b; "Fe" is the chemical symbol of iron.]]
Iron is an absolute requirement for most forms of life, including humans and most bacterial species. Because plants and animals all use iron, iron can be found in a wide variety of food sources. 

Iron is essential to life, because of its unique ability to serve as both an acceptor. 

Iron can also be potentially toxic. Its ability to donate and accept electrons means that if iron is free within the cell, it can catalyze the conversion of hydrogen peroxide into free radicals. Free radicals can cause damage to a wide variety of cellular structures, and ultimately kill the cell. To prevent that kind of damage, all life forms that use iron bind the iron atoms to proteins. That allows the cells to use the benefits of iron, but also limit its ability to do harm. 

The most important group of iron-binding proteins contain the heme molecules, all of which contain iron at their centers. Humans and most bacteria use variants of heme to carry out redox reactions and electron transport processes. These reactions and processes are required for oxidative phosphorylation. That process is the principal source of energy for human cells; without it, our cells would die. 

Humans also use iron in the hemoglobin of red blood cells, in order to transport oxygen from the lungs to the tissues and to export carbon dioxide back to the lungs. Iron is also an essential component of myoglobin to store oxygen in muscle cells. 

The human body needs iron for oxygen transport. That oxygen is required for the production and survival of all cells in our bodies. Human bodies tightly regulate iron absorption and excreting iron. Most humans prevent iron overload solely by regulating iron absorption. Those who cannot regulate absorption well enough get disorders of iron overload. In these diseases, the toxicity of iron starts overwhelming the body's ability to bind and store it. 

===Bacterial protection===
A proper iron metabolism protects against bacterial infection. If bacteria are to survive, then they must get iron from the environment. Disease-causing bacteria do this in many ways, including releasing iron-binding molecules called siderophores and then reabsorbing them to recover iron, or scavenging iron from hemoglobin and transferrin. The harder they have to work to get iron, the greater a metabolic price they must pay. That means that iron-deprived bacteria reproduce more slowly. So our control of iron levels appears to be an important defense against bacterial infection. People with increased amounts of iron, like people with hemochromatosis, are more susceptible to bacterial infection.  

Although this mechanism is an elegant response to short-term bacterial infection, it can cause problems when inflammation goes on for longer. Since the liver produces hepcidin in response to inflammatory cytokines, hepcidin levels can increase as the result of non-bacterial sources of inflammation, like viral infection, cancer, auto-immune diseases or other chronic diseases. When this occurs, the sequestration of iron appears to be the major cause of the syndrome of anemia of chronic disease, in which not enough iron is available to produce an adequate number of hemoglobin-containing red blood cells. 

==Body iron stores==
thumb|400px|right|1918 illustration of blood cell production in the [[bone marrow. In iron deficiency, the bone marrow produces fewer blood cells, and as the deficiency gets worse, the cells become smaller.]]
Most well-nourished people in industrialized countries have 3-4 grams of iron in their bodies. Of this, about 2.5 g is contained in the hemoglobin needed to carry oxygen through the blood. Another 400 mg is devoted to cellular proteins that use iron for important cellular processes like storing oxygen (plasma, bound to transferrin.  

Since so much iron is required for hemoglobin, iron deficiency anemia is the first and primary clinical manifestation of iron deficiency. Oxygen transport is so important to human life that severe anemia harms or kills people by depriving their organs of enough oxygen. Iron-deficient people will suffer or die from organ damage well before cells run out of the iron needed for intracellular processes like electron transport.

Some iron in the body is stored. Physiologically, most stored iron is bound by ferritin molecules; the largest amount of ferritin-bound iron is found in cells of the liver hepatocytes, the bone marrow and the spleen. The liver's stores of ferritin are the primary physiologic source of reserve iron in the body.

Macrophages of the reticuloendothelial system store iron as part of the process of breaking down and processing hemoglobin from engulfed red blood cells. 

Iron is also stored as a pigment called hemosiderin in an apparently pathologic process. This molecule appears to be mainly the result of cell damage. It is often found engulfed by macrophages that are scavenging regions of damage. It can also be found among people with iron overload due to frequent blood cell destruction and transfusions. 

Men tend to have more stored iron than women, particularly women who must use their stores to compensate for iron lost through menstruation, pregnancy or lactation.

==How the body gets its iron==
Most of the iron in the body is hoarded and recycled by the reticuloendothelial system which breaks down aged red blood cells. However, people lose a small but steady amount by sweating and by shedding cells of the skin and the mucosal lining of the gastrointestinal tract. The total amount of loss for healthy people in the developed world amounts to an estimated average of 1 mg a day for men, and 1.5&ndash;2 mg a day for women with regular menstrual periods. People in developing countries with gastrointestinal parasitic infections often lose more. 

This steady loss means that people must continue to absorb iron. They do so via a tightly regulated process that under normal circumstances protects against iron overload.

===Absorbing iron from the diet===
right|thumb|Meat contains heme, which is a source of iron. 

Like most mineral nutrients, iron from digested food or supplements is almost entirely absorbed in the duodenum by enterocytes of the duodenal lining. These cells have special molecules that allow them to move iron into the body. 

To be absorbed, dietary iron must be in its ferrous Fe<sup>2+</sup> form. A ferric reductase enzyme on the enterocytes' brush border, Dcytb, reduces ferric Fe<sup>3+</sup> to Fe<sup>2+</sup>. A protein called divalent metal transporter 1 DMT1, which transports all kinds of divalent metals into the body, then transports the iron across the enterocyte's cell membrane and into the cell. 