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An adult human contains only about 3 to 4 grams of iron, less than a small nail, yet without it you’d be dead within minutes. Iron carries oxygen in your blood, stores it in your muscles, and passes electrons along the chain that makes most of your ATP. Its usefulness comes from the same chemistry that makes it dangerous: iron readily switches between two oxidation states, and that ability must be tightly controlled.
Where the iron is
In a typical adult:
| Location | Approximate share of body iron | Role |
|---|---|---|
| Haemoglobin in red blood cells | about two-thirds | Carries oxygen in blood |
| Ferritin and haemosiderin (liver, spleen, bone marrow) | about a quarter | Storage |
| Myoglobin in muscle | a few percent | Stores oxygen in muscle |
| Enzymes (cytochromes, catalase and others) | a small amount | Electron transfer and catalysis |
| Transferrin in blood plasma | a tiny amount, in transit | Transport |
The chemistry: Fe²⁺ and Fe³⁺
Iron is a transition metal (see transition metals). It commonly exists in two oxidation states:
- Fe²⁺ (ferrous, iron(II)), electron configuration [Ar] 3d⁶;
- Fe³⁺ (ferric, iron(III)), [Ar] 3d⁵, a half-filled d subshell.
(See electron configuration of ions for why iron loses its 4s electrons first.)
The two are easily interconverted:
Fe³⁺ + e⁻ ⇌ Fe²⁺
This makes iron an ideal electron carrier in redox reactions (see oxidation and reduction). In the electron transport chain, cytochromes pass electrons along by switching their iron between Fe³⁺ and Fe²⁺ (see the electron transport chain).
But free iron is also dangerous. In the Fenton reaction, Fe²⁺ reacts with hydrogen peroxide to produce the hydroxyl radical, one of the most destructive reactive species known:
Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + •OH
Hydroxyl radicals attack DNA, proteins and membranes. That’s why the body almost never lets iron float around freely: it’s always bound to proteins.
Iron in haemoglobin and myoglobin
In haemoglobin and myoglobin, iron sits at the centre of a haem group: a flat porphyrin ring whose four nitrogen atoms hold the iron ion, similar to the magnesium in chlorophyll. A fifth bond links the iron to a histidine side chain of the protein, leaving a sixth position free to bind oxygen.
Crucially, only Fe²⁺ can bind O₂. If the iron is oxidised to Fe³⁺, the result is methaemoglobin, which can’t carry oxygen. Red blood cells contain an enzyme system that keeps haemoglobin iron in the Fe²⁺ state. The protein around the haem also shields the iron, stopping oxygen from simply oxidising it, as it would free Fe²⁺ in water.
How haemoglobin picks up oxygen in the lungs and releases it in tissues is covered in haemoglobin: how blood carries oxygen.
Getting iron in: absorption
Iron is absorbed mainly in the first part of the small intestine (the duodenum). Only a fraction of dietary iron is absorbed, and it depends on the form:
- Haem iron, from meat and fish (in haemoglobin and myoglobin), is absorbed relatively well, roughly 15–35%.
- Non-haem iron, from plants and fortified foods, is mostly Fe³⁺ and is absorbed less well, often only a few percent to about 20%.
Before absorption, Fe³⁺ must be reduced to Fe²⁺, which passes through a transporter protein into the gut cells. Absorption of non-haem iron is:
- increased by vitamin C, which reduces Fe³⁺ to Fe²⁺ and keeps it soluble (see vitamin C chemistry), and by meat in the same meal;
- decreased by tannins in tea and coffee, phytates in wholegrains and legumes, and calcium, which bind iron or compete with it.
Stomach acid also helps, by keeping iron dissolved (see stomach acid chemistry).
Transport and storage
- In the blood, iron is carried as Fe³⁺ bound to the protein transferrin, which holds two iron ions very tightly, keeping them safe and soluble.
- Cells take up transferrin through receptors on their surface.
- Excess iron is stored inside ferritin, a hollow protein shell made of 24 subunits that can hold up to about 4,500 iron atoms as a mineral core of iron(III) oxide-hydroxide, safely locked away. Blood ferritin levels are a common test of iron stores.
No way out: iron is recycled
Remarkably, the body has no active way to excrete iron. Small amounts are lost each day, about 1 mg in men, through shed skin and gut cells, plus blood loss (which is why menstruating women lose more). Instead:
- iron from old red blood cells, which live about 120 days, is recycled by the spleen and liver, supplying most of the iron needed to make new ones;
- iron balance is controlled at the point of absorption by the hormone hepcidin, made in the liver. When stores are high, hepcidin rises and blocks absorption; when they’re low, it falls.
When things go wrong
Iron-deficiency anaemia
The most common nutritional deficiency in the world, especially among young children, pregnant women and women with heavy periods. Without enough iron, the body can’t make enough haemoglobin. Red blood cells become small and pale, and less oxygen reaches the tissues, causing tiredness, breathlessness, pale skin and reduced ability to concentrate.
Treatment usually involves iron supplements such as iron(II) sulfate tablets. Iron(II) salts are used because Fe²⁺ is absorbed better than Fe³⁺. Their iron content can be measured in the lab by redox titration with potassium manganate(VII) (see iron in tablets lab).
Iron overload
In hereditary haemochromatosis, a genetic condition, too little hepcidin means the body absorbs too much iron. With no way to excrete it, iron builds up in the liver, heart and pancreas, damaging them through radical chemistry. Treatment is simple but effective: regularly removing blood.
Iron poisoning from swallowing too many supplement tablets is a serious danger to young children, which is why iron tablets come in child-resistant packaging.
Why iron matters beyond humans
Iron’s usefulness to life is ancient. In the early oceans, before oxygen built up in the atmosphere, iron existed mainly as soluble Fe²⁺ and was easy for organisms to obtain. Once photosynthesis filled the air with oxygen, most iron was oxidised to Fe³⁺, which forms highly insoluble oxides and hydroxides at neutral pH. That’s why iron is now hard to get: bacteria release special iron-grabbing molecules called siderophores, plants acidify the soil around their roots, and our own bodies guard every atom. In large parts of the open ocean, iron is so scarce that it limits the growth of phytoplankton, and experiments adding iron to seawater have triggered algal blooms.
Recommended intake
Typical recommendations for adults range from about 8–9 mg a day for men to about 15–18 mg a day for women of childbearing age, higher in pregnancy. Good sources include red meat, liver, fish, beans, lentils, tofu, fortified cereals and dark green vegetables.
Key takeaways
- The body contains about 3–4 g of iron, mostly in haemoglobin, with stores in ferritin.
- Iron switches between Fe²⁺ and Fe³⁺, making it a good electron carrier but also a source of damaging radicals (the Fenton reaction), so it’s always protein-bound.
- Only Fe²⁺ in haem binds oxygen; oxidised Fe³⁺ forms useless methaemoglobin.
- Haem iron is absorbed better than non-haem iron; vitamin C helps, tea and phytates hinder. Iron is recycled, not excreted, and absorption is controlled by hepcidin.
- Too little causes anaemia; too much causes iron overload. Explore iron’s properties on the iron element page.
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