What Is Iron in the Body and What Does It Do?

Iron is a trace mineral that your body uses for everything from ferrying oxygen through your bloodstream to copying DNA inside your cells. An adult carries roughly 3 to 5 grams of it, with men averaging about 55 milligrams per kilogram of body weight and women about 45 milligrams per kilogram.¹ That modest amount, barely enough to fill a small teaspoon, powers an astonishing range of biological jobs. The reason iron is so useful comes down to its chemistry: it flips easily between two electrical states, which lets it shuttle electrons and bind gases in ways few other elements can.

How Iron Carries Oxygen

The single biggest use of iron in your body is oxygen transport. About 60 to 70 percent of your total iron sits inside hemoglobin, the protein packed into red blood cells.¹ Each hemoglobin molecule holds four iron atoms, and each of those atoms can grab one molecule of oxygen. The iron stays in its reduced form when it binds oxygen, which is what allows it to pick the gas up in your lungs and release it smoothly in tissues that need it.²

A related protein called myoglobin handles oxygen once it arrives in muscle tissue. Myoglobin also contains iron, but it works as a local reserve and shuttle rather than a long-distance carrier. Research on exercising skeletal muscle showed that when myoglobin was chemically disabled, both oxygen consumption and force production dropped, confirming that functional myoglobin is essential for muscles working under load.³ Beyond simple storage, myoglobin also helps regulate reactive oxygen species and nitric oxide levels inside cells, which in turn fine-tunes how mitochondria produce energy in high-demand tissues like the heart and working muscles.⁴

Powering Your Cells

Oxygen delivery would be pointless without a way to use it, and that is where iron shows up again. Inside your mitochondria, small clusters made of iron and sulfur atoms act as tiny electron relays. These iron-sulfur clusters sit within the chain of protein complexes that ultimately produce most of your body’s energy currency. They pass electrons through Complexes I, II, and III, ultimately handing them off to oxygen at the end of the chain.⁵ Without functional iron-sulfur clusters, the whole energy-generating assembly line stalls. Heme groups, the same iron-containing structures found in hemoglobin, also participate in assembling and running these energy complexes, so iron contributes to cellular energy production through two distinct chemical forms.⁶

Iron in DNA Repair and Cell Division

Iron’s role extends into the nucleus. A wide range of enzymes involved in copying and repairing DNA depend on iron as a cofactor. These include helicases that unwind the double helix, nucleases that cut damaged strands, and the enzyme ribonucleotide reductase, which builds the raw materials for new DNA. Even DNA polymerases themselves, the molecular machines that assemble new DNA strands, contain iron-sulfur clusters that are essential for their stability and activity.⁷ This is one reason iron deficiency can slow the production of new cells, affecting everything from wound healing to the constant turnover of your gut lining.

How the Body Moves and Stores Iron

Because free iron is chemically reactive and potentially toxic, your body never lets it float around unattended. A dedicated transport protein called transferrin carries iron through the bloodstream. At any given moment, only about 3 milligrams of iron is bound to transferrin, but this small pool turns over roughly ten times a day to keep pace with demand.¹ Cells that need iron display transferrin receptors on their surface. When transferrin binds to these receptors, the complex gets pulled inside the cell. The interior of the resulting compartment is then acidified, causing iron to release from transferrin so the cell can use it.⁸

Surplus iron is tucked away in ferritin, a storage protein built from 24 subunits that assemble into a nearly spherical shell. Thousands of iron atoms can be stored inside this shell in a nontoxic form, preventing them from generating harmful free radicals in the cell’s cytoplasm.⁹ The iron within ferritin is stored as a mineral similar to ferrihydrite, and the protein’s channels allow iron to move in and out as the body’s needs shift.¹⁰ Adult men typically store between 0.5 and 1 gram of iron, mostly in liver cells and specialized immune cells. Children, adolescents, and women of childbearing age carry far less in reserve, which is why these groups are more vulnerable to deficiency.¹

Hepcidin, the Master Regulator

Your body has no dedicated pathway for excreting excess iron. Small amounts leave through shed skin cells, intestinal lining, and menstrual blood, but these losses are passive. Instead of controlling what goes out, the body controls what comes in, and the key to that control is a hormone called hepcidin. Produced in the liver, hepcidin acts on ferroportin, the only known protein that exports iron from cells into the bloodstream. When hepcidin levels rise, ferroportin is degraded and iron stays locked inside gut-lining cells and immune cells. When hepcidin falls, ferroportin works freely and more iron enters circulation.¹¹

This system responds to multiple signals. Inflammation drives hepcidin up, which is why people with chronic inflammatory conditions often develop low circulating iron even if their total stores are adequate. High iron stores and iron-rich meals also push hepcidin higher. Conversely, iron deficiency, increased red blood cell production, and low oxygen levels suppress hepcidin to let more iron through. It is an elegant feedback loop, but it also explains some frustrating clinical scenarios where iron is technically present in the body but sequestered where it cannot be used.

What Happens When You Run Low on Iron

Because most of your iron rides inside red blood cells, blood loss is the single most common trigger for iron depletion worldwide.¹² Heavy menstrual periods, gastrointestinal bleeding, frequent blood donation, and surgery can all drain iron stores faster than the diet replaces them. When stores drop far enough that hemoglobin production suffers, the result is iron deficiency anemia.

Mild cases are often symptomless, which is why iron deficiency frequently goes unnoticed until a routine blood test catches it. As it worsens, common signs include fatigue, poor appetite, exercise intolerance, dizziness, and irritability. Severe anemia can cause a rapid heart rate, shortness of breath, and visibly pale skin or nail beds.¹³ But the effects go beyond red blood cells. Because iron is embedded in energy-producing enzymes and DNA machinery, deficiency can impair concentration, slow cognitive development in children, and weaken immune responses well before full-blown anemia sets in.

When Iron Accumulates

Too much iron is genuinely dangerous. Unbound iron catalyzes chemical reactions that produce aggressive free radicals, a process that damages cell membranes, proteins, and DNA.¹⁴ This is not just a theoretical risk; excess iron has been linked to a specific form of cell death called ferroptosis, which is driven by runaway damage to the fat molecules in cell membranes and is biochemically distinct from other types of cell death.¹⁵

The most well-known cause of chronic iron overload is hereditary hemochromatosis, a genetic condition in which mutations in the HFE gene and related genes lead to inadequate hepcidin production. Without enough hepcidin, the intestine absorbs too much iron from every meal, and macrophages release too much stored iron. Over years, this drives iron accumulation in the liver, heart, pancreas, and joints.^{16 17} Repeated blood transfusions, as in thalassemia or sickle cell disease management, can produce a similar outcome because each unit of transfused blood delivers a large bolus of iron that the body has no efficient way to excrete.

Iron as an Immune Weapon

Your immune system has a clever trick up its sleeve: it starves invading bacteria of iron. Most pathogens need iron to grow and replicate, so during an infection your body rapidly sequesters iron inside cells and away from the bloodstream. This strategy has been given the name “nutritional immunity,” and it is an ancient defense that evolved over millions of years of coexistence between vertebrates and bacteria.¹⁸ Hepcidin plays a central role here too: inflammatory signals spike hepcidin, which traps iron inside cells and lowers the amount circulating in the blood. From the body’s perspective, temporarily limiting your own iron availability is a worthwhile trade-off if it slows bacterial growth.¹⁹

This is worth knowing because it explains why giving iron supplements during an active infection can sometimes backfire. If your body is deliberately withholding iron from a pathogen, flooding the system with supplemental iron can undermine that defense. It also explains why chronically ill patients sometimes show low serum iron despite having plenty of iron in storage: the body is in lockdown mode, not deficiency mode, and the treatment approach is very different from straightforward iron supplementation.

Iron and the Brain

The brain is one of the most iron-hungry organs relative to its size. Iron is needed there for producing neurotransmitters, maintaining the myelin coating on nerve fibers, and supporting mitochondrial energy production in neurons. But the same reactivity that makes iron useful also makes it dangerous when it accumulates in the wrong places. Abnormal iron deposition in specific brain regions has been associated with neurodegenerative diseases, especially Parkinson’s disease, for about a century.²⁰

In Parkinson’s disease, patients show disruptions in multiple iron-handling proteins, which leads to iron building up in areas like the substantia nigra. That excess iron fuels oxidative stress and may trigger ferroptosis in dopamine-producing neurons, worsening the disease’s progression.²¹ Researchers are still working out whether iron accumulation is a cause, a consequence, or a self-reinforcing contributor to neuronal death. That distinction matters because it determines whether therapies aimed at removing brain iron could slow the disease or would merely address a downstream marker.

Getting Iron From Food

Dietary iron comes in two forms. Heme iron, found in meat, poultry, and seafood, is already wrapped in the same molecular structure your body uses, so it is absorbed relatively efficiently and is not strongly affected by other dietary components. Non-heme iron, found in plant foods, dairy, and fortified products, is more sensitive to what you eat alongside it.

Vitamin C is probably the most well-known absorption booster for non-heme iron. It works by converting iron into a form that stays soluble in the gut even at the higher pH found further along the digestive tract, and it can counteract the inhibiting effects of compounds like tannins in tea and calcium-phosphate in dairy.²² On the inhibitor side, phytates found in whole grains and legumes and polyphenols found in tea, coffee, and some vegetables are the biggest players. They bind iron in the gut and carry it out of the body before it can be absorbed. Calcium also inhibits absorption, but uniquely, it affects both heme and non-heme iron, whereas most other inhibitors only interfere with non-heme iron.²³

A study looking at the relative weight of different dietary factors on non-heme iron absorption found that phosphorus accounted for the largest share of meal-to-meal variation, followed by vitamin C and then animal tissue.²⁴ The practical takeaway is that pairing iron-rich plant foods with a source of vitamin C and avoiding large calcium-rich foods or tea at the same meal makes a real difference in how much iron you absorb. But individual variation is large. Your own iron status is one of the strongest determinants: the more depleted your stores, the more aggressively your gut absorbs iron from whatever you eat.

Athletes, Pregnancy, and Other High-Demand States

Some groups face iron challenges that go beyond simple diet. Athletes, especially endurance runners, lose iron through sweat, gastrointestinal micro-bleeding from repeated impact, and destruction of red blood cells in the feet during prolonged exercise. On top of that, exercise temporarily raises hepcidin levels, which reduces iron absorption during the hours after a workout. A review of studies found that a single session of moderate-to-vigorous endurance exercise stimulated a hepcidin spike peaking around three hours afterward, and the size of that spike depended on pre-exercise iron stores and inflammation levels.²⁵ For female athletes, the combination of menstrual losses, exercise-driven hepcidin spikes, and often inadequate dietary intake creates a triple threat for iron depletion.

Pregnancy is another period of sharply increased demand. The growing fetus depends entirely on maternal iron transferred through the placenta, and this transfer has to supply enough iron to support the baby’s rapid red blood cell production and build stores that will last through early infancy.²⁶ Maternal blood volume also expands substantially during pregnancy, further increasing hemoglobin demand. These combined pressures are why iron supplementation is routinely recommended during pregnancy even for women who enter it with normal stores.

Iron Levels Follow a Daily Clock

One underappreciated feature of iron biology is that your serum iron levels are not static throughout the day. They follow a circadian rhythm, rising and falling on a roughly 24-hour cycle that is tied to the body’s internal clock rather than simply being a response to meals. Research in mice showed that serum iron, transferrin saturation, and even the rate of red blood cell production all oscillated in a circadian pattern. These rhythms persisted even in constant darkness, confirming they were driven by the internal clock rather than by light cues or feeding behavior. When the core clock gene Bmal1 was knocked out, the oscillations disappeared entirely.²⁷

This has a surprisingly practical implication. If you have ever had blood drawn to check your iron status and gotten a borderline result, the time of day the sample was taken could be part of the explanation. Clinicians have long noticed that serum iron values vary within a single day, and the circadian regulation of the iron transport system is likely a major reason why. Newer diagnostic markers are less affected by this fluctuation. Reticulocyte hemoglobin content, for example, reflects whether enough iron was available to newly produced red blood cells over the preceding few days, making it a more stable snapshot than a single serum iron measurement.²⁸ Soluble transferrin receptor is another promising marker that reflects overall red blood cell production and can help distinguish true iron deficiency from the iron sequestration that happens during chronic inflammation.²⁹

Why Iron Became So Central to Life

Iron’s dominance in biology is not an accident of chemistry alone. On the early Earth, dissolved iron was abundant in the primordial ocean, and the first living organisms built their metabolic machinery around it. When the atmosphere became oxygen-rich, much of that dissolved iron oxidized and precipitated out of the water, drastically reducing its availability.³⁰ Rather than abandon an element already woven into thousands of essential proteins and enzymes, life evolved increasingly sophisticated systems to scavenge, transport, store, and recycle the iron it had. The elaborate transferrin-ferritin-hepcidin system in your body is the modern result of billions of years of evolutionary pressure to hold onto a nutrient that became scarce.³¹ That same evolutionary pressure may have driven significant biological diversification, as organisms competed for a shrinking pool of bioavailable iron and developed new strategies to acquire it.