Iron Absorption and the Hepcidin Gate

Iron is the one mineral your body can absorb but cannot excrete — so the only place it can control the balance is the front door. Watch iron cross the gut wall, meet hepcidin (the liver hormone that destroys ferroportin, the body’s only iron exit channel), and either reach your blood or get trapped in the cell and shed in the stool a few days later.

Try this: press Inflammation (IL-6) and watch ferritin climb while haemoglobin falls. That dissociation is the whole reason you cannot read iron status from ferritin alone.

The Science in Plain Language

Iron is a one-way door: you can absorb it, but you cannot excrete it

Your body holds roughly 3–4 grams of iron. Most of it — about 2.5 g — is already in use, sitting inside the haemoglobin of your circulating red blood cells. The rest is stored in the liver and in the macrophages that recycle worn-out red cells.

Here is the strange part. You have no way to actively excrete iron. There is no iron equivalent of dumping excess potassium in your urine. You lose only about 1–2 mg a day, and every bit of that loss is passive and unregulated: cells shedding from your gut lining and skin, tiny amounts in bile and sweat. Men and postmenopausal women lose roughly 1 mg/day; menstruating women average closer to 1.5–2 mg/day, and considerably more with heavy periods.

Because the exit is fixed, the only place your body can control iron balance is the front door. That is why the gut gate is guarded so obsessively — and it is also why an excess, once it is in, has nowhere to go.

One more number worth holding on to: your macrophages recycle about 20–25 mg of iron every day from old red blood cells. That internal recycling, not your diet, is the main iron supply. Absorption is just the small top-up that covers losses. Keep that in mind — it is exactly why inflammation, which blocks recycling, is so devastating.

Two kinds of dietary iron — and only one of them cares what else is on your plate

Heme iron comes from meat, poultry and fish: iron locked inside the porphyrin ring of haemoglobin and myoglobin. It is absorbed as an intact molecule through a brush-border transporter (HCP1 is the leading candidate; the exact heme importer is still debated). Typically 15–35% is absorbed, and because the iron is wrapped inside the ring, heme is largely protected from the things in a meal that block iron. Once inside the cell, an enzyme called heme oxygenase-1 cracks the ring open and releases the iron.

Non-heme iron comes from plants, grains, legumes, fortified foods — and from nearly every iron supplement on the shelf. It arrives mostly as ferric iron (Fe³⁺), which is essentially rust: insoluble and useless at the pH of the small intestine. Typical absorption is only 2–20%, and that range is enormous because it depends almost entirely on what else is in the meal and on how iron-hungry you are.

Non-heme iron cannot cross the membrane at all until it is reduced from Fe³⁺ to Fe²⁺ by an enzyme sitting on the brush border, duodenal cytochrome b (DCYTB). Only then can DMT1 (divalent metal transporter 1) — a proton-powered pump — carry it into the cell. DMT1 needs an acidic microenvironment, which is one reason that acid-suppressing drugs such as PPIs reduce non-heme iron absorption.

Three things in the same meal blunt non-heme absorption, and you can toggle each of them in the animation:

• Polyphenols and tannins — black tea, coffee, cocoa, red wine. Drinking tea or coffee with a meal can cut non-heme absorption by roughly half. Move the drink to an hour or two after eating and most of that effect disappears.
• Phytates (phytic acid) — whole grains, bran, legumes, nuts, seeds. Potent, and dose-dependent: even fairly small amounts matter. Soaking, sprouting, leavening and fermenting all reduce phytate.
• Calcium — the only inhibitor that blunts both heme and non-heme iron. It matters most as a big calcium dose in the same meal; long-term studies suggest the effect on overall iron status is smaller than single-meal experiments imply.

Vitamin C: the electron that opens the door

Ascorbate does two useful things at once. It donates the electron that turns Fe³⁺ into Fe²⁺, feeding the DCYTB reaction, and it chelates iron, keeping it soluble as the pH climbs along the duodenum. In the animation, watch the green ascorbate particles collide with grey Fe³⁺ and flip it to orange Fe²⁺.

The effect is not subtle. Adding vitamin C to a meal can double or triple non-heme absorption, and it is dose-dependent — roughly 50 mg of ascorbic acid (about a small glass of orange juice, or a squeeze of lemon, or the vitamin C already sitting in peppers, broccoli and tomatoes) is enough to make a clear difference. This is why “take your iron with orange juice, not with tea” is genuinely good advice rather than folklore. Vitamin C does nothing at all for heme iron — heme does not need reducing.

Hepcidin: the hormone that decides how much iron actually reaches your blood

Getting iron into the gut cell is only half the journey. Once inside, iron faces a fork in the road:

• Stored in ferritin inside the cell — where it is effectively dead. Enterocytes live only 3–5 days before shedding into the gut, taking their ferritin with them. Iron parked here leaves in the stool. It never reaches you.
• Exported to the blood through ferroportin — the only iron exporter in the human body. The same protein is used by the macrophages recycling old red cells and by the liver cells releasing stored iron. On the way out, an enzyme called hephaestin (a copper-dependent ferroxidase) oxidises Fe²⁺ back to Fe³⁺, because transferrin, the blood’s iron taxi, only binds the ferric form. Each transferrin molecule carries two iron atoms — and the percentage of those seats that are filled is your transferrin saturation.

Which fork the iron takes is decided by a 25-amino-acid peptide hormone made in the liver: hepcidin. Hepcidin binds ferroportin and triggers its internalisation and destruction. Fewer ferroportin channels means less iron reaching the blood, which means iron piling up uselessly inside cells until they are shed. Hepcidin is the single master switch for iron in the entire body — watch it destroy the green channels one by one when you turn on Inflammation.

Hepcidin rises when iron stores are full (sensed by the liver through the BMP6/SMAD pathway) and when there is inflammation — interleukin-6 (IL-6) drives hepcidin transcription hard through JAK/STAT3.

Hepcidin falls when iron is low, and when the bone marrow is hungry for iron: erythroblasts release a hormone called erythroferrone that suppresses hepcidin production. That is the teal arrow running back from the marrow to the liver in the diagram.

Hepcidin can be measured, but the assays are not standardised between laboratories and reference ranges differ, so for now it remains largely a research test rather than a routine one. The panel shows it as a relative level.

Anaemia of chronic disease: high ferritin, low usable iron

This is the part that fools people — including clinicians in a hurry — and it is the single most important thing on this page.

In any chronic inflammatory state (rheumatoid arthritis, inflammatory bowel disease, chronic kidney disease, chronic infection, cancer), IL-6 pushes hepcidin high. Ferroportin is destroyed everywhere: in the gut, so you absorb almost nothing, and — far more importantly — in the macrophages that recycle 20–25 mg of iron a day from old red cells. That recycled iron, the body’s main supply, cannot get out. Serum iron falls. Transferrin saturation falls. The marrow starves and haemoglobin drifts down.

But look at the ferritin: it is HIGH. Two reasons, both real:

1. Iron genuinely is piled up inside cells, exactly where it cannot be used; and
2. Ferritin is an acute-phase reactant — inflammation raises its production directly, whatever the iron situation.

So you get the dissociation this animation is built to show: ferritin high, transferrin saturation low, haemoglobin low — and iron tablets that do almost nothing, because hepcidin has slammed shut the very gate those tablets need to cross. The body is not short of iron. The iron is locked away.

The practical rule: never read a ferritin without a CRP. If CRP is up, a “normal” or even high ferritin does not rule out iron deficiency. Guidelines commonly raise the deficiency cut-off in the presence of inflammation (often to around 100 ng/mL, higher still in chronic kidney disease) and lean instead on transferrin saturation — below about 20% suggests iron-restricted red-cell production — or on a soluble transferrin receptor measurement, which inflammation does not distort. Treating the underlying inflammation is what actually fixes the anaemia. Where iron truly is needed in this setting, intravenous iron works when oral iron will not, precisely because it bypasses the hepcidin-guarded gut gate.

Haemochromatosis: when the gate never closes

Now the mirror image. In hereditary haemochromatosis — most often two copies of the C282Y variant in the HFE gene — the liver’s iron sensor is broken. Stores fill up, but hepcidin never rises the way it should. Ferroportin stays on the membrane, wide open, and the gut keeps absorbing as though you were starving for iron.

Absorb 1–2 mg a day more than you lose, with no excretion route, and it accumulates — gram by gram, over decades. Normal total body iron is 3–4 g; untreated haemochromatosis can reach 20–40 g. The excess deposits where it does real damage: the liver (fibrosis, cirrhosis, liver cancer), the heart (cardiomyopathy, arrhythmias), the pancreas (destroying insulin-producing cells — hence the old name “bronze diabetes”), and also joints, pituitary and skin.

The first abnormal lab is usually transferrin saturation above ~45%, often years before ferritin climbs. Note what haemochromatosis does not do: it does not cause anaemia. Haemoglobin stays normal while the tissues quietly load up.

And the treatment is a blunt, beautiful illustration of the central fact of iron biology: because you cannot excrete iron, you remove it in blood. Regular therapeutic phlebotomy — roughly 200–250 mg of iron per 500 mL unit — is the standard therapy, and it works. (Penetrance is incomplete: many people who carry two C282Y copies never develop clinical iron overload.)

Why you should not take iron “just in case”

Iron supplements are cheap, unregulated and easy to think of as harmless. They are not. Iron supplementation without a documented deficiency can do real harm:

• There is no exit. Every milligram you absorb and do not need has to go somewhere, and “somewhere” means your tissues.
• Free iron is chemically dangerous. It catalyses the Fenton reaction, generating hydroxyl radicals that damage DNA, lipids and proteins. Your body keeps iron protein-bound at every single moment for exactly this reason.
• If you are inflamed it will not be absorbed anyway. You will simply be dosing your gut with unabsorbed iron — which commonly causes constipation, nausea and dark stools, and shifts the gut microbiome.
• Iron deficiency is a symptom, not a diagnosis. In an adult man, or in a postmenopausal woman, new iron deficiency needs its cause found — most often gastrointestinal blood loss, sometimes from a bleeding tumour. Quietly topping up with supplements can mask the very signal that would have caught it early.
• Iron is a leading cause of fatal poisoning in young children. A bottle of adult iron tablets can kill a toddler. Keep them locked away.

The right sequence is boring and correct: test first — ferritin, with a CRP, plus transferrin saturation — confirm that you really are deficient, find out why, and then treat with a dose and a route chosen for your situation. If you are already iron-replete, more iron will not give you more energy. It simply goes into a store you have no way to empty.

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Connections

• All Interactive Visualizations
• Iron — the mineral itself
• Ferritin Test — read it with a CRP
• Iron Panel — transferrin saturation
• Iron-Deficiency Anaemia
• Vitamin C — the reducing agent at the gate
• Digestion — how a meal is broken down