Story
Iron
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In one pass Food iron comes in two kinds, and the body absorbs one of them several times more easily than the other.
Educational content, not medical advice — consult a clinician.
DMT1 uptake Fe²⁺ in the gut lumen is pulled into the enterocyte by DMT1, which recognizes only Fe²⁺, so vitamin C or gastric acid must first reduce the iron.
Start in mitochondria In the mitochondria of red blood cell precursors, glycine + succinyl-CoA are combined by ALA synthase (ALAS) into δ-aminolevulinic acid (ALA) — the 5-carbon starting point of heme synthesis.
Hb tetramer · 4 subunits Hemoglobin is a tetramer of 2 α and 2 β chains, each holding a heme with an Fe²⁺ at its center, so one molecule carries 4 O₂.
Free Fe²⁺ + O₂ is dangerous Free Fe²⁺ in a cell makes hydroxyl radicals through the Fenton reaction, which attack DNA, proteins and lipids, so the body must lock iron inside the ferritin cage.
Normal HFE-hepcidin loop The body cannot actively excrete iron, so liver cells, sensing iron through HFE and other sensors, adjust hepcidin to set how much iron the gut lets in.
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Chapter 1
Two kinds of iron
Food iron comes in two kinds, and the body absorbs one of them several times more easily than the other.
Heme iron comes from the hemoglobin and myoglobin in meat, liver, blood and fish (see Beef · see Lamb · see Organ Meat). Its iron atom (Fe²⁺) sits inside a ring-shaped molecule called a porphyrin; the gut cell takes it up ring and all, and whatever else is in the meal affects it far less.
Non-heme iron comes from legumes, leafy greens, fortified grains and egg yolk, and is iron that no ring holds. It has to be reduced to Fe²⁺ before it can enter the gut cell through a surface transporter called DMT1, so it depends heavily on what else is on the plate: vitamin C helps it, while the polyphenols in tea and coffee, the phytate in whole grains and calcium eaten at the same meal hold it back. Lentils and black beans are everyday sources of plant iron (see Lentils · see Black Beans), and they absorb better with a serving of fruit or vegetables rich in vitamin C at the same meal.
Keep iron supplements where children cannot reach them. If you suspect a child has swallowed adult iron tablets, do not wait at home for symptoms — contact poison control or go to the emergency department now.
Heme iron comes from the hemoglobin and myoglobin in meat, liver, blood and fish (see Beef · see Lamb · see Organ Meat). Its iron atom (Fe²⁺) sits inside a ring-shaped molecule called a porphyrin; the gut cell takes it up ring and all, and whatever else is in the meal affects it far less.
Non-heme iron comes from legumes, leafy greens, fortified grains and egg yolk, and is iron that no ring holds. It has to be reduced to Fe²⁺ before it can enter the gut cell through a surface transporter called DMT1, so it depends heavily on what else is on the plate: vitamin C helps it, while the polyphenols in tea and coffee, the phytate in whole grains and calcium eaten at the same meal hold it back. Lentils and black beans are everyday sources of plant iron (see Lentils · see Black Beans), and they absorb better with a serving of fruit or vegetables rich in vitamin C at the same meal.
Keep iron supplements where children cannot reach them. If you suspect a child has swallowed adult iron tablets, do not wait at home for symptoms — contact poison control or go to the emergency department now.
Mechanism · Which door each kind of iron uses
Heme iron is Fe²⁺ held inside a porphyrin ring, and it comes from the hemoglobin and myoglobin in meat, liver, blood and fish. It is generally thought to be taken into the gut cell with the ring still attached. One candidate route is a protein called HCP1 (heme carrier protein 1), but exactly how heme gets through the door is still not fully worked out. Once inside the cell, an enzyme called heme oxygenase (HO-1) frees the iron from the ring.Non-heme iron is Fe³⁺ or Fe²⁺ that no ring holds. It comes from legumes, leafy greens, fortified grains and nuts, and from a few animal foods such as egg yolk. It must first be reduced to Fe²⁺ before a transporter on the gut-cell surface, divalent metal transporter 1 (DMT1), can take it in.
So before non-heme iron reaches that door, it has to stay dissolved in the gut. Acidic reducing agents — vitamin C, citric acid, lactic acid — reduce Fe³⁺ to Fe²⁺ and hold on to it so that it does not turn into a precipitate the gut cannot absorb, escorting it all the way to the door (Hallberg 1989). The polyphenols (tannins) in tea and coffee, the phytate in whole grains and raw legumes, and the oxalate in spinach bind iron first into insoluble complexes, so the iron is trapped before it ever reaches the door. Calcium is a special case: it lowers the absorption of non-heme iron and may lower heme-iron absorption somewhat too. Heme iron largely stays out of this contest, which is why its absorption is far steadier and several times more efficient.
These enhancers and inhibitors look large in single-meal isotope studies, but in studies of several meals of a varied, everyday diet their effect is usually much smaller; and whether the body is short of iron generally matters more for how much it absorbs than what was eaten (Hurrell & Egli 2010). Estimated over a whole diet, a mixed diet with plenty of meat, seafood and vitamin C lets about 14–18% of its iron be absorbed, and a vegetarian diet about 5–12%. Both figures are estimates for people whose iron stores are already empty, used to set dietary reference values; they are not the absorption rate of any single meal.
The US Recommended Dietary Allowance () is 8 mg a day for adult men, 18 mg for women aged 19–50 and 27 mg in pregnancy. Women need more than twice as much as men, mostly to replace what menstruation takes.
In practice · Choosing and taking iron tablets
Iron tablets are for people already found to be short of iron: check hemoglobin and (the marker of how much iron is in storage) first, then decide whether to take them. How the diagnosis is made, and when the cause of blood loss must be found first, is covered in Iron-Deficiency Anemia. When iron really is needed, the wrong form or the wrong timing wastes a good share of it.Common forms (elemental iron = the part of a tablet that is actually iron):
| Form | Elemental iron | Absorption | Gut side effects |
|---|---|---|---|
| Ferrous sulfate | 20% (about 65 mg per tablet) | Standard, cheap | Stomach upset most common |
| Ferrous fumarate | 33% | Similar to ferrous sulfate | Similar |
| Ferrous gluconate | 12% | Slightly gentler | Less stomach upset |
| Ferrous bisglycinate | 20% | Gentler | Pricier; worth trying for a sensitive stomach |
| Polysaccharide-iron complex | 100% (the label states elemental iron directly) | Lower than the ionic salts | Very little stomach upset |
| Ferric carboxymaltose (intravenous) | Not applicable | About 100% (bypasses the gut) | Fast correction; only in a medical setting |
The elemental-iron shares for ferrous sulfate, fumarate and gluconate match the figures of the US National Institutes of Health Office of Dietary Supplements (NIH ODS); the absorption and side-effect columns are practical comparisons.
How to take it. The usual practice used to be several doses a day. Moretti 2015 gave 54 young women with low iron stores but no anemia isotope-labeled ferrous sulfate, which shows exactly how much of each dose is absorbed. Twenty-four hours after a dose of 60 mg or more, hepcidin (the liver hormone that turns iron absorption down) was still raised, and the fraction absorbed from the next day's dose fell by 35–45%. A sixfold increase in dose raised the iron absorbed only about threefold. Stoffel 2017 compared schedules directly in two open-label randomized trials: with 60 mg doses, cumulative absorption was about 21.8% on alternate days versus about 16.3% on consecutive days. Taking 120 mg at once versus two 60 mg doses a day made no significant difference to absorption over three days, but the split doses raised hepcidin more. The authors concluded that alternate-day single doses may be the better schedule and that this needs confirming in patients with anemia. Both trials studied young women with low iron stores and measured how much iron was absorbed, not how fast hemoglobin recovered.
A few points: do not take tea, coffee, milk or calcium tablets around the time of an iron dose. Supplementation usually continues for a while after hemoglobin recovers, to refill the stores; how long the course lasts, when to recheck ferritin and when to stop are your doctor's call.
If your stomach objects, you can switch to the gluconate or bisglycinate form, move to every other day, or take the tablet with food — less is absorbed that way, but it is easier to keep going.
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In practice · Building an iron-friendly meal
| Helps non-heme iron (reduces it to Fe²⁺ and holds on to it) | Holds it back (traps the iron or competes with it) |
|---|---|
| Vitamin C: citrus, bell peppers, tomatoes, kiwifruit | Black-tea polyphenols: one cup with a meal cuts that meal's non-heme iron absorption by 79–94% |
| Meat, fish or poultry in the same meal | Phytate: whole grains, unsoaked raw legumes |
| Citric and lactic acid: citrus, fermented foods | Oxalate: spinach |
| — | Calcium: milk, calcium tablets |
Non-heme iron = the iron in plants and fortified foods that no porphyrin ring holds.
The number in the table comes from a single-meal study: adults ate the same bread meal with black tea or with water, and tea cut non-heme iron absorption by 79–94%; the more polyphenols a drink held, the bigger the cut (Hurrell 1999). Coffee also contains polyphenols, but this study did not test coffee. Across several meals of a varied, everyday diet, effects like this usually shrink (Hurrell & Egli 2010), so they matter for each meal more than for long-term iron status.
In practice: if you eat a plant-based diet, pair the plant iron at every meal with a serving of fruit or vegetables rich in vitamin C. Drink tea and coffee between meals rather than with an iron-rich meal or an iron tablet, and take calcium and iron tablets at different times.
Chapter 2
The body controls absorption by need
The body has no active way to get rid of iron, so the only place to control the total is the way in: absorb more when short, less when there is enough.
The gatekeeper is a small hormone made by the liver, hepcidin. Iron leaving the gut cell for the blood has only one exit, a channel called ferroportin. When hepcidin rises, it grabs that exit and has it pulled into the cell and broken down, so the door shuts. Iron already inside the gut cell cannot get out and leaves in the stool when the lining cells are shed.
Full iron stores and inflammation (the inflammatory signal ) both raise hepcidin and shut the door; empty stores and a demand for new red cells lower it and open the door.
That means two things for you. In healthy people, this door keeps excess iron out. In people with long-term inflammation, the door can stay shut even when the stores are empty, so iron tablets do not get in — which is why anemia of chronic disease is hard to treat with iron. When the door itself is faulty and fails to close when it should, the result is hereditary hemochromatosis.
The gatekeeper is a small hormone made by the liver, hepcidin. Iron leaving the gut cell for the blood has only one exit, a channel called ferroportin. When hepcidin rises, it grabs that exit and has it pulled into the cell and broken down, so the door shuts. Iron already inside the gut cell cannot get out and leaves in the stool when the lining cells are shed.
Full iron stores and inflammation (the inflammatory signal ) both raise hepcidin and shut the door; empty stores and a demand for new red cells lower it and open the door.
That means two things for you. In healthy people, this door keeps excess iron out. In people with long-term inflammation, the door can stay shut even when the stores are empty, so iron tablets do not get in — which is why anemia of chronic disease is hard to treat with iron. When the door itself is faulty and fails to close when it should, the result is hereditary hemochromatosis.
Mechanism · How hepcidin shuts the iron exit
Unlike most nutrients, iron is regulated in reverse: the body has no active route for excreting iron, so it controls the total by controlling what comes in.The core regulator is hepcidin, a small peptide of 25 amino acids made by the liver. It works in three steps:
1. Ferroportin (FPN), on the underside of the gut cell (the side facing the blood), is the only channel by which iron leaves the cell for the blood; the macrophages that recycle old red cells use the same kind of channel to release iron back into the blood
2. Hepcidin binds ferroportin and has it taken into the cell and broken down, which closes the channel
3. Iron piled up inside the gut cell leaves in the stool as the lining is naturally shed, so net absorption falls
Hepcidin itself answers to several signals:
When iron stores are full (high ) it rises and shuts the door; when stores are low it falls and opens the doorWhen oxygen is short, or the marrow urgently needs new red cells, the maturing red cells release a signal called erythroferrone that pushes hepcidin down and opens the doorInflammation () drives hepcidin up sharply, locking iron inside macrophages and gut cells; this is the main mechanism of anemia of inflammation (anemia of chronic disease)
So healthy people do not over-absorb: hepcidin throttles automatically. In hereditary hemochromatosis (a mutation in the HFE gene), hepcidin fails to rise when it should, the body over-absorbs for life, and iron piles up in the liver, heart and pancreas. In chronic inflammation (inflammatory bowel disease, rheumatoid arthritis, chronic kidney disease), hepcidin stays high, so even with low stores, oral iron is poorly absorbed and the anemia is hard to fix with tablets.
In the end, how much iron is absorbed is decided by the body, not by how much you swallow.
Clinical · Anemia of inflammation or iron lack?
Anemia of chronic disease (also called anemia of inflammation) is a common diagnostic trap, and the second most common type of anemia after iron-deficiency anemia.The mechanism: long-term inflammation — rheumatoid arthritis, inflammatory bowel disease, chronic kidney disease, cancer, chronic infection — raises , which drives hepcidin up sharply and locks iron inside macrophages and gut cells. Plasma iron falls and the marrow cannot get iron to build red cells, even though the body's iron stores are normal or even high.
Telling it apart from true iron deficiency:
Iron-deficiency anemia: falls (below about 30 µg/L), transferrin saturation falls, serum iron fallsAnemia of chronic disease: ferritin is normal or high (inflammation itself pushes it up), transferrin saturation falls, serum iron fallsSoluble transferrin receptor (sTfR) helps separate the two: it rises in iron deficiency and is usually not raised in anemia of chronic disease alone
The two can also coexist: someone with long-term inflammation can also be losing blood and be truly iron-deficient.
The treatment points in different directions. Iron-deficiency anemia needs iron, plus a search for why it happened. Anemia of chronic disease needs the underlying disease treated first, because while hepcidin is high, most oral iron is not absorbed, so iron alone often does not work.
So when an anemia is investigated, ferritin is usually read together with an inflammation marker (C-reactive protein, , or the erythrocyte sedimentation rate). Low ferritin points to true iron deficiency; high ferritin with high inflammation markers looks more like anemia of chronic disease; results in between need a doctor to weigh them with other tests.
Chapter 3
Eight steps to make heme
Before iron can go into hemoglobin, the body has to build the ring that holds it: heme. Heme comes off an 8-step production line whose first and last steps run inside the mitochondria and whose middle steps run in the cytoplasm. If any step jams, red cells cannot be made properly.
Step 1, in the mitochondria: glycine and succinyl-CoA are joined into a small 5-carbon molecule, δ-aminolevulinic acid (). This is the rate-limiting step of the whole line, and its enzyme needs vitamin B6 to work.
Step 2, in the cytoplasm: a zinc-containing enzyme joins 2 ALA molecules. Lead knocks the zinc out of this enzyme and stops it — lead's first punch inside the heme factory.
The middle steps assemble the building blocks into a complete porphyrin ring and send it back into the mitochondria. In the last step, an enzyme called ferrochelatase sets iron into the center of the ring, and only then is heme finished. Lead inhibits this enzyme too: iron cannot be fitted in, and zinc is fitted in its place — the second punch.
What that means for you: an anemia with small red cells is not always iron deficiency. When neither iron nor vitamin B6 is short, lead poisoning is one of the causes to rule out, alongside thalassemia and anemia of chronic disease.
Step 1, in the mitochondria: glycine and succinyl-CoA are joined into a small 5-carbon molecule, δ-aminolevulinic acid (). This is the rate-limiting step of the whole line, and its enzyme needs vitamin B6 to work.
Step 2, in the cytoplasm: a zinc-containing enzyme joins 2 ALA molecules. Lead knocks the zinc out of this enzyme and stops it — lead's first punch inside the heme factory.
The middle steps assemble the building blocks into a complete porphyrin ring and send it back into the mitochondria. In the last step, an enzyme called ferrochelatase sets iron into the center of the ring, and only then is heme finished. Lead inhibits this enzyme too: iron cannot be fitted in, and zinc is fitted in its place — the second punch.
What that means for you: an anemia with small red cells is not always iron deficiency. When neither iron nor vitamin B6 is short, lead poisoning is one of the causes to rule out, alongside thalassemia and anemia of chronic disease.
Clinical · Porphyrias: where the heme line breaks
Porphyrias are a group of metabolic diseases caused by a fault at one step of the heme production line. There are 8 hereditary porphyrias, each tied to one step of the line; 7 come from a partial loss of that step's enzyme activity, and one from an enzyme that is too active (Puy 2010). Wherever the line jams, the intermediate made just before that step piles up — and that is where the symptoms come from.Acute porphyrias show up as attacks: severe abdominal pain, nausea, constipation, confusion and seizures. The most common is acute intermittent porphyria (AIP). The enzyme for step 3, porphobilinogen deaminase, keeps only part of its activity, and the person may be fine most of the time. When certain drugs, fasting or hormonal changes make the liver speed up heme production, and porphobilinogen (PBG) build up in the liver and an attack follows. Attacks can be life-threatening: anyone known to have an acute porphyria, or with a family history of it, who develops severe abdominal pain with confusion or seizures should get medical care quickly.
Cutaneous porphyrias show up in the skin. In porphyria cutanea tarda (PCT), the activity of uroporphyrinogen decarboxylase falls, porphyrins are deposited in the skin, and sun exposure brings blisters, fragile skin and extra hair growth. In erythropoietic protoporphyria (EPP), the last-step enzyme, ferrochelatase, is deficient; protoporphyrin builds up in red cells, and bright light makes the skin burn painfully, usually without blisters.
Porphyrias are uncommon overall and often missed; once one is suspected, a few simple first-line tests can confirm the diagnosis in anyone with symptoms (Puy 2010). Screening relatives can find carriers who have not yet had symptoms, so they can avoid the triggers.
They are worth teaching because they run the heme line in reverse: whichever step's enzyme is broken, the product in front of that step piles up.
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Mechanism · The whole line, and copper's part
Walk the heme production line from start to finish and the 8 steps divide the work cleanly, each one handing the previous step's product forward.Step 1 is in the mitochondria: glycine and succinyl-CoA are joined into δ-aminolevulinic acid (). The enzyme is ALA synthase, the master switch of the whole pathway: when there is plenty of heme, heme turns it down (negative feedback), and the enzyme needs the active form of vitamin B6 (PLP) as a helper. That is why a shortage of B6 can also produce an anemia with small, pale red cells that looks like iron deficiency when the real problem is stalled synthesis. The tuberculosis drug isoniazid (INH) works against B6 and causes the same thing.
Steps 2 to 5 are in the cytoplasm: step 2 joins two ALA molecules into porphobilinogen (PBG); then 4 PBG molecules link head to tail, first into hydroxymethylbilane (HMB), then into uroporphyrinogen III and coproporphyrinogen III. The porphyrin ring closes during these steps and its side chains are finished.
Steps 6 to 8 are back in the mitochondria: coproporphyrinogen III becomes protoporphyrinogen IX, which is oxidized to protoporphyrin IX; finally ferrochelatase sets iron into the ring, and heme is complete.
Iron only reaches this last step with help from copper further upstream. Ceruloplasmin and hephaestin are two copper-containing enzymes that oxidize Fe²⁺ to Fe³⁺, and only Fe³⁺ can load onto transferrin to be carried to the marrow. When copper is short, this step falters: even with plenty of stored iron, the iron cannot reach the places that make blood, and anemia follows (copper-deficiency anemia), which is sometimes mistaken for iron deficiency.
On throughput: the marrow makes about 2 million red cells a second and needs about 20 mg of iron a day for new heme; about 95% of that is recycled from old red cells, so the net amount absorbed from food each day only needs to be 1–2 mg.
The two steps lead jams each leave a mark on lab results. When step 2 is blocked, ALA builds up in blood and urine. When the last step is blocked, zinc is fitted into the porphyrin ring instead of iron, and zinc protoporphyrin (ZPP) in red cells rises. ZPP reflects cumulative exposure over the past few months and is used in occupational health monitoring alongside blood lead as a supplementary marker; the decisive test for lead exposure is the blood lead level, and ZPP is insensitive to recent or acute exposure.
The harm from childhood lead exposure is often hard to undo. As tests have grown more sensitive, the lowest blood lead level at which researchers can see harm has kept falling until it approaches zero — in other words, no safe lower limit for children has been found (Needleman 2004). Taking lead out of gasoline cut the lead in the environment dramatically; the main remaining source is lead paint in older housing. So if you suspect a child has been exposed to lead, ask a doctor for a blood lead test rather than waiting for problems to appear.
Chapter 4
How red cells carry oxygen
Once heme is made, it is loaded into hemoglobin, and hemoglobin's shape decides how it picks up oxygen in the lungs and drops it off in the tissues.
One hemoglobin molecule is built from 2 alpha (α) chains and 2 beta (β) chains. Each chain holds one heme with an iron atom at its center, so there are 4 seats for oxygen, and each seat holds 1 oxygen molecule.
The 4 seats do not work independently. The first oxygen binds with some difficulty, but once it is on, the whole molecule loosens slightly and each following seat binds more easily — this is called cooperativity. So in the oxygen-rich lungs, hemoglobin fills almost completely, and in oxygen-poor tissues it lets go of its oxygen quickly.
The surroundings help too. Busy tissues produce carbon dioxide and acid, which push hemoglobin to release more oxygen; back in the lungs, carbon dioxide is breathed out and hemoglobin is ready to load up again.
So hemoglobin is not just a container for iron; it is a molecular machine that switches with its surroundings, and some mutations that change a single amino acid are enough to make it fail — sickle cell disease is one example.
One hemoglobin molecule is built from 2 alpha (α) chains and 2 beta (β) chains. Each chain holds one heme with an iron atom at its center, so there are 4 seats for oxygen, and each seat holds 1 oxygen molecule.
The 4 seats do not work independently. The first oxygen binds with some difficulty, but once it is on, the whole molecule loosens slightly and each following seat binds more easily — this is called cooperativity. So in the oxygen-rich lungs, hemoglobin fills almost completely, and in oxygen-poor tissues it lets go of its oxygen quickly.
The surroundings help too. Busy tissues produce carbon dioxide and acid, which push hemoglobin to release more oxygen; back in the lungs, carbon dioxide is breathed out and hemoglobin is ready to load up again.
So hemoglobin is not just a container for iron; it is a molecular machine that switches with its surroundings, and some mutations that change a single amino acid are enough to make it fail — sickle cell disease is one example.
Clinical · Classify an anemia before treating it
Anemia means hemoglobin below the lower reference limit, but it is a finding, not a diagnosis: you need to work out which kind it is before you know what to give and what to look for.The usual first cut is the average size of the red cells (mean corpuscular volume, MCV):
Small, pale cells (MCV below 80 fL): common causes are iron deficiency, anemia of chronic disease and thalassemia (beta-thalassemia). In iron deficiency, , transferrin saturation and serum iron all fall; in anemia of chronic disease, ferritin is normal or high (hepcidin has locked the iron away) while serum iron falls. The two respond to iron in opposite ways: iron deficiency needs iron, anemia of chronic disease needs the underlying disease treated first. In thalassemia the problem lies in making the globin chains, not in iron.
Large cells (MCV above 100 fL): common causes are vitamin B12 or folate deficiency, also called megaloblastic anemia. The raw materials for making DNA run short, so red cells grow large but few are made, and a blood smear can show neutrophils with too many lobes. B12 deficiency raises both and ; folate deficiency raises only homocysteine. Folate alone can bring the blood count back but cannot repair the nerve damage caused by B12 deficiency.
Normal-sized, normally colored cells (MCV 80–100 fL): common causes are acute blood loss, hemolysis, chronic disease, kidney disease and marrow suppression. In kidney failure, the kidney makes too little erythropoietin (EPO), and the anemia is treated with recombinant EPO. In hemolysis, reticulocytes (young red cells just out of the marrow) increase, indirect bilirubin and lactate dehydrogenase (LDH) rise, and haptoglobin falls.
So the starting point for any anemia is a with MCV, ferritin and a reticulocyte count. If iron deficiency is found, the next question is where the iron is being lost: in adult men and women past menopause, iron deficiency has to be investigated as gastrointestinal (GI) bleeding until shown otherwise (see Iron-Deficiency Anemia).
Anemia, so take iron is a dangerous shortcut: it can hide bowel cancer, nerve damage from B12 deficiency, kidney failure, thalassemia and other key diagnoses.
Mechanism · Allostery and hemoglobin disorders
How hemoglobin fills almost completely in the lungs and empties quickly in the tissues involves more than cooperativity; several finer layers of control sit on top of it.R/T allostery: hemoglobin switches between two shapes. At low oxygen the four chains pull tight against each other (the T, or tense, state) and bind oxygen poorly; once oxygen starts to bind, the structure relaxes into the R (relaxed) state, which binds oxygen more easily. That tightening and relaxing is the physical mechanism behind cooperativity.
The Bohr effect (named for Christian Bohr, who described it in 1904): the carbon dioxide, hydrogen ions and heat produced by working tissue push hemoglobin toward the T state, so it releases more oxygen at the same oxygen level. In the lungs, carbon dioxide is breathed out, acidity falls, hemoglobin is pushed back toward the R state, and it loads more oxygen. In numbers: in the lungs (partial pressure of oxygen, PO₂, 100 mmHg) hemoglobin is nearly 100% saturated; in the tissues (40 mmHg) saturation falls to about 75% — so at rest it unloads only about a quarter, and the other three-quarters is reserve. During exercise, the local Bohr effect, rising temperature and 2,3-BPG add up and call on that reserve too, and oxygen extraction can reach 70–80%.
2,3-BPG (2,3-bisphosphoglycerate) is a by-product of the red cell's own glycolysis. It stabilizes the T state and helps release more oxygen. At high altitude or with long-term low oxygen, 2,3-BPG rises within hours as an adaptation; stored blood-bank red cells slowly lose their 2,3-BPG, so in the first few hours after a transfusion, those cells deliver oxygen less efficiently.
Fetal hemoglobin (HbF): the fetus uses hemoglobin made of α₂γ₂. Gamma chains bind 2,3-BPG poorly, so the oxygen dissociation curve of HbF sits further left, with a higher affinity for oxygen than the mother's HbA — which is how the fetus wins oxygen at the placenta. In the first 6 months after birth, HbF is gradually replaced by HbA.
A small structural error can have large effects. Sickle cell disease (HbS) is a change at position 6 of the beta chain from glutamate to valine: when oxygen is low, the hemoglobin polymerizes into long fibers that twist red cells into sickles, block small blood vessels and cause hemolysis. In beta-thalassemia too little beta chain is made; the excess alpha chains precipitate and the spleen destroys the red cells before they mature. In alpha-thalassemia too little alpha chain is made; its severity depends on how many alpha genes are missing, and losing all 4 causes death before birth.
Production and recycling: the marrow makes about 2 million red cells a second, about 200 billion a day; red cells live about 120 days and are recycled by macrophages in the spleen and liver, where about 95% of the iron is reused, so only 1–2 mg is lost each day.
Chapter 5
Ferritin shows your iron stores
is the body's iron warehouse: 24 subunits form a hollow protein cage that locks iron away in its oxidized (Fe³⁺) form. It keeps iron in reserve and stops free iron from driving the Fenton reaction, which makes free radicals that damage cells. The main warehouse is the liver; the spleen, bone marrow and muscle also hold some.
The ferritin measured in a blood test is the small share that leaks from cells into the blood. It rises and falls with the body's stores, so it is the most direct number for reading the warehouse. Reading it low is reliable: low ferritin almost always means the stores are empty. Reading it high takes care: ferritin is also an acute-phase reactant, pushed up by inflammation and liver disease, so a normal or even high ferritin cannot rule out iron deficiency, and it cannot on its own show iron overload.
The stretch when the stores are already empty but hemoglobin has not yet fallen is called iron deficiency without anemia. Many people in it are exhausted, yet are sent away with a lab report that says they are not anemic.
The ferritin measured in a blood test is the small share that leaks from cells into the blood. It rises and falls with the body's stores, so it is the most direct number for reading the warehouse. Reading it low is reliable: low ferritin almost always means the stores are empty. Reading it high takes care: ferritin is also an acute-phase reactant, pushed up by inflammation and liver disease, so a normal or even high ferritin cannot rule out iron deficiency, and it cannot on its own show iron overload.
The stretch when the stores are already empty but hemoglobin has not yet fallen is called iron deficiency without anemia. Many people in it are exhausted, yet are sent away with a lab report that says they are not anemic.
Mechanism · What the cage holds and where
is a spherical protein cage built from 24 subunits. Each cage can hold up to about 4,500 iron atoms, stored in the middle as a hydrated iron phosphate of oxidized iron (Fe³⁺).It does two jobs. The first is storage: it locks iron away safely so that free iron cannot drive the Fenton reaction, which produces hydroxyl radicals (•OH) that damage DNA, proteins and cell membranes. The second is buffering: it releases iron when the body needs it and takes it back when there is extra.
Stored iron sits mainly as ferritin and its breakdown product hemosiderin, in the liver, spleen and bone marrow; the iron in muscle is mostly in myoglobin.
A blood test measures serum ferritin: the small share that leaks from cells into the blood. It rises and falls with the iron stored in the tissues, which makes it the most direct marker of iron stores.
Reading it downward is straightforward: the lower it is, the emptier the warehouse. Reading it upward takes a second number, because ferritin is also an acute-phase reactant — inflammation, liver disease and metabolic syndrome all push it up — so a high ferritin on its own does not mean too much iron. What really prompts a work-up for hemochromatosis is transferrin saturation and ferritin raised together. The European Association for the Study of the Liver (EASL) 2022 guideline judges iron overload in genetically confirmed patients with these lines: in women, saturation above 45% and ferritin above 200 µg/L; in men and postmenopausal women, saturation above 50% and ferritin above 300 µg/L. The chapter on iron overload in this story uses the same lines.
A state that is often overlooked is iron deficiency without anemia: ferritin has already fallen below about 30 while hemoglobin is still normal. The body is already rationing iron, with low energy, poor exercise tolerance, restless legs, thinning hair and poorer concentration — yet a lab report that says hemoglobin normal often sends the person home. So if you suspect iron deficiency, ferritin is a far more sensitive test than hemoglobin alone.
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Clinical · Why restless legs start with an iron check
The link between restless legs syndrome (RLS) and low iron is a clue that clinicians often miss.The mechanism: the first step in making dopamine in the brain depends on an enzyme called tyrosine hydroxylase (TH), which needs iron. When the brain regions that control movement (the substantia nigra and basal ganglia) run short of iron, dopamine signaling at night goes awry. The result is a hard-to-describe discomfort in the legs and an urge to move them that gets worse toward night and makes falling asleep hard.
The basis is an evidence-based and consensus guideline from a task force of the International Restless Legs Syndrome Study Group (IRLSSG) (Allen 2018): people with restless legs have low iron in the brain, even when serum iron and hemoglobin are normal.
The guideline uses serum to decide on iron: oral iron is recommended when ferritin is 75 µg/L or lower and transferrin saturation is under 45%, and intravenous iron can be considered when ferritin is 100 µg/L or lower. Both lines sit far above the roughly 30 commonly used to judge that iron stores are empty. Within that range, the guideline considers that iron may improve symptoms; for people whose iron is already sufficient, it does not recommend iron as a treatment for restless legs.
Restless legs is especially common in pregnancy, and the steep rise in iron needs late in pregnancy is one reason.
So for someone whose legs need to move at night and who struggles to fall asleep, if ferritin is low, checking iron and taking iron under a doctor's guidance is the first step — do not jump straight to dopamine agonists.
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Clinical · Reading a ferritin result
| Ferritin (µg/L) | What it means |
|---|---|
| < 15 | Absolute iron deficiency (empty warehouse) |
| 15–30 | Low stores, borderline |
| 30–100 | Generally adequate |
| 100–300 | Replete |
| > 300 | High: think inflammation, liver disease or metabolic syndrome first; this number alone cannot establish iron overload |
| Transferrin saturation and ferritin both raised | Work up iron overload; next step is HFE gene testing |
Transferrin saturation = the share of the blood's iron-carrying protein that is currently loaded with iron.
The last two rows are not two marks on one ruler; they are two different things. A high on its own is most often explained by inflammation, not iron overload; moving toward hemochromatosis needs transferrin saturation as a second marker. The chapter on iron overload in this story uses the same threshold.
The low bands also need an inflammation marker beside them: chronic inflammation pushes ferritin up, so someone with a raised C-reactive protein () whose ferritin sits at 30–100 may still be truly iron-deficient. For any anemia, the first pair of tests is ferritin plus CRP or the sedimentation rate.
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Chapter 6
Low iron before anemia shows
Anemia is the last stop of iron deficiency, not the first. The stores empty first and the body starts to ration iron: tiring easily, lower exercise tolerance, poor concentration, restless legs and thinning hair, often while hemoglobin is still in the normal range. Only later come thin, spoon-shaped nails, cravings for things like ice, pallor and breathlessness.
The people most likely to run short are women with heavy periods, pregnant women, people who get most of their iron from plants, people with slow bleeding in the gut, and babies and toddlers whose foods after weaning do not supply enough iron. For them, a blood test before deciding whether to take iron matters more than buying iron tablets on their own.
The people most likely to run short are women with heavy periods, pregnant women, people who get most of their iron from plants, people with slow bleeding in the gut, and babies and toddlers whose foods after weaning do not supply enough iron. For them, a blood test before deciding whether to take iron matters more than buying iron tablets on their own.
Clinical · Who runs short, and how it feels
Iron deficiency is one of the most common nutritional deficiencies in the world.The people most likely to run short:
Women of reproductive age and anyone with heavy periods: 30–80 mL of blood lost a month means 15–40 mg of iron lostPregnancy and breastfeeding: building blood volume for the fetus and the baby sharply raises the need (the recommended intake in pregnancy is 27 mg a day, against 18 mg otherwise)Fully or nearly vegan diets: non-heme iron is poorly absorbed, and there is no meat to boost itEndurance athletes: foot-strike hemolysis, raised hepcidin after training and diluted blood plasma are three different things, which the chapter on endurance athletes in this story takes apart — do not merge them into one diagnosisPeople with chronic inflammation or bleeding in the gut: for example bowel cancer, hemorrhoids, or long-term aspirin useBabies and toddlers aged 6–24 months: after weaning, their foods often do not supply enough iron
Symptoms run roughly from mild to severe, though they vary from person to person and do not follow a strict order:
1. Tiring easily and lower exercise tolerance ( is often already below 30)
2. Poorer concentration and memory (low stores may affect the making of neurotransmitters)
3. Restless legs syndrome: uncomfortable legs at night that have to move
4. Thinning hair that falls out easily
5. Thin, brittle nails that hollow in the middle into spoon nails (koilonychia)
6. Pica: cravings for ice, soil or paper, a classic sign of iron deficiency
7. Pale skin, palpitations and breathlessness (frank anemia)
So anemia is already the late stage, and many people are worn down through the earlier stages for a long time without a diagnosis.
In practice: people at high risk are better off having hemoglobin and ferritin checked once than buying iron tablets on their own. Test first, then supplement.
Numbers · How much iron each life stage needs
Iron needs change a great deal across life, so the recommended intake is not one fixed number.| Stage | Recommended intake (mg/day) | Key risk |
|---|---|---|
| Infants 0–6 months | 0.27 (Adequate Intake, AI) | Breast milk has little iron, but it is well absorbed; preterm and formula-fed babies depend on fortification |
| Infants 7–12 months | 11 | Foods after weaning must contain iron, such as puréed meat or fortified rice cereal |
| Toddlers 1–3 years | 7 | Iron deficiency at this age is linked with later delays in cognitive development |
| Children 4–8 years | 10 | |
| Boys 9–13 years | 8 | |
| Boys 14–18 years | 11 | Growth and gains in muscle |
| Girls 14–18 years | 15 | Needs rise sharply after the first period |
| Men 19–50 years | 8 | Usually no need for extra iron; excess is the bigger worry |
| Women 19–50 years | 18 | Menstruation |
| Pregnancy | 27 | The fetus, the placenta and the extra blood volume; hard to reach from food alone, so whether and how much to supplement is for the prenatal-care doctor to decide |
| Breastfeeding | 9 | Periods have often not returned yet, so needs actually fall |
| Women past menopause | 8 | Periods have stopped; the same as men |
| Endurance training | Separate the three mechanisms before talking about dose | Foot-strike hemolysis, raised hepcidin after training and plasma dilution, taken apart in the chapter on endurance athletes in this story |
The table gives the US Recommended Dietary Allowances (RDAs); the figure for 0–6 months is an Adequate Intake ().
Iron matters especially in infancy and early childhood. Roughly 6 months to 3 years is a critical period for myelination in the brain, the making of neurotransmitters and the development of the hippocampus, and iron takes part in all of it (Lozoff & Georgieff 2006). In a follow-up cohort in Costa Rica, children who had severe, chronic iron deficiency as infants and were treated for it still did worse on arithmetic, writing, motor function and some cognitive tests more than 10 years later than children who had good iron status as infants, and the gaps remained after adjusting for family background (Lozoff 2000, published in *Pediatrics*). This is an observational study: it shows these children carry a higher long-term risk, but it cannot fully rule out other factors.
So the cost of iron deficiency in babies and toddlers may be far greater than the anemia visible at the time, and preventing it matters more than making it up later.
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Chapter 7
Why endurance athletes test low
Endurance sport often labels a low hemoglobin on a blood test sports anemia and then prescribes iron. That one result may actually be three mechanically different things stacked together.
The first is the feet crushing red cells, as the soles strike the ground again and again during running. The second is the gut's iron exit shutting for a few hours after training: exercise raises hepcidin, which closes the gut cell's iron-exit channel, ferroportin. The third is not a loss at all: endurance training first expands the plasma volume, so hemoglobin concentration is diluted even though the total number of red cells may not have fallen.
All three can stack in one person, but each is read differently on a lab report: the warehouse is read from , not from a diluted hemoglobin concentration. So runners should check ferritin first and then decide whether to take iron.
The first is the feet crushing red cells, as the soles strike the ground again and again during running. The second is the gut's iron exit shutting for a few hours after training: exercise raises hepcidin, which closes the gut cell's iron-exit channel, ferroportin. The third is not a loss at all: endurance training first expands the plasma volume, so hemoglobin concentration is diluted even though the total number of red cells may not have fallen.
All three can stack in one person, but each is read differently on a lab report: the warehouse is read from , not from a diluted hemoglobin concentration. So runners should check ferritin first and then decide whether to take iron.
Mechanism · Running's extra hemolysis comes from the feet
Telford 2003 had 10 male triathletes run for an hour and cycle for an hour at the same intensity (75% of peak oxygen uptake). Free hemoglobin in the plasma rose after both, but about four times as much after running; haptoglobin (the blood protein that mops up free hemoglobin and is used up during hemolysis) fell only after the hour of running. Cycling still broke a few red cells, which shows that circulation itself wears some cells down; but the large extra amount in running comes from the feet striking the ground.This was a small crossover comparison with no control group. It answers where the hemolysis comes from, not whether runners become anemic because of it.
Mechanism · The same door, shut for a few hours after
Iron leaving the gut cell for the blood has only one exit, ferroportin. Hepcidin grabs it and has it taken into the cell and broken down, so iron inside the gut cell can only leave when the lining is shed. Moretti 2015, in young women with low iron stores, found that a larger oral iron dose itself raises hepcidin and cuts absorption of the next dose — the basis for alternate-day dosing. Exercise uses a different trigger: working muscle releases , the liver reads it as an inflammation signal, and hepcidin rises in the same way.The timing is the point. In Peeling 2009, 8 trained athletes did one 60-minute hard run and one session of seated rest (a crossover design): IL-6 was already up the moment the run ended, and urinary hepcidin was higher than before the run at 3, 6 and 24 hours of recovery. In other words, the absorption discount falls in the hours after the run, not during it. Barney 2022, in trained runners with low iron stores, timed an isotope-labeled iron meal to the post-exercise hepcidin peak: hepcidin was about half again as high as on a rest day, and the fraction of dietary iron absorbed was about a third lower. The paper's own exploratory analysis suggested the rise may be larger in men. Putting an iron tablet or an iron-rich meal into that window is feeding a door that has just closed.
The mechanism predicts that in people whose iron stores are very low, hepcidin is already pushed down and the window may be smaller; but Barney 2022's participants were runners with low iron stores, and they still showed reduced absorption. So this step explains why absorption is worse in the hours right after training; it is not a dosing table that fits everyone.
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Clinical · Low concentration is not an empty store
Convertino 1991, a review, describes the training-driven expansion of blood volume as an adaptation: for the first two to four weeks it is almost all plasma, the heart fills more fully, and there is extra water for shedding heat. Hemoglobin concentration therefore looks a notch lower, while the total number of red cells may not have fallen. Weight 1992 compared distance runners, triathletes, ballet dancers and people who did not exercise: average hemoglobin was in the normal range in every group, and true iron-deficiency anemia was no more common in the athletes than in the non-exercisers; some people fell below the usual hemoglobin cut-off, which an expanded plasma volume can explain. Weight therefore concluded that sports anemia does not name a disease of its own.So a low hemoglobin needs at least three questions first. Did the feet crush red cells (haptoglobin will fall)? Is it a few hours after training, with the door still shut (serum iron is then held back by hepcidin and is not a reading of the stores)? Or has the plasma expanded ( is what reflects the stores)?
The rule of the rest of this story holds here too: read ferritin first, then decide whether to supplement. Do not collapse the three mechanisms into runner's anemia, prescribe iron.
Chapter 8
Too much iron is possible
Iron is not like a water-soluble vitamin, where the excess is simply flushed out — the body has no active exit for iron. Net daily loss is only 1–2 mg, so balance is kept entirely by adjusting absorption.
Overload comes in two kinds.
Acute poisoning (a child swallowing adult iron tablets): counted as elemental iron, the dose falls into three bands — under 20 mg/kg is usually uneventful; 20–60 mg/kg is mild-to-moderate poisoning; over 60 mg/kg can be fatal. The band to remember is the middle one: at 40 mg/kg or more, or with clear, persistent symptoms, the child must be assessed in hospital.
The arithmetic shows how big the gap is: one 325 mg ferrous sulfate tablet holds about 65 mg of elemental iron. A 12 kg toddler passes 20 mg/kg after 5 tablets, while 60 mg/kg takes 11 — remember only the top band and you will read a child who needs hospital care as one to wait and see.
If you suspect a child has swallowed iron tablets, do not wait at home for symptoms — contact poison control or go to the emergency department now.
Chronic overload takes a different route: in hereditary hemochromatosis the hepcidin response is faulty, so the body over-absorbs iron for life; screening looks at whether transferrin saturation and are both raised.
Test ferritin before you supplement — that is the one rule with iron.
Overload comes in two kinds.
Acute poisoning (a child swallowing adult iron tablets): counted as elemental iron, the dose falls into three bands — under 20 mg/kg is usually uneventful; 20–60 mg/kg is mild-to-moderate poisoning; over 60 mg/kg can be fatal. The band to remember is the middle one: at 40 mg/kg or more, or with clear, persistent symptoms, the child must be assessed in hospital.
The arithmetic shows how big the gap is: one 325 mg ferrous sulfate tablet holds about 65 mg of elemental iron. A 12 kg toddler passes 20 mg/kg after 5 tablets, while 60 mg/kg takes 11 — remember only the top band and you will read a child who needs hospital care as one to wait and see.
If you suspect a child has swallowed iron tablets, do not wait at home for symptoms — contact poison control or go to the emergency department now.
Chronic overload takes a different route: in hereditary hemochromatosis the hepcidin response is faulty, so the body over-absorbs iron for life; screening looks at whether transferrin saturation and are both raised.
Test ferritin before you supplement — that is the one rule with iron.
Clinical · Swallowed iron tablets and hemochromatosis
Iron tablets are one of the most common preventable causes of drug poisoning in children. Poisoning shows up as severe bleeding in the gut, metabolic acidosis and liver failure; serious poisoning usually declares itself within 6 hours of swallowing, but that observation window is for medical staff to watch, not for parents to count out at home.A few principles you can use directly: men and women past menopause should not routinely take a multivitamin with iron or an iron supplement without clear evidence of iron deficiency; multivitamins made for men usually contain no iron, and that is by design; and iron supplements at home belong where children cannot possibly reach them.
Hereditary hemochromatosis (HH, homozygous for the C282Y variant of the HFE gene) is the most common autosomal recessive disease in people of European descent. About 1 in 200–300 people of Northern European descent are C282Y homozygous, and about 10% are heterozygous carriers; it is very rare in Asian and African populations. The mechanism is loss of HFE function: hepcidin fails to rise when it should, the body over-absorbs iron for life, and iron builds up in the liver, heart, pancreas, skin, joints and sex glands.
But carrying two copies does not mean certain disease: many more people show raised iron on lab tests than ever develop organ damage, and men develop the disease more often than women (European Association for the Study of the Liver, EASL, 2022 guideline). Symptoms usually appear after age 40–60: fatigue, joint pain and an enlarged liver, progressing to cirrhosis, heart muscle disease, diabetes (formerly called bronze diabetes, because the skin darkens at the same time) and shrinking of the sex glands. Women, because of menstrual blood loss, usually develop it about 10 years later.
Screening: transferrin saturation and raised together call for HFE gene testing. For people with C282Y homozygosity, EASL 2022 judges iron overload with these lines: in women, saturation above 45% and ferritin above 200 µg/L; in men and postmenopausal women, saturation above 50% and ferritin above 300 µg/L. These are the lines a doctor diagnoses with; when you read your own results, saturation above 45% with raised ferritin is already a reason to see a doctor, without waiting for these lines.
Treatment: regular removal of blood from a vein (phlebotomy) is the main treatment. How often, and down to what level, is set by the doctor from blood tests: usually more often at first to bring the stores down, then maintenance every few months, for life. People who start treatment before cirrhosis develops have a near-normal life expectancy; those who already have cirrhosis still face a clearly higher risk of liver cancer (hepatocellular carcinoma) even with treatment, and need lifelong monitoring.
If someone with a family history newly develops arthritis, a raised (a liver enzyme on the liver-function panel) and diabetes, perhaps with darkening skin, think of checking for iron overload rather than putting each one down to another common disease.
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References · 13
- National Institutes of Health, Office of Dietary Supplements. (2024). Iron — Fact Sheet for Health Professionals. Fact sheet (updated September 4, 2025; Wayback snapshot 21 September 2026): RDAs 8 mg/day for men and for women 51+, 18 mg women 19-50, 27 mg pregnancy; UL 45 mg/day from age 14; bioavailability about 14%-18% from mixed diets with meat, seafood and vitamin C and 5%-12% from vegetarian diets; serum ferritin below 30 mcg/L suggests iron deficiency and below 10 mcg/L IDA, but inflammation can raise ferritin; supplemental iron of 45 mg/day or more may cause nausea and constipation; people with hereditary hemochromatosis are at risk of iron overload (fact sheet). Heme vs nonheme: heme iron (lean meat and seafood are the richest sources) has higher bioavailability than nonheme iron, and other dietary components affect it less; calcium might reduce the bioavailability of both forms; heme iron is about 10%-15% of total iron intake in western populations. The sheet gives no separate heme and nonheme absorption percentages (fact sheet, Wayback 2026 snapshot). ods.od.nih.gov/factsheets/Iron-HealthProfessional
- Hallberg, L., Brune, M., & Rossander, L. (1989). The role of vitamin C in iron absorption. International Journal for Vitamin and Nutrition Research. Supplement, 30, 103–108. Short review: ascorbic acid has a key role in non-heme iron absorption by preventing insoluble, unabsorbable iron compounds and by reducing ferric to ferrous iron. The abstract gives no dose-response numbers (e.g. no mg of vitamin C or fold increase) (abstract, PMID 2507689).
- Hurrell, R., & Egli, I. (2010). Iron bioavailability and dietary reference values. The American Journal of Clinical Nutrition, 91(5), 1461S-1467S. Abstract only (the AJCN supplement full text could not be retrieved): iron bioavailability is estimated at 14-18% for mixed diets and 5-12% for vegetarian diets in subjects with no iron stores, the values used to set dietary reference values. Phytate, polyphenols, calcium, ascorbic acid and muscle tissue shift absorption strongly in single-meal isotope studies but only modestly in multimeal studies with a varied diet; the nature of the meat factor is unresolved; iron status generally has a greater effect than diet composition. The abstract gives no separate heme vs non-heme absorption percentages (abstract, PMID 20200263). 10.3945/ajcn.2010.28674F
- Hurrell, R. F., Reddy, M., & Cook, J. D. (1999). Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. British Journal of Nutrition, 81(4), 289-295. Black tea reduced non-heme iron absorption 79-94% vs a water-control bread meal; peppermint 84%, cocoa 71%, camomile 47%. Dose-dependent with total polyphenols. Green tea was not tested. 10.1017/S0007114599000537
- Camaschella, C. (2015). Iron-deficiency anemia. The New England Journal of Medicine, 372(19), 1832–1843. 10.1056/NEJMra1401038
- Institute of Medicine. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academies Press. www.ncbi.nlm.nih.gov/books/NBK222310
- Ajioka, R. S., Phillips, J. D., & Kushner, J. P. (2006). Biosynthesis of heme in mammals. Biochimica et Biophysica Acta (BBA) — Molecular Cell Research, 1763(7), 723–736. 10.1016/j.bbamcr.2006.05.005
- Needleman, H. (2004). Lead poisoning. Annual Review of Medicine, 55, 209–222. Narrative review in Annual Review of Medicine (not Pediatrics, whatever the id says): the least observable effect level of lead has fallen until it approaches zero; removing lead from gasoline cut environmental lead; the remaining major source is older housing (lead paint). The abstract gives no blood-lead thresholds or IQ figures (abstract, PMID 14746518). 10.1146/annurev.med.55.091902.103653
- Telford, R. D., Sly, G. J., Hahn, A. G., Cunningham, R. B., Bryant, C., & Smith, J. A. (2003). Footstrike is the major cause of hemolysis during running. Journal of Applied Physiology, 94(1), 38-42. Ten male triathletes, 1 h running vs 1 h cycling at the same 75% peak oxygen uptake. Plasma free hemoglobin rose after both, but about fourfold more after running; haptoglobin fell 1 h after running only. Methemoglobin (oxidative stress marker) rose similarly in both modes. The authors' own conclusion: circulatory trauma can hemolyze some red cells, but footstrike is the major contributor during running. 10.1152/japplphysiol.00631.2001
- Peeling, P., Dawson, B., Goodman, C., Landers, G., Wiegerinck, E. T., Swinkels, D. W., & Trinder, D. (2009). Effects of exercise on hepcidin response and iron metabolism during recovery. International Journal of Sport Nutrition and Exercise Metabolism, 19(6), 583-597. Eight moderately trained athletes, crossover: 60 min hard running vs seated rest. Interleukin-6 rose immediately after the run (~6.9×); urinary hepcidin was higher at 3, 6 and 24 h of recovery than pre-run and immediate post-run. Serum iron rose immediately after the run. The authors conclude high-intensity exercise raises hepcidin after IL-6 and serum iron have already moved. n is small; the load-bearing fact is the delayed hepcidin window, not a dosing schedule. 10.1123/ijsnem.19.6.583
- Barney, D. E., Ippolito, J. R., Berryman, C. E., & Hennigar, S. R. (2022). A prolonged bout of running increases hepcidin and decreases dietary iron absorption in trained female and male runners. The Journal of Nutrition, 152(9), 2039-2047. Trained runners with low iron stores: a long run vs rest, then a labelled iron meal timed to the post-exercise hepcidin peak. Plasma hepcidin was 51% higher after exercise than rest; fractional iron absorption was 36% lower. Plasma IL-6 was also higher after exercise. The paper's own limit: exploratory splits suggested the hepcidin rise may be driven more in males than females in this sample. 10.1093/jn/nxac129
- Convertino, V. A. (1991). Blood volume: its adaptation to endurance training. Medicine and Science in Sports and Exercise, 23(12), 1338-1348. Narrative review of cross-sectional and longitudinal work: endurance training expands blood volume. For the first 2-4 weeks nearly all of that expansion is plasma; later the extra volume is shared more evenly with red cell mass. The extra plasma is a training adaptation (filling pressure, heat dump), not a disease. It will make hemoglobin concentration look lower even when total red cell mass has not fallen. 10.1249/00005768-199112000-00004
- Weight, L. M., Klein, M., Noakes, T. D., & Jacobs, P. (1992). 'Sports anemia' — a real or apparent phenomenon in endurance-trained athletes? International Journal of Sports Medicine, 13(4), 344-347. Male and female distance runners, male triathletes, female ballet dancers vs non-exercising controls. Mean hemoglobin sat in the normal range and did not differ by training status within sex. Frank iron-deficiency anemia was uncommon (about 1.7-3.3% of the runner groups). Some athletes had hemoglobin below the conventional cut; the authors attribute that to an expanded plasma volume (pseudoanemia). Their conclusion: athletes are at no greater risk of frank anemia than non-exercisers, and 'sports anemia' does not name a specific clinical entity. 10.1055/s-2007-1021278