Place · Level 3
Iron
两种形式 · 红血球的核心 · 储存有名片 · 不该乱补
Story path
- 1Two kinds of ironTwo kinds of iron
- 2Gut · gated by needGut · gated by need
- 3Heme biosynthesis · 8 stepsHeme biosynthesis · 8 steps
- 4Red cells · oxygen courierRed cells · oxygen courier
- 5Ferritin · the storage name tagFerritin · the storage name tag
- 6Deficiency ≠ anemia (yet)Deficiency ≠ anemia (yet)
- 7Don't supplement blindlyDon't supplement blindly
Chapter 1
Two kinds of iron
Two kinds of iron
Dietary iron comes in two forms; chemical state decides its absorption fate.
Heme iron is Fe²⁺ wrapped in a porphyrin ring, sourced from animal hemoglobin and myoglobin (meat, liver, blood, fish). Absorption 15–35%, stable, barely affected by other foods at the same meal. It enters enterocytes directly through HCP1 (heme carrier protein), then HO-1 (heme oxygenase) releases the iron from the porphyrin ring inside the cell.
Non-heme iron is free Fe³⁺/Fe²⁺ from plants (legumes, leafy greens, fortified grains, nuts) plus egg yolk and a few animal sources. Absorption 2–20%, highly variable, dominated by what else is on the plate. It must first be reduced to Fe²⁺, then transported into the enterocyte via DMT1.
Meal-table enhancers and inhibitors:
Practical: vegetarians should pair every meal's plant iron with vitamin C, and push tea/coffee an hour after meals.
RDA: men 8, women 18, pregnancy 27 mg/day — women need more than double men, mostly because of menstrual loss.
Heme iron is Fe²⁺ wrapped in a porphyrin ring, sourced from animal hemoglobin and myoglobin (meat, liver, blood, fish). Absorption 15–35%, stable, barely affected by other foods at the same meal. It enters enterocytes directly through HCP1 (heme carrier protein), then HO-1 (heme oxygenase) releases the iron from the porphyrin ring inside the cell.
Non-heme iron is free Fe³⁺/Fe²⁺ from plants (legumes, leafy greens, fortified grains, nuts) plus egg yolk and a few animal sources. Absorption 2–20%, highly variable, dominated by what else is on the plate. It must first be reduced to Fe²⁺, then transported into the enterocyte via DMT1.
Meal-table enhancers and inhibitors:
| Enhancers (Fe³⁺ → Fe²⁺ reduction + chelation) | Inhibitors (lock-up / competition) |
|---|---|
| Vitamin C ~25 mg, doubles absorption | Tea/coffee tannins, ~60% inhibition |
| Meat/fish at same meal (heme synergy) | Phytate (raw legumes, whole grains) |
| Citric/lactic acid (fermented foods) | Oxalate (spinach) |
| Mild acid (lemon water, vinegar) | Calcium/milk (strong non-heme block) |
Practical: vegetarians should pair every meal's plant iron with vitamin C, and push tea/coffee an hour after meals.
RDA: men 8, women 18, pregnancy 27 mg/day — women need more than double men, mostly because of menstrual loss.
Iron supplements in practice
When dietary iron isn't enough and ferritin is already low, supplements are a useful tool — but picking the wrong form or wrong timing wastes most of the dose.Main forms:
| Form | Elemental Fe | Absorption | Side effects |
|---|---|---|---|
| Ferrous sulfate | 20% (~65 mg/tablet) | Standard, cheap | GI upset most common |
| Ferrous fumarate | 33% | Similar to sulfate | Similar |
| Ferrous gluconate | 12% | Slightly gentler | Less GI upset |
| Bisglycinate | 20% | Better, gentler | Pricier, good for sensitive gut |
| Polysaccharide-iron complex | 100% (labeled) | Inferior to ionic | Virtually no GI upset |
| Ferric carboxymaltose (IV) | n/a | ~100% | Rapid correction, medical setting |
Dose principle (2020s refresh): the old idea of 'split into 2–3 doses/day (60 mg × 2–3)' has been overturned. Moretti 2015 and Stoffel 2017 show that alternate-day single doses of 60–120 mg work better — each iron pulse raises hepcidin for 24 hours, so next-day absorption drops to ~1/3. So 'every-other-day fasting pill + 250 mg vitamin C' delivers more absorbed iron than 'three times a day with meals'.
Key points: avoid tea, coffee, milk, and calcium supplements at the same meal; treat deficiency for at least 3–6 months until ferritin climbs back to ~50 µg/L before stopping.
For GI intolerance, switch to gluconate or bisglycinate, move to every-other-day, or take with food (absorption drops ~30% but adherence improves).
Chapter 2
Gut · gated by need
Gut · gated by need
Unlike most nutrients, iron absorption is reverse-regulated — the body has no active iron excretion route, so it controls total iron by controlling uptake.
The regulatory core is hepcidin (a 25-amino-acid peptide secreted by the liver). The mechanism:
1. Ferroportin (FPN) on the enterocyte basolateral membrane is the only exit channel from cell into blood
2. Hepcidin binds ferroportin, internalizes and degrades it, closing the channel
3. Iron piled up inside enterocytes leaves with normal epithelial shedding into stool — net absorption drops
Hepcidin itself responds to several signals: iron stores full (high ferritin) → hepcidin up → door shut; stores low → hepcidin down → door open; hypoxia or high erythropoietic demand → erythroferrone lowers hepcidin → door open; inflammation (interleukin-6: A pro-inflammatory signal molecule (cytokine) released by immune cells during inflammation.) → hepcidin spikes sharply, iron locked inside macrophages and enterocytes — this is the chemical root of anemia of chronic disease.
So healthy people don't overload — hepcidin throttles automatically. Hereditary hemochromatosis (HFE mutation) breaks hepcidin response, causing lifelong over-absorption with iron accumulating in liver, heart, and pancreas. Chronic inflammation (IBD, RA, CKD) sustains high hepcidin so that even with low stores, iron can't enter — refractory anemia.
A counter-intuitive but elegant system — the body decides whether to absorb iron, not you.
The regulatory core is hepcidin (a 25-amino-acid peptide secreted by the liver). The mechanism:
1. Ferroportin (FPN) on the enterocyte basolateral membrane is the only exit channel from cell into blood
2. Hepcidin binds ferroportin, internalizes and degrades it, closing the channel
3. Iron piled up inside enterocytes leaves with normal epithelial shedding into stool — net absorption drops
Hepcidin itself responds to several signals: iron stores full (high ferritin) → hepcidin up → door shut; stores low → hepcidin down → door open; hypoxia or high erythropoietic demand → erythroferrone lowers hepcidin → door open; inflammation (interleukin-6: A pro-inflammatory signal molecule (cytokine) released by immune cells during inflammation.) → hepcidin spikes sharply, iron locked inside macrophages and enterocytes — this is the chemical root of anemia of chronic disease.
So healthy people don't overload — hepcidin throttles automatically. Hereditary hemochromatosis (HFE mutation) breaks hepcidin response, causing lifelong over-absorption with iron accumulating in liver, heart, and pancreas. Chronic inflammation (IBD, RA, CKD) sustains high hepcidin so that even with low stores, iron can't enter — refractory anemia.
A counter-intuitive but elegant system — the body decides whether to absorb iron, not you.
Anemia of chronic disease vs IDA
Anemia of chronic disease (ACD) is a common clinical diagnostic trap.Mechanism: chronic inflammation (RA, IBD, CKD, cancer, chronic infection) raises interleukin-6: A pro-inflammatory signal molecule (cytokine) released by immune cells during inflammation., which strongly upregulates hepcidin and locks iron inside macrophages and enterocytes — plasma iron is low, red cells can't be made, but iron stores are normal or elevated.
Differentiating true deficiency from ACD:
Iron deficiency anemia (IDA): ferritin drops (<30), transferrin saturation drops, plasma iron dropsACD: ferritin normal or elevated (acts as an inflammation marker), transferrin saturation drops, plasma iron dropsSoluble transferrin receptor (sTfR) is the gold-standard discriminator: elevated in IDA, normal in ACD
Treatment is opposite: IDA gets iron supplements; ACD requires treating the underlying disease first (plain iron supplementation is ineffective and can worsen inflammation and oxidative stress).
Practical: any anemia should start with ferritin + C-reactive protein: A liver protein that rises with inflammation — a common blood marker for 'is the body inflamed'./ESR — high ferritin plus high CRP suggests ACD; low ferritin suggests true deficiency.
Chapter 3
Heme biosynthesis · 8 steps
Heme biosynthesis · 8 steps
Loading iron into hemoglobin isn't a single step — it's an 8-step assembly line spanning mitochondria and cytosol. Any block stalls red cell production.
Step 1 (mitochondria, rate-limiting): glycine + succinyl-CoA → δ-aminolevulinic acid (ALA). The rate-limiting enzyme is ALA synthase (ALA-S), the master switch for the whole pathway, negatively regulated by heme (high heme shuts it down). Cofactor: vitamin B6 (PLP). This is why B6 deficiency produces microcytic hypochromic anemia (looks like iron deficiency but the block is synthesis), and why anti-TB drug isoniazid (INH), which antagonizes B6, does the same.
Step 2 (cytosol): 2× ALA → porphobilinogen (PBG), via ALA dehydratase (ALA-D), a zinc enzyme. This is lead's first target — lead displaces Zn²⁺, ALA piles up in blood and urine.
Steps 3–6 (cytosol): 4× PBG → hydroxymethylbilane (HMB) → uroporphyrinogen III → coproporphyrinogen III, closing the porphyrin ring and finishing side-chain modifications.
Steps 7–8 (back into mitochondria): coproporphyrinogen III → protoporphyrinogen IX → protoporphyrin IX (oxidative finish). The final step is ferrochelatase, which inserts Fe²⁺ into the porphyrin ring — heme is complete. This is lead's second target — lead takes the Fe²⁺ seat, Fe²⁺ can't enter, Zn²⁺ substitutes, erythrocyte zinc-protoporphyrin (ZPP) rises. ZPP is the gold-standard screen for occupational lead exposure.
Iron's upstream supply depends on copper as a hidden cofactor. Ceruloplasmin and hephaestin are multicopper oxidases that convert Fe²⁺ to Fe³⁺ so iron can load onto transferrin and be exported. Copper deficiency paralyzes iron mobilization — even with full iron stores, heme can't be made. This is 'copper anemia', often misdiagnosed as IDA.
On throughput: marrow makes ~2 million red cells per second, needing ~20 mg new iron per day for new heme (95% from recycled old red cells; net absorption is only 1–2 mg).
Lead poisoning is a double hit: ALA-D inhibited (blood and urine ALA rise) and ferrochelatase inhibited (ZPP rises). If you see 'anemia + sufficient iron + sufficient B6', check for lead. Chronic low-dose childhood lead exposure permanently damages cognition — after the US banned leaded gasoline in 1973 and lead paint in 1978, mean childhood blood lead dropped from 15 µg/dL to <1, and mean IQ rose 2–5 points.
Step 1 (mitochondria, rate-limiting): glycine + succinyl-CoA → δ-aminolevulinic acid (ALA). The rate-limiting enzyme is ALA synthase (ALA-S), the master switch for the whole pathway, negatively regulated by heme (high heme shuts it down). Cofactor: vitamin B6 (PLP). This is why B6 deficiency produces microcytic hypochromic anemia (looks like iron deficiency but the block is synthesis), and why anti-TB drug isoniazid (INH), which antagonizes B6, does the same.
Step 2 (cytosol): 2× ALA → porphobilinogen (PBG), via ALA dehydratase (ALA-D), a zinc enzyme. This is lead's first target — lead displaces Zn²⁺, ALA piles up in blood and urine.
Steps 3–6 (cytosol): 4× PBG → hydroxymethylbilane (HMB) → uroporphyrinogen III → coproporphyrinogen III, closing the porphyrin ring and finishing side-chain modifications.
Steps 7–8 (back into mitochondria): coproporphyrinogen III → protoporphyrinogen IX → protoporphyrin IX (oxidative finish). The final step is ferrochelatase, which inserts Fe²⁺ into the porphyrin ring — heme is complete. This is lead's second target — lead takes the Fe²⁺ seat, Fe²⁺ can't enter, Zn²⁺ substitutes, erythrocyte zinc-protoporphyrin (ZPP) rises. ZPP is the gold-standard screen for occupational lead exposure.
Iron's upstream supply depends on copper as a hidden cofactor. Ceruloplasmin and hephaestin are multicopper oxidases that convert Fe²⁺ to Fe³⁺ so iron can load onto transferrin and be exported. Copper deficiency paralyzes iron mobilization — even with full iron stores, heme can't be made. This is 'copper anemia', often misdiagnosed as IDA.
On throughput: marrow makes ~2 million red cells per second, needing ~20 mg new iron per day for new heme (95% from recycled old red cells; net absorption is only 1–2 mg).
Lead poisoning is a double hit: ALA-D inhibited (blood and urine ALA rise) and ferrochelatase inhibited (ZPP rises). If you see 'anemia + sufficient iron + sufficient B6', check for lead. Chronic low-dose childhood lead exposure permanently damages cognition — after the US banned leaded gasoline in 1973 and lead paint in 1978, mean childhood blood lead dropped from 15 µg/dL to <1, and mean IQ rose 2–5 points.
Porphyria: rare but dramatic
Porphyrias are metabolic diseases caused by genetic defects in any of the heme synthesis enzymes — 8 steps means at least 7 subtypes, with vastly different clinical pictures.Acute intermittent porphyria (AIP, the classic): partial PBG deaminase deficiency. Under stress (drugs, fasting, hormones), ALA and PBG accumulate in the liver, producing severe abdominal pain plus neuropsychiatric symptoms (hallucinations, anxiety, seizures). King George III's 'madness' is suspected to be AIP attacks.
Porphyria cutanea tarda (PCT): uroporphyrinogen decarboxylase deficiency. Porphyrins deposit in skin; sun exposure produces blisters, skin fragility, and hypertrichosis.
Erythropoietic protoporphyria (EPP): ferrochelatase deficiency. Red cells accumulate protoporphyrin; bright light produces burning skin pain (but no blisters).
Legend vs reality: medieval werewolf myths (nocturnal, photophobic, abnormal hair, receding gums exposing teeth) have long been speculatively linked to porphyrias — weak evidence but dramatic. AIP prevalence is ~1/20,000 in European descent; other subtypes are rarer.
The reason to teach porphyrias is that they are the perfect reverse-tutorial of the heme synthesis pathway — seeing what each step actually does through gene knockouts is the gold standard of mechanism learning.
Chapter 4
Red cells · oxygen courier
Red cells · oxygen courier
Heme is already built (previous scene). Now, once loaded into hemoglobin (Hb), the protein geometry decides how it loads O₂ in the lung and unloads in tissue. This is a triumph of protein engineering, not a chemical reaction.
Tetramer geometry: Hb is an α₂β₂ tetramer — 2 α and 2 β chains, each cradling one heme and one Fe²⁺. One Hb has 4 oxygen binding sites; each Fe²⁺ reversibly binds 1 O₂. Only Fe²⁺ (ferrous) can carry oxygen — oxidation to Fe³⁺ produces methemoglobin, which cannot (caused by nitrites, benzocaine, aniline toxicity).
R/T allostery and cooperativity: the T (tense) state at low O₂ keeps subunits pulled tight, low O₂ affinity; the R (relaxed) state at high O₂ loosens subunits, high O₂ affinity. The first O₂ binds with difficulty; each subsequent O₂ binds more easily — this is cooperativity. The result is an S-shaped (sigmoidal) oxygen dissociation curve — near-100% saturated in the lung (PO₂ 100 mmHg), rapidly unloading in tissue (40 mmHg).
Bohr effect (Christian Bohr, 1904): tissue metabolism produces CO₂, H⁺, and heat — the curve shifts right, so Hb releases more O₂ at the same PO₂; in the lung CO₂ is exhaled, pH rises, the curve shifts left, more O₂ loads. One molecule, two environments, auto-tuned — an evolutionary marvel.
2,3-BPG (bisphosphoglycerate), a byproduct of red cell glycolysis, stabilizes the T state and shifts the curve right (easier unloading). High altitude or chronic hypoxia raises 2,3-BPG within hours as adaptation; stored blood loses 2,3-BPG, so newly transfused blood actually delivers oxygen poorly for several hours.
Fetal hemoglobin (HbF, α₂γ₂): γ chains bind 2,3-BPG poorly, so HbF's curve is left-shifted with higher affinity than maternal HbA — the fetus can outcompete the mother for oxygen at the placenta. HbF is progressively replaced by HbA in the first 6 months of life.
Geometric-defect diseases:
Sickle cell (HbS): β chain position 6 Glu → Val. On deoxygenation, Hb polymerizes into long fibers; red cells twist into sickles, blocking microvasculature and hemolyzing.β-thalassemia: insufficient β chain production; excess α precipitates, red cells are destroyed by the spleen before maturing — microcytic hypochromic anemia plus marrow hyperplasia.α-thalassemia: insufficient α chain; severity graded by how many α genes are missing (loss of all 4 is fatal in utero).
Production and recycling: marrow makes ~2 million red cells per second, ~200 billion per day; lifespan 120 days; recycled by spleen and liver macrophages; 95% of the iron is reused, only 1–2 mg lost per day.
Hemoglobin isn't just a 'container for iron' — it's a breathing molecular machine, where a single amino-acid mutation can collapse the entire oxygen transport system.
Tetramer geometry: Hb is an α₂β₂ tetramer — 2 α and 2 β chains, each cradling one heme and one Fe²⁺. One Hb has 4 oxygen binding sites; each Fe²⁺ reversibly binds 1 O₂. Only Fe²⁺ (ferrous) can carry oxygen — oxidation to Fe³⁺ produces methemoglobin, which cannot (caused by nitrites, benzocaine, aniline toxicity).
R/T allostery and cooperativity: the T (tense) state at low O₂ keeps subunits pulled tight, low O₂ affinity; the R (relaxed) state at high O₂ loosens subunits, high O₂ affinity. The first O₂ binds with difficulty; each subsequent O₂ binds more easily — this is cooperativity. The result is an S-shaped (sigmoidal) oxygen dissociation curve — near-100% saturated in the lung (PO₂ 100 mmHg), rapidly unloading in tissue (40 mmHg).
Bohr effect (Christian Bohr, 1904): tissue metabolism produces CO₂, H⁺, and heat — the curve shifts right, so Hb releases more O₂ at the same PO₂; in the lung CO₂ is exhaled, pH rises, the curve shifts left, more O₂ loads. One molecule, two environments, auto-tuned — an evolutionary marvel.
2,3-BPG (bisphosphoglycerate), a byproduct of red cell glycolysis, stabilizes the T state and shifts the curve right (easier unloading). High altitude or chronic hypoxia raises 2,3-BPG within hours as adaptation; stored blood loses 2,3-BPG, so newly transfused blood actually delivers oxygen poorly for several hours.
Fetal hemoglobin (HbF, α₂γ₂): γ chains bind 2,3-BPG poorly, so HbF's curve is left-shifted with higher affinity than maternal HbA — the fetus can outcompete the mother for oxygen at the placenta. HbF is progressively replaced by HbA in the first 6 months of life.
Geometric-defect diseases:
Sickle cell (HbS): β chain position 6 Glu → Val. On deoxygenation, Hb polymerizes into long fibers; red cells twist into sickles, blocking microvasculature and hemolyzing.β-thalassemia: insufficient β chain production; excess α precipitates, red cells are destroyed by the spleen before maturing — microcytic hypochromic anemia plus marrow hyperplasia.α-thalassemia: insufficient α chain; severity graded by how many α genes are missing (loss of all 4 is fatal in utero).
Production and recycling: marrow makes ~2 million red cells per second, ~200 billion per day; lifespan 120 days; recycled by spleen and liver macrophages; 95% of the iron is reused, only 1–2 mg lost per day.
Hemoglobin isn't just a 'container for iron' — it's a breathing molecular machine, where a single amino-acid mutation can collapse the entire oxygen transport system.
Anemia is not one disease
'Anemia' is hemoglobin below the lower reference limit — but anemia is a symptom, not a diagnosis. You need to classify before you can treat.By red cell size (MCV):
Microcytic hypochromic (MCV <80 fL): common causes — iron deficiency, anemia of chronic disease, β-thalassemia. Iron deficiency drops ferritin, transferrin saturation, and serum iron; ACD has normal-or-high ferritin (hepcidin locks iron away) but low serum iron. Iron supplementation works opposite ways here — deficiency needs iron; chronic disease needs the primary disease treated first.
Macrocytic (MCV >100 fL): common causes — B12 or folate deficiency, also called megaloblastic anemia. DNA synthesis stalls, red cells grow large but few, with hypersegmented neutrophils. B12 deficiency elevates both homocysteine and methylmalonic acid; folate deficiency elevates only homocysteine. Folate alone cannot repair B12's neural damage (see folate/B12 story).
Normocytic normochromic (MCV 80–100): common causes — acute blood loss, hemolysis, chronic disease, renal failure, marrow suppression. Renal failure produces inadequate erythropoietin (EPO) → recombinant EPO treatment. Hemolysis raises reticulocytes, raises indirect bilirubin and LDH, drops haptoglobin.
Diagnostic starting point: CBC + MCV + ferritin + reticulocyte count. Iron deficiency demands investigation of GI bleeding sources — in adult men and post-menopausal women, iron deficiency means GI bleeding until proven otherwise.
'Anemia → take iron' is one of the most dangerous oversimplifications, masking colon cancer, B12 deficiency with neural damage, renal failure, thalassemia, and other critical diagnoses.
Chapter 5
Ferritin · the storage name tag
Ferritin · the storage name tag
Ferritin is a spherical protein cage built from 24 subunits — each cage can hold about 4,500 iron atoms (as a hydrated phosphate of oxidized Fe³⁺).
It does two things: stores iron safely so it doesn't roam free and drive Fenton chemistry (•OH radicals that damage DNA, proteins, membranes); and buffers — releases on demand, recycles when excess.
Main storage sites: liver ~60% (primary depot), marrow ~25%, spleen ~10%, muscle ~5%.
Clinically, the ferritin we measure in blood is serum ferritin — a small fraction leaked from cells, correlating positively with tissue iron, the most sensitive marker of body iron status:
A frequently-overlooked insight: iron deficiency without anemia is a clinically very common subclinical state — ferritin already <30 while hemoglobin is still normal. The body is already running on 'rationing mode' — symptoms include fatigue, poor exercise tolerance, restless legs, hair thinning, declining focus — but 'normal Hb' makes doctors dismiss it. If you suspect iron deficiency, testing ferritin is much more sensitive than testing Hb.
It does two things: stores iron safely so it doesn't roam free and drive Fenton chemistry (•OH radicals that damage DNA, proteins, membranes); and buffers — releases on demand, recycles when excess.
Main storage sites: liver ~60% (primary depot), marrow ~25%, spleen ~10%, muscle ~5%.
Clinically, the ferritin we measure in blood is serum ferritin — a small fraction leaked from cells, correlating positively with tissue iron, the most sensitive marker of body iron status:
| Ferritin (µg/L) | Meaning |
|---|---|
| < 15 | Absolute deficiency (empty warehouse) |
| 15–30 | Low stores, borderline |
| 30–100 | Normal/adequate |
| 100–300 | Replete |
| > 300 | Elevated / overload / inflammation |
| > 500 (no inflammation) | Suspect hemochromatosis |
A frequently-overlooked insight: iron deficiency without anemia is a clinically very common subclinical state — ferritin already <30 while hemoglobin is still normal. The body is already running on 'rationing mode' — symptoms include fatigue, poor exercise tolerance, restless legs, hair thinning, declining focus — but 'normal Hb' makes doctors dismiss it. If you suspect iron deficiency, testing ferritin is much more sensitive than testing Hb.
Restless legs & low iron
The link between restless legs syndrome (RLS) and low iron is one of the most-missed diagnoses in clinical practice.Mechanism: dopamine synthesis requires iron (as a TH enzyme cofactor); low iron in the substantia nigra and basal ganglia disrupts nighttime dopamine signaling, presenting as the urge to move the legs and difficulty falling asleep.
Key evidence (Allen 2018, American Academy of Sleep Medicine): RLS patients have low brain iron even when serum iron and Hb are normal.
Diagnostic marker: plasma ferritin. The target in RLS patients is ferritin > 75–100 µg/L (far above the standard lower reference of 30).
Treatment: patients with ferritin <75 typically improve significantly with oral or IV iron; supplementation has no effect once stores are replete.
RLS is common in pregnancy because iron demand spikes — third-trimester RLS incidence reaches 25%.
So for 'urge to move legs at night / trouble falling asleep / restless legs' plus ferritin <75, iron is step one — don't jump straight to dopamine agonists.
Chapter 6
Deficiency ≠ anemia (yet)
Deficiency ≠ anemia (yet)
Iron is the world's most common micronutrient deficiency — WHO estimates ~30% of the global population is iron-insufficient.
High-risk groups:
Reproductive-age women and heavy menstruators — 30–80 mL monthly loss = 15–40 mg of ironPregnancy and lactation — demand spikes to build blood volume for fetus and infant (pregnancy RDA 27 mg vs baseline 18)Pure vegan or near-vegan diets — non-heme absorption low and lacking enhancersEndurance athletes — 'sports anemia' from sweat losses and foot-strike hemolysis (mechanical stress under the foot lyses red cells)Chronic inflammation or GI bleeding — colon cancer, hemorrhoids, aspirin abuseInfants 6–24 months — after weaning, complementary foods often under-fortified
Symptom staircase (mild to severe):
1. Fatigue, declining exercise tolerance (ferritin <30)
2. Reduced focus, foggy memory (low stores affect neurotransmitter synthesis)
3. Restless legs syndrome (RLS), legs needing to move at night
4. Hair thinning, increased shedding
5. Thin, brittle, spoon-shaped nails (koilonychia)
6. Pica (appetite distortion, craving ice, dirt, paper) — a classic signal of iron deficiency
7. Pale skin, palpitations, shortness of breath (frank anemia)
Frank anemia is the late stage — many people are chronically depleted through stages 1–5 without diagnosis.
Practical priority: high-risk groups should test annual Hb + ferritin rather than self-supplementing. 'Test before treat' is the unchangeable discipline of iron.
High-risk groups:
Reproductive-age women and heavy menstruators — 30–80 mL monthly loss = 15–40 mg of ironPregnancy and lactation — demand spikes to build blood volume for fetus and infant (pregnancy RDA 27 mg vs baseline 18)Pure vegan or near-vegan diets — non-heme absorption low and lacking enhancersEndurance athletes — 'sports anemia' from sweat losses and foot-strike hemolysis (mechanical stress under the foot lyses red cells)Chronic inflammation or GI bleeding — colon cancer, hemorrhoids, aspirin abuseInfants 6–24 months — after weaning, complementary foods often under-fortified
Symptom staircase (mild to severe):
1. Fatigue, declining exercise tolerance (ferritin <30)
2. Reduced focus, foggy memory (low stores affect neurotransmitter synthesis)
3. Restless legs syndrome (RLS), legs needing to move at night
4. Hair thinning, increased shedding
5. Thin, brittle, spoon-shaped nails (koilonychia)
6. Pica (appetite distortion, craving ice, dirt, paper) — a classic signal of iron deficiency
7. Pale skin, palpitations, shortness of breath (frank anemia)
Frank anemia is the late stage — many people are chronically depleted through stages 1–5 without diagnosis.
Practical priority: high-risk groups should test annual Hb + ferritin rather than self-supplementing. 'Test before treat' is the unchangeable discipline of iron.
Iron across the lifespan
RDA isn't a static number — iron demand varies sharply across life stages.| Stage | RDA (mg/day) | Key risk |
|---|---|---|
| Infant 0–6 mo | 0.27 (AI) | Breast milk iron low but bioavailable; preterm/formula need fortification |
| Infant 7–12 mo | 11 | Weaning foods must contain iron (meat purée, fortified rice cereal) |
| Toddler 1–3 y | 7 | Deficiency damages cognitive development — irreversible window |
| Children 4–8 y | 10 | |
| Boys 9–13 | 8 | |
| Boys 14–18 | 11 | Growth + muscle gain |
| Girls 14–18 | 15 | Sharp rise post-menarche |
| Men 19–50 | 8 | Excess is the bigger problem |
| Women 19–50 | 18 | Menstruation |
| Pregnancy | 27 | Fetus + placenta + blood volume; supplementation nearly unavoidable |
| Lactation | 9 | Menses often paused, demand drops |
| Post-menopausal women | 8 | Menses ended, same as men |
| Endurance athletes | RDA +30–70% | Foot-strike hemolysis + sweat losses |
Childhood deficiency leaves irreversible cognitive damage. The critical window of brain myelination, neurotransmitter synthesis, and hippocampal development runs from 6 months to 3 years. Deficiency during this window — even if later replenished — leaves permanent deficits in cognition, attention, and learning ability (Lozoff 2006, *Pediatrics*, long-term follow-up).
This is why WHO ranks infant iron deficiency as the top global child-nutrition issue — the consequences are far worse than the visible short-term anemia.
Chapter 7
Don't supplement blindly
Don't supplement blindly
Iron isn't like water-soluble vitamins where excess just gets excreted — the body has no active iron excretion mechanism. Daily net loss is only 1–2 mg (gut epithelial shedding + sweat + trace urine); balance is controlled by regulating absorption.
Overload risk splits into two scenarios.
Acute toxicity (child swallowing adult iron pills): a single dose over 60 mg/kg can be fatal — iron pills are one of the most common preventable pediatric poisonings. Presentation: severe GI bleeding + metabolic acidosis + liver failure.
Chronic overload: hereditary hemochromatosis (HFE C282Y homozygous, ~1/200 in Northern European descent) breaks the hepcidin response, causing lifelong over-absorption. Iron deposits in order — liver, heart, pancreas, skin, joints — producing irreversible damage. Clinical picture: iron-overload cardiomyopathy, cirrhosis, diabetes ('bronze diabetes'), gonadal atrophy, arthritis. Treatment is the ancient but effective remedy: phlebotomy.
A few principles: men and post-menopausal women should NOT routinely take iron-containing multivitamins or single iron supplements without documented deficiency; multivitamins for men typically contain no iron — this is science, not skimping; iron supplements at home must be kept absolutely out of reach of children.
Test ferritin before supplementing — this is the unchangeable discipline of iron.
Overload risk splits into two scenarios.
Acute toxicity (child swallowing adult iron pills): a single dose over 60 mg/kg can be fatal — iron pills are one of the most common preventable pediatric poisonings. Presentation: severe GI bleeding + metabolic acidosis + liver failure.
Chronic overload: hereditary hemochromatosis (HFE C282Y homozygous, ~1/200 in Northern European descent) breaks the hepcidin response, causing lifelong over-absorption. Iron deposits in order — liver, heart, pancreas, skin, joints — producing irreversible damage. Clinical picture: iron-overload cardiomyopathy, cirrhosis, diabetes ('bronze diabetes'), gonadal atrophy, arthritis. Treatment is the ancient but effective remedy: phlebotomy.
A few principles: men and post-menopausal women should NOT routinely take iron-containing multivitamins or single iron supplements without documented deficiency; multivitamins for men typically contain no iron — this is science, not skimping; iron supplements at home must be kept absolutely out of reach of children.
Test ferritin before supplementing — this is the unchangeable discipline of iron.
Hemochromatosis screening
Hereditary hemochromatosis (HH, HFE C282Y homozygous) is the most common autosomal recessive disease in European-descent populations.On frequency: ~1/200–300 of Northern European descent are C282Y homozygous, about 10% are heterozygous; extremely rare in Asian and African populations. Mechanism: HFE gene loss-of-function, hepcidin can't be properly upregulated, lifelong over-absorption, iron accumulates in liver, heart, pancreas, skin, joints, gonads.
Symptoms typically appear after age 40–60: fatigue, joint pain, hepatomegaly → progression to cirrhosis, cardiomyopathy, diabetes ('bronze diabetes'), gonadal atrophy, plus skin hyperpigmentation. Women, due to menstrual loss, typically present about 10 years later.
Screening: transferrin saturation >45% AND ferritin >200 (men) / 150 (women) prompts HFE genotyping; C282Y homozygous plus clinical evidence is diagnostic.
Treatment: phlebotomy is the ancient but only curative approach — start weekly until ferritin <50, then maintenance every 3–4 months. Early treatment gives normal lifespan; patients who have progressed to cirrhosis still face ~100× elevated HCC risk even after treatment.
If you see 'family history + new-onset arthritis + elevated ALT + diabetes, possibly with skin darkening', think iron overload — don't default to other common causes.