Iron.
The mineral your body cannot excrete — and the most common deficiency on earth?
Iron is the atom at the center of every hemoglobin molecule — the oxygen-binding core of the protein that makes blood red and delivers oxygen to every cell. It is also the nutrient with the strangest regulatory story in this library: the body has no excretion route for iron. No kidney filtration, no biliary dumping — once absorbed, iron stays until blood is lost or cells are shed. All regulation happens at a single gate: the duodenum's absorption channels, controlled by the hormone hepcidin. This design explains everything on this page: why deficiency develops over months of drain, why a toddler can die from a handful of tablets, why hemochromatosis carriers accumulate organ-damaging iron for decades, and why — the newest discovery — the dosing schedule itself is pharmacology.
Iron (Fe)
The most abundant transition metal in the human body — ~3–4 grams in an adult: ~2 grams in hemoglobin, ~1 gram in storage (ferritin), the remainder in muscle myoglobin and iron-dependent enzymes. Every red blood cell carries ~270 million hemoglobin molecules, each with four iron atoms at its center — roughly a quarter of your body's iron turning over every 120 days.
The hemoglobin core — each iron atom binds one oxygen molecule; ~2 grams of iron carries your entire oxygen economy. No iron, no delivery.
Iron sits at the center of the electron transport chain's cytochromes and is required for ATP production — the reason deficiency feels like the power has been cut.
Muscle's oxygen reservoir — another iron-centered protein, storing oxygen where contraction needs it. Iron-deficient muscle tires early.
Myelination and neurotransmitter synthesis are iron-dependent — infancy's iron needs are why pediatric deficiency is taken so seriously.
Iron is a two-front nutrient for immunity: immune cells need it, AND the body deliberately hides it from invading bacteria (hepcidin's second job).
Zero regulated exit route — the body's iron economy is regulated entirely at absorption. The design that makes deficiency slow and excess permanent.
The hepcidin gate is the whole story of iron pharmacology. When stores are full, the liver releases hepcidin, which destroys ferroportin (the iron-export channel) — the gut cell's iron is trapped, absorbed iron drops, and the mineral stays out. When stores are low, hepcidin falls and the gate opens. The 2020 dosing discovery (Chapter 3): each iron dose RAISES hepcidin for up to 48 hours — closing the gate against the next dose. Daily consecutive dosing fights the body's own regulation; alternate-day dosing works with it. The gate also explains the infection logic: bacteria need iron, so illness RAISES hepcidin — the body hiding its iron from invaders (and why iron supplementation during active infection is a clinical conversation, not a default).
Chelated Capsules
Bisglycinate — the tolerable form
Sulfate Tablets
The clinical standard, cheap
Heme Iron Polypeptide
Bypasses the non-heme gate
Liquid Drops
Pediatric dosing — lock-stored
Ferrous Sulfate — The Standard
The form the treatment literature was built on: cheap, dense in elemental iron (65 mg per 325 mg tablet), and the GI trade-off — constipation and nausea are common enough that treatment adherence fails on this form more than any other. Take with vitamin C; take with a small meal if the stomach objects.
Iron Bisglycinate — The Tolerable
Chelated to glycine: comparable absorption at lower elemental doses, dramatically less GI upset — the form that keeps people on treatment. The trade-off is capsule math (25 mg per capsule means multiple capsules for full treatment doses) and price per mg. The form our anemia page leads with.
The No-Exit Economy
Every other mineral in this library has a regulated exit: the kidneys filter excess sodium, magnesium, potassium, and water-soluble vitamins continuously. Iron has none. The body loses iron only through blood loss, the sloughing of gut cells (a trickle), and — for women — menstruation (the reason premenopausal women are the world's highest-risk deficiency group). This design made evolutionary sense in an iron-scarce, parasite-rich world where the mineral was precious and hoarded — and it makes modern iron pharmacology uniquely cautious: there is no undo button. Absorbed iron stays until it is bled out or recycled.
The recycling system is the economy's efficiency marvel: ~90% of the body's daily iron needs are met by recycling — old red cells (120-day lifespan) are dismantled in the spleen, their iron loaded back onto transferrin, and shipped straight to the bone marrow for new cells. Only 1–2 mg must come from diet daily — a razor-thin margin that works beautifully until losses outrun it (heavy periods, GI bleeding, pregnancy's doubled demand). The margin's narrowness is why deficiency is common — and why the body guards absorption so tightly with hepcidin.
- The daily absorption target: just 1–2 mg — against a dietary requirement of 8 mg (men) to 18 mg (premenopausal women), because only a fraction of food iron absorbs.
- The deficiency timeline: months. Ferritin (stores) drains first, then transferrin saturation falls, then hemoglobin finally drops — the three-stage path our anemia page maps.
- The toxicity timeline: years to decades. Hemochromatosis — the genetic failure of the hepcidin gate — deposits iron in liver, heart, pancreas, and joints silently until the organ damage announces it.
- The pediatric arithmetic: a 65 mg elemental tablet is toxic to a toddler in small numbers — the reason child-resistant caps and locked storage are part of this product's pharmacology, not its packaging.
What It Does NOT Do
The honest negative space — iron marketing's persistent inflations:
- It does not boost energy in iron-replete people. The trials are consistent: no benefit, real risk. 'Tired' is not an indication — ferritin is.
- It does not work through 'stronger blood' folklore. It works through a measured deficiency and the marrow's hemoglobin production, full stop. The anemia page's diagnosis-first rule IS the claim.
- It does not pair safely with every infection. Bacteria need iron; the body raises hepcidin during illness to hide it. Supplementing through an active infection is a clinical conversation, not a default.
- It does not absorb better just because more is taken daily. The hepcidin discovery inverted the old logic: each dose closes the gate against the next. Alternate-day dosing can OUTPERFORM daily — Chapter 3's story.
- It is not spinach's nutrient. Spinach carries iron but binds it with oxalates — Popeye's vegetable delivers a fraction of its label. The honest plant sources and their pairing rules are in Chapter 2.