Carries oxygen and builds stable collagen. Too little and too much are both common, so test before you supplement.
Mechanisms, studies and evidence grades for each claim above.
Most of the body's iron sits in hemoglobin, where each heme group binds one oxygen molecule for transport from lungs to tissue; a smaller share is in myoglobin (oxygen storage in muscle) and in the iron-sulfur and heme proteins of the mitochondrial electron transport chain. Iron deficiency progresses in stages — depleted stores (low ferritin), then iron-deficient red cell production, then frank anemia — so symptoms can precede a low hemoglobin. (Strong)
↑ back to summaryA double-blind placebo-controlled trial in 144 Swiss women aged 18–55 with unexplained fatigue and normal hemoglobin gave 80 mg/day elemental iron or placebo for four weeks. Fatigue fell 29% on iron versus 13% on placebo (p=0.004) — but the benefit was confined to women with ferritin ≤50 µg/L.1 That subgroup finding is the useful part: it says low-normal stores can matter before anemia appears, and that iron does nothing for fatigue when stores are already adequate. (Moderate — one well-run trial, subgroup-dependent)
↑ back to summaryMechanistically closer to the rest of this series: reduced iron (Fe²⁺) is the cofactor that prolyl and lysyl hydroxylase need to hydroxylate collagen and stabilize the triple helix. Vitamin C's job in that pathway is specifically to keep iron in this reduced, active form — see Vitamin C and Glycine / Collagen for the full mechanism. (Strong — enzymology)
↑ back to summaryIron is unusual among minerals: the body has no regulated way to excrete it, so balance is controlled almost entirely at the point of absorption.2 Estimated bioavailability is 14–18% from mixed diets and 5–12% from vegetarian diets. Heme iron from meat is absorbed far better than non-heme iron from plants, and meat also enhances non-heme absorption from the same meal; phytate, polyphenols (tea, coffee) and calcium inhibit it, and ascorbic acid enhances it.2 These effects are large in single-meal studies and more modest across a varied diet. (Strong)
Alternate-day dosing. Each oral iron dose raises the hormone hepcidin, which blocks absorption of the next dose for roughly a day. In isotope-labelled trials in iron-depleted women, 60 mg on alternate days for 28 days delivered more total iron than the same dose on consecutive days for 14 days (fractional absorption 21.8% vs 16.3%; 175 mg vs 131 mg absorbed), and splitting a dose twice daily raised hepcidin without improving absorption.3 The authors note this needs confirming in anemic patients. (Moderate — mechanistically clean, small trials)
↑ back to summaryRDA 8 mg/day for adult men and postmenopausal women; 18 mg/day for menstruating women; 27 mg/day in pregnancy.5