Iron is an essential mineral critical for haemoglobin synthesis, oxygen transport and energy metabolism. Iron deficiency anaemia is the most widespread nutritional deficiency globally, disproportionately affecting women of reproductive age, vegetarians and endurance athletes. Unlike most supplements, iron should only be taken if deficiency is confirmed — excess iron is harmful.
Iron supplementation is highly effective at correcting deficiency. Do not supplement without a blood test — excess iron is oxidative and can cause organ damage. If you are deficient, the energy, cognition and performance benefits of correcting it are substantial.
Each row grades the claimed effect by strength of human evidence, not mechanism or marketing.
Iron deficiency is the most common nutritional deficiency globally. Supplementation is standard of care for confirmed deficiency. Testing before supplementation is essential.
Sometimes used by athletes trying to self-supplement perceived 'performance iron' without testing — risky and counterproductive if replete.
The deficiency case is unambiguous. The key gap is routine misuse — supplementing based on fatigue alone without testing. Iron overload (haemochromatosis, secondary iron overload) is a real and serious condition.
Test ferritin and haemoglobin before supplementing — serum ferritin below 30 µg/L is generally considered iron deficiency, below 12 µg/L is severely deficient.
Iron deficiency can exist without anaemia ('iron deficiency without anaemia') and still cause fatigue and cognitive impairment.
Women of reproductive age, vegetarians, vegans and endurance athletes are highest-risk for deficiency.
Vitamin C taken alongside iron supplements improves non-haem iron absorption significantly.
Tea, coffee, calcium and phytates (from wholegrains) reduce iron absorption — separate from iron supplements by 1–2 hours.
Mechanism is not outcome. Each mechanism is labelled by how far it has been validated in humans.
Iron is central to haem in haemoglobin and myoglobin, which transport and store oxygen in blood and muscle. Deficiency directly impairs oxygen delivery.
Iron is required by multiple enzymes in the electron transport chain and Krebs cycle. Deficiency impairs cellular energy production independently of anaemia.
Iron is a cofactor for tyrosine hydroxylase (dopamine synthesis) and tryptophan hydroxylase (serotonin). Deficiency impairs neurotransmitter production — a likely route for cognitive and mood effects.
Safe and necessary when deficiency is confirmed. Potentially harmful when not deficient. GI side effects are common at therapeutic doses.
This page is educational and not medical advice. Do not supplement iron without blood testing. Iron overload is dangerous and irreversible in some conditions.
A small, curated set — not a literature dump. Each reference comes with a single-line takeaway.
Iron deficiency is the most common and widespread nutritional disorder in the world. Supplementation is effective and essential where deficiency is confirmed.
Iron supplementation in women with low ferritin but normal haemoglobin significantly reduced fatigue scores over 12 weeks.
Iron supplementation in deficient athletes consistently improves VO2 max and exercise efficiency, with larger effects the more severe the deficiency.