What Is Iron?
Iron is an essential trace mineral required to make haemoglobin and myoglobin, the proteins that carry oxygen in blood and muscle. It is also needed for many enzymes and for energy metabolism. Dietary iron comes as heme iron (from meat, more bioavailable) and non-heme iron (from plants and fortified foods, less bioavailable); supplements are sold in several salt forms, including ferrous sulfate and the gentler-tolerated iron bisglycinate. Iron-deficiency anaemia is the most common nutritional deficiency worldwide, and correcting a genuine deficiency is iron’s core, well-supported use.
Evidence
Iron’s role in oxygen transport and the deficiency disease iron-deficiency anaemia are firmly established, and reference intakes are set by NIH’s Office of Dietary Supplements: an RDA of 8 mg/day for adult men and post-menopausal women, 18 mg/day for women aged 19–50, and 27 mg/day in pregnancy 1Reference 1ReviewIron — Health Professional Fact Sheet — review/referenceView study →. People following vegetarian diets are advised to aim higher because non-heme iron is less bioavailable 1Reference 1ReviewIron — Health Professional Fact Sheet — review/referenceView study →. Supplementation clearly benefits people with diagnosed iron deficiency or deficiency anaemia; this is the evidence-backed indication. Routine iron supplementation in people who are not deficient is not beneficial and carries risk, and specific comparative-efficacy or tolerability claims for the bisglycinate form (often marketed as better tolerated) were not verified against trials in this quick pass.
Forms of Iron
Supplemental iron is sold as several salts that differ mainly in elemental iron (how much iron per tablet) and gut tolerance — the older salts cause more GI upset, which is the usual reason people switch. Each form below has its own page; the comparison:
| Form | Elemental iron | Tolerance | Best for |
|---|---|---|---|
| Ferrous bisglycinate | ~20% (chelate) | Gentlest of the common forms | When sulfate causes GI upset |
| Ferrous sulfate | ~20% (heptahydrate) | Most GI upset | Cheap, first-line, most-studied |
| Ferrous gluconate | ~12% | Often gentler than sulfate | A milder, lower-dose alternative |
| Ferrous fumarate | ~33% | Intermediate | More iron per tablet |
Form choice is mostly about tolerance and adherence — the best iron supplement is the one a person can actually keep taking. Absorption between the salts is broadly similar; the older sulfate is effective but the most likely to cause side effects. All are dosed as elemental iron, and none should be taken without a diagnosed need.
Dosage & Safety
The RDA is 8–18 mg/day for most adults (27 mg/day in pregnancy) 1Reference 1ReviewIron — Health Professional Fact Sheet — review/referenceView study →. Therapeutic doses used to treat diagnosed deficiency are higher and should be a clinical decision, not self-directed; these figures are informational and not a personal recommendation. NIH sets a tolerable upper intake level of 45 mg/day for adults 1Reference 1ReviewIron — Health Professional Fact Sheet — review/referenceView study →. Iron is not a “take it just in case” supplement: high doses commonly cause gastrointestinal effects, and acute overdose can cause severe toxicity — including corrosive gut injury, multi-organ failure, and death — with iron being a leading cause of poisoning fatalities in young children 1Reference 1ReviewIron — Health Professional Fact Sheet — review/referenceView study →. This is the basis for the moderate toxicity rating. Specific drug interactions (iron can bind certain medications) and individual medical cautions were not fully assessed in this pass.
References
- National Institutes of Health, Office of Dietary Supplements. Iron — Health Professional Fact Sheet — review/reference. NIH ODS. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/