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Types of Iron: Forms & Bioavailability

Evidence:Strong
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This content is for informational purposes only and does not constitute medical advice. Statements about dietary supplements have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease. Individual results may vary — consult your healthcare provider before starting any supplement. Full disclaimer

Forms Comparison

FormBioavailabilityBest ForElemental Content
Ferrous Bisglycinate (Iron Bisglycinate Chelate)HighBest overall — high absorption with minimal GI side effects20% elemental iron by weight
Ferrous SulfateModerateMost commonly prescribed — inexpensive but more GI side effects20% elemental iron by weight
Ferrous FumarateModerateHigh elemental iron content, commonly used in prenatal vitamins33% elemental iron by weight
Carbonyl IronModerateSlow-release option with lower toxicity risk98% elemental iron by weight

Ferrous Bisglycinate (Iron Bisglycinate Chelate)

Bioavailability: High. Best for: Best overall — high absorption with minimal GI side effects. Elemental content: 20% elemental iron by weight.

Ferrous Sulfate

Bioavailability: Moderate. Best for: Most commonly prescribed — inexpensive but more GI side effects. Elemental content: 20% elemental iron by weight.

Ferrous Fumarate

Bioavailability: Moderate. Best for: High elemental iron content, commonly used in prenatal vitamins. Elemental content: 33% elemental iron by weight.

Carbonyl Iron

Bioavailability: Moderate. Best for: Slow-release option with lower toxicity risk. Elemental content: 98% elemental iron by weight.

Full Iron Guide

References

  1. RCTVaucher P, Druais PL, Waldvogel S, Favrat B (2012). Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. DOI PubMed
  2. RCTStoffel NU, Cercamondi CI, Brittenham G, et al. (2017). Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. The Lancet Haematology. DOI PubMed
  3. Meta-analysisRehman T, Agrawal R, Ahamed F, Das S, et al. (2025). Optimal dose and duration of iron supplementation for treating iron deficiency anaemia in children and adolescents: A systematic review and meta-analysis.. PloS one. DOI PubMed
  4. Watt A, Eaton H, Eastwick-Jones K, Thomas ET, et al. (2025). The benefits and harms of oral iron supplementation in non-anaemic pregnant women: a systematic review and meta-analysis.. Family practice. DOI PubMed
  5. Meta-analysisFiani D, Chahine S, Zaboube M, Solmi M, et al. (2025). Psychiatric and cognitive outcomes of iron supplementation in non-anemic children, adolescents, and menstruating adults: A meta-analysis and systematic review.. Neuroscience and biobehavioral reviews. DOI PubMed
  6. Meta-analysisFinkelstein JL, Cuthbert A, Weeks J, Venkatramanan S, et al. (2024). Daily oral iron supplementation during pregnancy.. The Cochrane database of systematic reviews. DOI PubMed
  7. Garofalo V, Condorelli RA, Cannarella R, Aversa A, et al. (2023). Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis.. Nutrients. DOI PubMed