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SupplementScience

Types of Iron: Forms & Bioavailability

This content is for informational purposes only and does not constitute medical advice. 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. (). Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. DOI
  2. (). Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split doses. The Lancet Haematology. DOI
  3. (). Iron deficiency. The Lancet. DOI