Carbonyl Iron vs Ferrous Sulfate
How carbonyl iron and ferrous sulfate differ in elemental content, absorption, tolerance, and cost.
This page contains affiliate links. Titrate earns a small commission when you purchase through them, at no cost to you. This does not affect our recommendations.
Side by Side
| Carbonyl Iron | Ferrous Sulfate | |
|---|---|---|
| Elemental content | Varies by product | 20% elemental iron |
| Absorption | Moderate | High |
| How it is absorbed | Microparticles of nearly pure elemental iron that must be dissolved by gastric acid before absorption, which slows and limits uptake. | Ferrous (Fe2+) iron is absorbed by divalent metal transporter 1 (DMT1) in the duodenum. Absorption rises when stores are low and falls as hepcidin rises after a dose. |
| Typical dose | 45-65 mg elemental iron daily | 40-65 mg elemental iron daily or on alternate days for deficiency |
| Common uses | iron deficiency, households with young children | iron deficiency anemia, first-line oral repletion |
| Side effects | GI effects similar to salts. Acid-dependent dissolution makes it less effective with acid-suppressing medications. | Nausea, constipation, dark stools, epigastric pain; dose-dependent. |
| Cost | Low | Low |
| Also called | None | iron sulfate |
| Labels in index | 68 | 101 |
When to Choose Carbonyl Iron
Choose carbonyl iron when accidental-ingestion risk in the household is a concern. Rate-limited dissolution gives it lower acute toxicity in accidental overdose than iron salts.
Carbonyl Iron
Choose carbonyl iron when accidental-ingestion risk in the household is a concern.
What to look for: Carbonyl Iron named on the Supplement Facts panel, with the amount per serving stated.
Find on iHerb Affiliate linkWhen to Choose Ferrous Sulfate
Choose ferrous sulfate when you want the cheapest proven repletion and tolerate it. The reference form against which others are compared. A single morning dose on alternate days improved fractional absorption versus daily dosing in Stoffel 2017.
Ferrous Sulfate
The reference standard: cheapest, best studied, and the form used in the hepcidin dosing trials. GI side effects are the price.
What to look for: 65 mg elemental iron (325 mg tablet), not enteric coated, taken once in the morning on alternate days.
Find on iHerb Affiliate linkProducts on Titrate
In Titrate's index of NIH label data, 68 labels named as iron products list carbonyl iron and 101 list ferrous sulfate.
Iron does not have its own search category yet. Each product above links to its full label in the NIH database.
Related
References
NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals.
Hallberg L, Brune M, Rossander L. Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate. Am J Clin Nutr. 1989;49(1):140-144.
Moretti D, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981-1989.
Tolkien Z, et al. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383.
Stoffel NU, et al. 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. Lancet Haematol. 2017;4(11):e524-e533.
Stoffel NU, et al. Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica. 2020;105(5):1232-1239.
World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO; 2020.