Form Comparison

Carbonyl Iron vs Iron Bisglycinate

How carbonyl iron and iron bisglycinate differ in elemental content, absorption, tolerance, and cost.

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Quick verdict. Bisglycinate is the better choice for most people who struggle with iron: it is less affected by dietary phytates than other forms, and trials report fewer GI complaints. Carbonyl iron depends on stomach acid, so it falters with acid suppression, and its GI side effects resemble the salts. Choose carbonyl only when accidental ingestion by a child is a real risk in the household.

Side by Side

Carbonyl Iron Iron Bisglycinate
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. Iron chelated to two glycine molecules. The chelate is less affected by dietary phytates than iron salts, and absorption remains regulated by iron status.
Typical dose 45-65 mg elemental iron daily 25-50 mg elemental iron daily
Common uses iron deficiency, households with young children iron deficiency with GI intolerance to salts, pregnancy
Side effects GI effects similar to salts. Acid-dependent dissolution makes it less effective with acid-suppressing medications. Typically fewer GI complaints than ferrous sulfate in trials, partly due to lower doses.
Cost Low Moderate
Also called None ferrous bisglycinate chelate, Ferrochel, gentle iron
Labels in index 68 283

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.

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When to Choose Iron Bisglycinate

Choose iron bisglycinate when ferrous salts cause GI side effects. Often the switch when sulfate causes constipation or nausea.

Iron Bisglycinate

Easier on the stomach for many people. Fewer head-to-head data than sulfate.

What to look for: Elemental iron stated per capsule, often 25-36 mg. Ferrochel is the most studied raw material.

Find on iHerb Affiliate link

Products on Titrate

In Titrate's index of NIH label data, 68 labels named as iron products list carbonyl iron and 283 list iron bisglycinate.

Open supplement search

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.