Carbonyl Iron Contains 100% Elemental Iron
18 grams of carbonyl iron contains 18 grams of elemental iron, as carbonyl iron is pure elemental iron (100% iron content).
Understanding Carbonyl Iron Composition
Carbonyl iron is pure elemental iron in powder form, distinct from iron salts that contain varying percentages of elemental iron 1. This is a critical distinction when calculating iron dosing:
Comparison with Iron Salts
The guidelines clearly demonstrate the difference between carbonyl iron and traditional iron salts 2:
- Ferrous sulfate (325 mg tablet): Contains only 65 mg elemental iron (~20%)
- Ferrous fumarate (325 mg tablet): Contains 108 mg elemental iron (~33%)
- Ferrous gluconate (325 mg tablet): Contains 35 mg elemental iron (~11%)
- Carbonyl iron: Contains 100% elemental iron
Clinical Implications
The FDA drug labels confirm that carbonyl iron formulations list "iron (as carbonyl iron)" with the full weight representing elemental iron content 1. For example, a product containing 75 mg of carbonyl iron provides 75 mg of elemental iron.
Research supports that carbonyl iron at doses of 1,000-3,000 mg daily (providing equivalent elemental iron) effectively treats iron deficiency anemia with comparable efficacy to iron salts 3, 4. The key advantage is that the entire weight of carbonyl iron represents bioavailable elemental iron, simplifying dosing calculations.
Important Caveat
While carbonyl iron is 100% elemental iron by weight, its absorption rate may differ from iron salts due to its unique physical properties as a fine powder that requires gastric acid for dissolution 3. However, this does not change the fact that 18 grams of the compound contains 18 grams of elemental iron—the bioavailability is a separate consideration from the chemical composition.