What can you tell me about carbonyl iron?

[Zyada]

This is the iron supplement I use, and I was wondering if anyone knew how it compares to ferrous sulfate in terms of absorption, usability, etc.

[astro]

Iron Deficiency Anemia

The most economical and effective medication in the treatment of iron deficiency anemia is the oral administration of ferrous iron salts. Among the various iron salts, ferrous sulfate most commonly is used. Claims are made that other iron salts are absorbed better and have less morbidity. Generally, the toxicity is proportional to the amount of iron available for absorption. If the quantity of iron in the test dose is decreased, the percentage of the test dose absorbed is increased, but the quantity of iron absorbed is diminished. There are advocates for the use of carbonyl iron because of the greater safety with children who ingest their mothers’ medication. Decreased gastric toxicity is claimed but not clearly demonstrated in human trials. Bioavailability is approximately 70% of a similar dose of ferrous sulfate

Carbonyl iron (Feosol) – Used as a substitute for ferrous sulfate. Has a slower release of iron and is more expensive than ferrous sulfate. Slower release affords the agent greater safety if ingested by children. On an mg basis, it is 70% as efficacious as ferrous sulfate. Claims are made that there is less gastrointestinal toxicity, prompting use when ferrous salts are producing intestinal symptoms and in patients with peptic ulcers and gastritis. Tablets are available containing 45 mg and 60 mg of iron.

[WarmNPrickly]

I would guess that the absorption of an iron carbonyl complex would be faster because it is less polar than an iron sulfate complex. In other words, an iron carbonyl complex would be happier in the fatty lipids of the body so it would be more likely to get where it needs to go.
I find it interesting that they would use a carbonyl iron complex in an iron suppliment, but i am not a doctor so I do not know how the body would process this species. The reason I find this interesting, is that a carbonyl group is just a molecule of carbon monoxide bound to the metal. Carbon monoxide is so extraordinarily toxic because it binds so tightly to the iron in your hemoglobin that it never lets go therby depriving your body of the ability to transport oxygen to your cells. That said, I very highly doubt that any iron suppliment would contain anything that toxic, so the CO probably gets processed before causing any harm.
What it may do is slow the processing of iron so that your body doesn’t get too high a dose too quickly. I used to take an Iron Sulfate suppliment and can remember getting a bit of a rush if I followed it with a cup of coffee.

[Squink]

It seems that Iron carbonyl has about the same toxicity as forms of iron:
1997:

The Food and Drug Administration (FDA) is issuing regulations
to require label warning statements on products taken in solid oral dosage form to supplement the dietary intake of iron
…
FDA is temporarily exempting one form of elemental iron, carbonyl iron, from the packaging requirements of this final rule. The temporary exemption will automatically expire 1 year from the effective date of this final rule. If, during the temporary exemption period, FDA receives animal data that establish that carbonyl iron is significantly less toxic than at least one commonly used iron salt, FDA will consider permanently exempting carbonyl iron from the packaging requirements of this final rule.

1998:

The rule provided a temporary exemption to the unit-dose packaging requirement for products that contained carbonyl iron as the iron source; the temporary exemption expired January 15, 1998 and the requirement for unit-dose packaging for carbonyl iron-containing products became effective July 15, 1998.

Iron ions are the toxic agent here, so it must be getting absorbed similarly, regardless of whether it’s a carbonyl, carbonate, or whatever.