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Description of key information

The studies presented provide information on: 1) the bioavailability of metallic iron after oral exposure, 2) the adaptation of iron homeostasis after oral exposure to iron from soluble salts, 3) the iron turn-over in humans and 4) the acidity of human sweat. They are included in the dossier because they provide important information on the toxicokinetics of metallic iron. Human information is important in this case because the regulation of iron absorption differs strongly between humans and the experimental animal used most often: the rat. The study on the acidity of sweat provides information that is necessary to assess systemic bioavailability of metallic iron after skin contact.    

Additional information

The absorption by humans of dietary iron or supplement iron from various sources has been studied extensively, Reasons for this are obvious. Iron deficiency is an important human pathological condition that goes with serious symptoms and effects. These studies included metallic iron particles, including carbonyl iron. Studies summarized here are Gordeuk et al. (1987), Hallberg et al. (1986), Hoppe et al. (2006) and Cook et al. (1973). A review paper by Hurrell (2002) contains a table in which the bioavailability of a large series of iron species is compared. The interpretation of these data, including their comparability, is hampered by the following factors. First of all, different methods are used to demonstrate absorption, which will not be discussed here in detail. They are based on changes in iron status (serum ferritin, serum iron, haematocrit etc.) or on a more absolute quantification through the use of labelled iron. Healthy replete individuals are used in one study and iron deplete individuals in another. Moreover, every change in iron intake will result in a temporary change in iron status, which may return to normal values. On top of that comes the discussion about the reliability of serum ferritin as an indicator for total stored iron. What is most important here is how much of the metallic iron is absorbed by the healthy, non-replete human. The study with radiolabelled iron by Hallberg et al. appears to be the best one to address this question. Combination of this study with the data listed in the table in Hurrell indicates 20% relative bioavailability of carbonyl iron in comparison to readily absorbable iron from soluble salts. This percentage will depend on the average surface/mass ratio of the particles ingested, the total mass ingested and the residence time in the acid gastric juice.