Iron absorption is affected by the form in which iron is presented to the digestive tract, and inorganic iron ions change oxidation state during the absorption process.
There are two major forms of dietary iron.
Heme iron, found primarily in red meats, is the most easily absorbed form.
Other forms of iron are bound to some other organic constituent of the food. Cooking tends to break these interactions and increase iron availability.
Some iron-rich foods are poor sources of the mineral because other compounds render it nonabsorbable.
The classic example is spinach. It contains iron, but it also contains considerable oxalate, which chelates it and renders it nonabsorbable.
Phytates, present in whole grains that have not been subjected to fermentation by yeast (for example, during bread making), have a similar effect.
Iron ions undergo two important changes of oxidation state during digestion and absorption.
The first change occurs in the stomach.
Here iron (III) is reduced to iron (II).
This reduction is favored by the low pH. Reducing agents, such as ascorbic acid, assist this process.
Reduction is important because iron (II) dissociates from ligands more easily than iron (III).
The second change occurs in the duodenum.
The duodenum is bicarbonate-rich, and alkaline.
In the alkaline environment
heme is absorbed directly by the mucosal cells. Within the cells, the iron dissociates from it.
free iron (II) ions are oxidized to iron (III), which is taken up by the mucosal cells in substantial amounts under all circumstances of nutritional iron status.
