Before iron can be absorbed, it must be soluble (able to dissolve). Different types of iron are more soluble than others. For example, the heme iron in meat and animal products is easily absorbed and generally not affected by other foods.1
Plant products contain non-heme iron. Vitamin C or other acids and sugars may aid in iron absorption, whereas tannins and phytates may inhibit iron absorption.2 For this reason, absorption of non-heme iron can be helped or hindered by other foods you eat during the same meal.1,3
One type of iron that is not immediately soluble when ingested is carbonyl iron. Carbonyl iron can only be absorbed when it is converted to soluble ferrous iron by gastric acid.4
1 Gordeuk VR, Brittenham GM, McLaren CE, Hughes MA, Keating LJ. Carbonyl iron therapy for iron deficiency anemia. Blood. 1986;67(3):745-752.
2 Brittenham GM, Klein HG, Kushner JP, Ajioka RS. Preserving the national blood supply. Hematol Am Soc Hematol Educ Program. 2001:422-432.
3 Gordeuk VR, Brittenham GM, Hughes M, Keating LJ, Opplt JJ. High-dose carbonyl iron for iron deficiency anemia: a randomized double-blind trial. Am J Clin Nutr. 1987;46(6):1029-1034.
4 Ferronyl iron. ISP Corp Web site. http://www.ispcorp.com/products/pharma/content/brochure/ferronyl/. Accessed April 8, 2008.