Transfer of heme iron (foods of animal origin) and nonheme iron (plant foods) through the intestinal mucosa into blood. Nonheme iron is taken up by divalent metal transporter-1 (DMT-1), while heme iron enters the epithelial cell by endocytosis. Nonheme iron as well as iron released from heme is either transported by iron regulatory protein 1 (IREG 1) from the epithelial cell to the blood or deposited in ferritin. Isotope effects (isotope fractionation) may occur during transport of preferentially lighter isotopes by DMT-1 or IREG 1 and/or during deposition of heavier iron isotopes into ferritin. In contrast, endocytosis of heme isotopomers (small relative mass differences) and release of iron from the heme molecule (quantitative process) are not expected to be isotope selective.