Alternative pathways for absorption of iron from foods

Abstract

Iron is known to be absorbed from foods in two major forms, heme iron and non-heme iron. Iron status as well as dietary factors known to affect iron absorption has limited effect on heme iron absorption, whereas inhibitors and enhancers of iron absorption have pronounced effects on non-heme iron absorption. The enterocyte transporter for non-heme iron, DMT1, is strongly up-regulated during iron deficiency and down-regulated during iron overload. A transporter for heme iron, HCP1, was recently characterized and is present on the apical membrane of enterocytes. Two other pathways for iron absorption have been discovered and may serve to facilitate uptake of iron from two unique iron-binding proteins, lactoferrin and ferritin. Lactoferrin is an iron-binding protein in human milk and known to survive proteolytic digestion. It mediates iron uptake in breast-fed infants through endocytosis via a specific lactoferrin receptor (LfR). Recently, lactoferrin has become popular as a food additive and may enhance iron status in several age groups. Ferritin is present in meat, but also in plants. The ferritin content of plants can be enhanced by conventional breeding or genetic engineering, and thereby increase iron intake of populations consuming plant-based diets. Ferritin is a bioavailable source of iron, as shown in recent human studies. Ferritin can be taken up by intestinal cells via endocytosis, suggesting a receptor-mediated mechanism.