Dietary Iron Absorption

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The efficiency of iron absorption depends on both the bioavailability of dietary iron and iron status.

Figure 1 Iron metabolism and balance: inputs, losses, and recycling of iron through the reticuloendothelial system. Fe, iron; Tf, transferrin; Hb, hemoglobin; RBC, red blood cell; RE, reticuloendothelial.

Typically, 5-20% of the iron present in a mixed diet is absorbed. Dietary iron exists in two forms, heme and non-heme. Heme iron is derived from animal source food and is more bioavailable than non-heme iron, with approximately 20-30% of heme iron absorbed via endocytosis of the entire heme molecule. Iron is then released into the enterocyte by a heme oxidase.

Non-heme iron exists in plant products and its bioavailability is compromised by the concurrent ingestion of tannins, phytates, soy, and other plant constituents, that decrease its solubility in the intestinal lumen. Bioavailability of non-heme iron is increased by concurrent ingestion of ascorbic acid and meat products. Non-heme iron is reduced from the ferric to the ferrous form in the intestinal lumen and transported into enterocytes via the divalent metal transporter (DMT-1). Once inside the entero-cyte, iron from heme and non-heme sources is similarly transported through the cell and across the basolateral membrane by the ferroportin transporter in conjunction with the ferroxidase hephaestin after which it can be taken up by transferrin into the circulation. The regulation of iron across the baso-lateral membrane of the enterocyte is considered the most important aspect of iron absorption.

The absorption efficiency of non-heme iron in particular is also inversely related to iron status. The factor responsible for communicating body iron status to the enterocyte to allow for the up- or downregula-tion of iron absorption remained elusive until recently, when the hormone hepcidin was identified. Hepcidin declines during iron deficiency, and its decline is associated with an increased production of the DMT-1 and ferroportin transporters in a rat model, although its exact mode of action is unknown. Hepcidin may also regulate iron absorption and retention or release of iron from body stores during conditions of enhanced erythropoiesis and inflammation.

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