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IRON

iron stores to meet metabolic demands for the first 4 months of life. Breast milk contains 0.2 mg iron/liter; breast-feeding infants receive about 0.27 milligrams per day.

There are two natural dietary forms of iron: (1) in- organic salts of ferric iron, and (2) iron bound to a cyclic carbon ring called heme in the form of hemoglobin and myoglobin in meat products. Inorganic iron is readily lib- erated from food in the acidic lumen of the stomach but is not absorbed well in the small intestine because of its poor solubility at physiological pH and because it is se- questered by many dietary components that hinder ab- sorption, including phytates, polyphenols, calcium, and fiber. Therefore, only a small percentage of injected iron salts are actually absorbed into the body, thereby indi- cating that iron salts have a low bioavailability, or ability to be effectively absorbed. However, other low-molecu- lar-weight dietary components bind inorganic iron and facilitate its absorption. These compounds, which include vitamin C and lactic acids, are commonly found in citrus and deciduous fruits and are known as metal chelators. In addition, an unidentified “meat factor” present in an- imal tissue also enhances the absorption of iron salts. Fi- nally, heme iron has a much greater bioavailability than iron salts because fewer factors interfere with its absorp- tion and it displays greater solubility in water. Hence, heme iron can account for up to 35 percent of absorbed iron in diets when accounting for only 10 percent of to- tal dietary iron intake. In the United States, artificially fortified foods in the form of fortified grain products are a major source of dietary iron and account for nearly 50 percent of all iron consumed.

Iron absorption and transport from the intestinal lu- men to the circulatory system is tightly regulated and complex. Enterocyte cells, which are responsible for the uptake and transport of nutrients from the intestinal mu- cosa, mediate the uptake and transport of iron to the plasma. These cells, once mature, function for only 48 to 72 hours before they are shed and excreted. The capac- ity of the mature enterocyte to transport inorganic iron is determined very early in its development and is in- versely proportional to plasma iron status. The entero- cyte iron transport protein, DMT1 (divalent metal transporter), facilitates iron uptake from the intestinal lu- men into the enterocyte. DMT1 concentrations at the cell surface are increased when whole-body iron stores are depleted, which increases the rate of cellular iron ac- cumulation into the enterocyte once it is matured. The induction of DMT1 protein synthesis results from in- creased DMT1 messenger RNA levels. During iron de- ficiency, the iron regulatory protein (IRP) binds to the 3' untranslated region of the DMT1 messenger RNA and increases its stability. Heme iron is transported into the enterocyte from the intestinal lumen by an unidentified heme iron receptor, and cellular enzymes in the entero- cyte release iron from the heme ring. Iron is exported from the basolateral surface of the enterocyte to plasma

ENCYCLOPEDIA OF FOOD AND CULTURE

by the iron transport protein ferroportin1 (Fp1). Fp1 is believed to assist in the direct transfer of iron to a solu- ble plasma iron transport protein called transferrin. Transferrin facilitates the delivery of two molecules of iron among the sites of absorption and storage and to all tissues and organs. The transferrin-iron complex enters the cell by binding to a specific protein, the transferrin receptor, which is present on the plasma membrane of all cells. Once transferrin binds to its receptor, the re- ceptor-transferrin complex is engulfed by the cell, form- ing an internal vesicle called an endosome. Once in the cell, iron is released from transferrin by the acidification of the endosome, and the transferrin receptor is recycled to the cell surface where it can bind additional transfer- rin molecules.

Iron Physiology

Intestinal absorption is the primary mechanism that reg- ulates whole body iron concentrations. There are no spe- cific mechanisms to remove excess iron from mammals. Inorganic iron excretion is limited because of its low sol- ubility in aqueous environments and therefore daily iron loss is minimal in the absence of blood loss. Fecal (from shed enterocytes and biliary heme products), urogenital, and integumental losses account for 4 mg/day of iron loss. Menstruation, blood donation, and pregnancy also can cause significant iron loss. Variations in iron status and requirements are influenced by individual genetic makeup as well as by differences in menstrual losses. The latter averages 0.6 mg/day but can greatly exceed that value in the individual, resulting in a need to absorb an additional 3 to 4 mg/day to maintain adequate iron status. An addi- tional 4 to 5 mg/day of iron must be absorbed during pregnancy. States of rapid growth during childhood through adolescence also increase iron requirements.

Most absorbed iron is used by the bone marrow to make hemoglobin, an abundant protein that binds and distributes oxygen throughout the body. The remaining iron is distributed to other tissues where it is incorpo- rated into iron-requiring proteins or stored. Nearly 70 percent of total body iron is present in red blood cells bound to hemoglobin. Another 15 percent is bound to metabolic enzymes and numerous other proteins, in- cluding muscle myoglobin, which transports oxygen to the mitochondria, and cytochromes, which act as elec- tron carriers during respiration. The remaining iron is stored in the liver, spleen, and macrophages and can be distributed to other cells during states of dietary iron de- ficiency. The primary iron storage protein is ferritin, which is a hollow sphere comprised of 24 protein sub- units. One ferritin molecule can store about 3,000 fer- ric iron molecules that can be mobilized readily when required. There are two types of ferritin subunits, heavy- chain and light-chain ferritin. Heavy-chain ferritin se- questers Fe+2

and oxidizes it to Fe+3 ; light-chain ferritin

aids in the formation of the mineral iron core within the protein. Tissue, gender, hormones, and iron status can

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