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Iron.

Article Excerpt
KEY POINTS

* Iron is required for the transport of oxygen and numerous metabolic functions, including the production of energy.

* Dietary iron exists in two forms: haem iron is found in foods from animal sources, especially meat; and non-haem iron is found mainly in plant foods. Haem iron is absorbed with much greater efficiency than non-haem iron. Absorption of non-haem iron is enhanced by vitamin C or the presence of meat, poultry or fish; and inhibited by phytate and polyphenols.

* The combination of increased iron requirement for growth, iron loss through bleeding, and poor food selection, increases the risk of iron deficiency, especially in young women and children. Diets that are low in iron, high in inhibitors, with little haem iron intake, are the most common causes of iron deficiency.

* Symptoms of iron deficiency are anaemia, decreased aerobic fitness, and in children, behavioural disturbances, and impairment of both cognitive function and psychomotor development.

* The Recommended Dietary Intake for iron is 18 and 8 mg/day for women and men, respectively. The Dietary Guidelines for Australian Adults, Children and Adolescents identify red meat as the best source of bioavailable iron in the Australian diet.

INTRODUCTION

Adult humans contain approximately 2-4 g of iron, in the ferric ([Fe.sup.3+]) or ferrous ([Fe.sup.2+]) form. The majority (50-70%) of body iron is located in red blood cells in association with haemoglobin, and the remainder is distributed in myoglobin (7-10%), in a range of enzymes (3-5%), stored in the liver (25%), or appears in the plasma (less than 1%). The essentiality of iron relates to its ability to bind oxygen for transport to tissues (haemoglobin) and for short-term storage in muscles (myoglobin). The presence of iron in mitochondrial enzymes (e.g. dehydrogenases or cytochromes) facilitates the flow of electrons that is needed for the production of metabolic energy Other iron-containing enzymes are involved in numerous metabolic pathways, including protein and DNA metabolism, collagen synthesis and metabolism of [beta]-carotene. (1-3)

Dietary iron is absorbed from the small intestine, mainly the duodenum, by a process that requires specific iron transporters. Once absorbed, iron is either bound to ferrtin in the intestinal cell, or transported to tissues by transferrin. Most of the absorbed iron is transported to the bone marrow for the synthesis of haemoglobin and red blood cells. Body iron is tightly conserved, and through recycling of old (senescent) red blood cells, a large percentage of its requirements can be met. Small amounts of iron are excreted through the gastrointestinal tract, skin and kidneys. Significant losses of iron occur when salvage mechanisms are bypassed, primarily by direct loss of intact red blood cells, for example through menstrual blood losses or in people with gastrointestinal infections or ulcers. Iron is stored as ferritin or haemosiderin, with the principal sites of storage being the liver, bone marrow and spleen. (1-3)

In view of the widespread involvement of iron in metabolism, it is not surprising that iron deficiency results in a broad range of adverse effects. Iron deficiency may occur at any stage of the life cycle; however, children, adolescents, pregnant and post-partum women are the most commonly affected individuals because of the high iron requirements of growth, pregnancy and lactation. The most common symptom of iron deficiency is anaemia, which, in its severest form, is associated with weakness and eventually heart failure. Iron deficiency decreases aerobic fitness and...

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