Iron is essential to all organisms, except for a few bacteria. It is mostly stably
incorporated in the inside of metalloproteins, because in exposed or in free form it
causes production of free radicals that are generally toxic to cells. To say that
iron is free doesn't mean that it is free floating in the bodily fluids. Iron binds
avidly to virtually all biomolecules so it will adhere nonspecifically to cell
membranes, nucleic acids, proteins etc.
Animals incorporate iron into the heme complex, an essential component of
cytochromes, which are proteins involved in
redox reactions (including but not limited to respiration), and of oxygen carrying
proteins hemoglobin and myoglobin. Inorganic iron involved in redox reactions is
also found in the iron-sulfur clusters of many enzymes, such as nitrogenase
(involved in the synthesis of ammonia from nitrogen and hydrogen) and hydrogenase. A
class of non-heme iron proteins is responsible for a wide range of functions within
several life forms, such as enzymes methane monooxygenase (oxidizes methane to
methanol), ribonucleotide reductase (reduces ribose to deoxyribose; DNA
biosynthesis), hemerythrins (oxygen transport and fixation in marine invertebrates)
and purple acid phosphatase (hydrolysis of phosphate esters). When the body is
fighting a bacterial infection, the body sequesters iron in the transporter protein
transferrin so it cannot be used by bacteria.
Iron distribution is heavily regulated in mammals. The iron absorbed from duodenum
binds to transferrin, and carried by blood it reaches different cells. There it gets
by an as yet unknown mechanism incorporated into target proteins. [1]
(http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.p
bio.0000079).
Good sources of dietary iron include meat, fish, poultry, lentils, beans, spinach,
tofu, chickpeas, black-eyed pea, strawberries and farina.
Iron provided by dietary supplements is often found as Iron (II) fumarate. The RDA for iron
varies considerably based on the age, gender, and source of dietary iron (heme-based
iron has higher bioavailability)[2]
(http://www.iom.edu/Object.File/Master/7/294/0.pdf). Also note the section below on
precautions.
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