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In developed countries where meat is an important constituent of the diet, much of the dietary iron is in the heme-iron form as hemoglobin and myoglobin. Heme-iron is absorbed more efficiently than inorganic iron by the human intestine. Thus, it is important to know how the dose of heme-iron affects iron absorption. The purpose of this study was to establish the dose-effect of heme-iron on the percentage and absolute amount of iron absorbed. Twenty-seven healthy women (28- to 50-y-old) were selected to participate in two iron absorption studies. Through the use of iron isotopes ([sup.59]Fe and [sup.55]Fe), the studies were performed to characterize the dose-response curve of non-heme-iron absorption (ferrous sulfate), and to establish the dose-response curve of heme-iron absorption (hemoglobin). The labeled hemoglobin was prepared by use of red blood cells from rabbits. The geometric means ([+ or -] 1 SEM range) of non-heme iron absorbed were 0.2 (0.2-0.3), 1.2 (1.0-1.5), 6.7 (5.7-8.0) and 13.0 (11.5-14.6) mg of iron for doses of 0.5, 5, 50 and 100 mg of iron as ferrous sulfate, respectively; and 0.1 (0.1-0.2), 0.4 (0.3-0.4), 2.2 (2.0-2.4) and 2.2 (1.7-3.0) mg of iron for doses of 0.5, 3, 15 and 30 mg of heme-iron as hemoglobin, respectively. The fitted curves for heme and non-heme iron differed (P < 0.04). These results strongly suggest that the heme-iron absorption pathway is saturable. KEY WORDS: * heme-iron absorption * iron bioavailability * iron fortification * iron status

There are similarities between mammals and plants in the absorption and transport of trace elements. The chemistry of trace element uptake from food sources in both cases is based on the thermodynamics of adsorption on charged solid surfaces embedded in a solution phase of charged ions and metal-binding ligands together with redox systems in the case of iron and some other elements. Constitutive absorption systems function in nutrient uptake during normal conditions, and inducible 'turbo' systems increase the supply of a particular nutrient during deficiency. Iron uptake is the most studied of the micronutrients, and divides the plant kingdom into two groups: dicotyledonous plants have a turbo system that is an upregulated version of the constitutive system, which consists of a membrane-bound reductase and an ATP-driven hydrogen ion extrusion pump; and monocotyledonous plants have a constitutive system similar to that of the dicots, but with an inducible system remarkably different that uses the mugeneic acid class of phytosiderophores (PS). The PS system may in fact be an important port of entry for iron from an iron-rich but exceedingly iron-insoluble lithosphere into the iron-starved biosphere. Absorption of trace metals in these graminaceous plants is normally via divalent ion channels after reduction in the plasma membrane. Once absorbed, iron can be stored in plants as phytoferritin or transported to active sites by transport-specific ligands. The transport of iron and zinc into seeds is dominated by the phloem sap system, which has a high pH that requires chelation of heavy metals. Loading into grains involves three or four genes each that control chelation, membrane transport and deposition as phytate. KEY WORDS: * micronutrients * iron * zinc * absorption * transport * plants * animals * genetics