Iron absorption determination

Iron is, as part of several proteins, such as hemoglobin, essential for vertebrates. The element is not available as ion but mostly as the protein ligands transferrin (transport), lactoferrin (milk), and ferritin (storage), and cytochromes (electron transport) (Alexander 1994). Toxicity due to excessive iron absorption caused by genetic abnormalities exists. For the determination of serum Fe a spectrophoto-metric reference procedure exists. Urine Fe can be determined by graphite furnace (GF)-AAS, and tissue iron by GF-AAS and SS-AAS (Alexander 1994 Herber 1994a). Total Iron Binding Capacity is determined by fuUy saturated transferrin with Fe(III), but is nowadays mostly replaced by immunochemical determination of transferrin and ferritin. 

Figure 8.2 Rat duodenal cells divide in the crypts of Lieberktihn and differentiate while migrating to the villus tips within approximately 48 h. The crypt cells take up iron from the blood, and are thereby able to sense the body s state of iron repletion. They migrate to the villus tips where this information determines their iron absorption capacity from the intestinal lumen. Adapted from Schumann et al., 1999, by permission of Blackwell Science.

In the enterocyte as it enters the absorptive zone near to the villus tips, dietary iron is absorbed either directly as Fe(II) after reduction in the gastrointestinal tract by reductants like ascorbate, or after reduction of Fe(III) by the apical membrane ferrireductase Dcytb, via the divalent transporter Nramp2 (DCT1). Alternatively, haem is taken up at the apical surface, perhaps via a receptor, and is degraded by haem oxygenase to release Fe(II) into the same intracellular pool. The setting of IRPs (which are assumed to act as iron biosensors) determines the amount of iron that is retained within the enterocyte as ferritin, and that which is transferred to the circulation. This latter process is presumed to involve IREG 1 (ferroportin) and the GPI-linked hephaestin at the basolateral membrane with incorporation of iron into apotransferrin. (b) A representation of iron absorption in HFE-related haemochromatosis. 

More detailed knowledge of the mechanisms of iron absorption from the gastrointestinal tract of mammals has advanced by leaps and bounds in the last few years, with the cloning of key participants at both the apical and basolateral faces of the mucosal endothelial cells. The extraordinarily far-sighted contribution of William Crosby and Marcel Conrad, nearly 40 years ago (Conrad and Crosby, 1963 Crosby, 1963), reminded us of the key role of body iron status in determining the level at which iron absorption will be set within the cells of the crypts of Lieberkiihn when they differentiate and move up to the villus tips to play their key role in... 

Spencer and Sachs [29] determined particulate aluminium in seawater by atomic absorption spectrometry. The suspended matter was collected from seawater (at least 2 litres) on a 0.45 tm membrane filter, the filter was ashed, and the residue was heated to fumes with 2 ml concentrated hydrofluoric acid and one drop of concentrated sulfuric acid. This residue was dissolved in 2 ml 2 M hydrochloric acid and the solution was diluted to give an aluminium concentration in the range 5-50 pg/1. Atomic absorption determination was carried out with a nitrous oxide acetylene flame. The effects of calcium, iron, sodium, and sulfate alone and in combination on the aluminium absorption were studied. 

Schall192) recommended that the atomic absorption determination of magnesium, calcium, manganese, iron, and copper in fertilizers should be adopted as official, first action. 

The total sulfur content of the coal samples was determined by the Eschka method. The sulfate sulfur content of the test samples was determined by extraction of a one-gram sample with dilute hydrochloric acid followed by turbidimetric determination of sulfate (24). The pyritic sulfur content was determined by extraction of the weighed coal samples with 2N nitric acid followed by titrimetric or atomic absorption determination of iron in the extract. (25). 

Dominated by Iron Absorption. Regression analyses were performed on D2 of the absorbance vs. measured water content at the wavelength determined to be optimal for the correlation with iron (as above). The results of these analyses on the crude SWy clay and its 100% Fe- and Ca-forms are shown in Figures 8 a-c. 

Through calibration and re-calibration techniques possible interelement effects or influences from the matrix can be largely eliminated. Because of its worldwide importance the introduction of standard methods by atomic absorption started first with iron ores [67] and refractories. The ISO/TC 102 is presently occupied with draft proposals for the atomic absorption determinations of Na/K Ca/Mg, Zn, V, Pb and Al. 

The meals were extrinsically labelled by added 65 Zn. The rationale for this method is that a complete isotope exchange takes place between the added radioactive zinc isotope and the zinc present in the meal. Measurements of the uptake of radioactive iron isotopes in blood or in the whole body have been used for many years in studies of iron absorption. (12, 13, 14). The absorption in the present study is determined from measurement of the whole body retention of the radioisotope. However, this can not be done until the non-absorbed fraction of the isotope has left the body. During this periode of time some of the initially absorbed has been extrected. A correction of retention data... 

The remaining cations were analyzed by atomic absorption spectrophotometry (AAS) except ferrous iron which was done by a modification of the Ferrozine method (29, 30, 3j ). Total iron was determined by AAS and Fe " by difference. Sulfate was... 

Determination of stable isotopes in other tissues requires additional considerations. To measure iron absorption via isotopic enrichment of erythrocytes requires labeling of 2.5 to 3.0 g of erythrocyte iron. To double the natural ratio of 100 mg of erythrocyte iron would require less then 0.4 mg of Fe or... 

Two critical aspects of fecal monitoring for absorption determinations are complete collection of unabsorbed isotopes and sample homogeneity. The times required for complete collections of unabsorbed isotopes vary markedly between subjects and also within the same subject. Therefore fecal makers are required to assure complete collections (9). The times required for complete elimination of a fecal marker have ranged from 6 to 18 days and included from 3 to 23 fecal samples in the experiments discussed later in this paper. Collection of only 80% of unabsorbed isotopes, particularly when absorption is low, will result in serious overestimation of absorption. In the case of iron, absorption of only 10% would appear to be nearly 3(, if 20% of the unabsorbed iron was not collected. A marker such as PEG, which can be measured quantitatively, must be used. Stools are collected until no trace of the inert marker can be detected. Recovery of less than 90% of the PEG... 

To determine iron absorption with tracers, a known amount of an iron tracer is given orally either by itself or with a meal. The appearance of the tracer, either in the feces or in whole blood, is monitored. In the fecal monitoring method, the feces are collected quantitatively. The collection period may last from as few as 3 (5) to as long as H days (6). Iron absorption is then estimated from the difference between the total amounts of tracer ingested and excreted. Probably, the most common and serious error in this method is introduced by incomplete fecal collection. 

Iron balance is determined by the difference between iron absorption and iron loss. Humans lack a mechanism to excrete excess iron and physiological control of iron balance is achieved by regulation of absorption. There is a reciprocal relationship between stores and absorption so that, as stores decline absorption increases and vice versa. 

Other conditions previously mentioned in this chapter, such as cancer, immunocompetence and interferon production, antihistamine eflFects in colds, increased iron absorption, eflFects on cholesterol metabolism, and nitrosamine blocking, were studied at higher than RDA dosage levels. These many situations require additional research study to determine what the minimum levels are to attain the optimal health condition. 

Such chemical changes will obviously effect bioavailability and perhaps explain some of the results reported. For instance, Brise and Hallberg (kO) determined that 200-500 mg ascorbic acid more than tripled the bioavailability of 30 mg of iron administered as ferrous sulfate while 100 mg or less had little effect. Similarly, Cook and Monsen (Ul) determined that the increase in iron absorption from a semisynthetic meal was directly proportional to... 

Level of ascorbic acid in the diet has been found to be an important factor in determining non-heme iron absorption (6,10,11). Ascorbic acid intake has been found to be more closely correlated to several biochemical parameters of iron nutritional status than was total iron Intake (12). However, timing of consumption is equally important. If non-heme iron absorption is to be increased via this factor, then both the non-heme iron and the ascorbic acid must be consumed at the same time. Considering that important sources of ascorbic acid are all of plant origin, the chances that a shift from more animal-based foods to more plant-based foods will lead to Increased consumption of ascorbic acid are good indeed. However, this is not necessarily the case if the shift moves toward a diet based solely, for example, on highly polished cereals. 

Some authors caution that tannins in St. John s wort (as well as those in fever few, borage, saw palmetto, and chamomile) might inhibit iron absorption. Study is needed to determine if this is a concern. 

J.A. Sweileh, E.M. El-Nemma, On-line elimination of spectral interference of iron matrix in the flame atomic absorption determination of zinc by anion-exchange separation, Anal. Chim. Acta 523 (2004) 287. 

J.A. Sweileh, On-line flow injection solid sample introduction digestion and analysis spectrophotometric and atomic absorption determination of iron, copper and zinc in multi-vitamin tablets, Microchem. J. 65 (2000) 87. 

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