Iron absorption measuring

Absorption of iron from the diet is an inefficient process which may be enhanced or inhibited by the iron status of the individual consuming the diet, the form of iron in individual foods, and interactions between foods consumed in a single meal (1-4) Because of this, estimates of iron bioavailability obtained from iron absorption measurements are necessary in... 

Dosing for iron should be divided equally into two to three doses daily. An empty stomach (1 hour before or 2 hours after a meal) is preferred for maximal absorption. After absorption, iron binds to transferrin in the plasma and is transported to the muscles (for myoglobin), liver (for storage), or bone marrow (for red cell production). Iron is not actively excreted from the body but is lost through other measures already described.7 Some studies suggest that iron absorption may be... 

Results from some recent studies (19-36) on the effects of fiber are summarized in Table I. For the most part, the results are from multi-day balance studies. However, Turnland et al. (36) used the stable isotope fecal monitoring method to assess zinc utilization and Simpson et al. (24) measured iron absorption from a single test meal. 

Lynch et al. (5 7) found non-heme iron absorptions, as measured by single test meals, for black beans, lentils, mung beans split peas, and whole soybeans to be low (0.84 to 1.91%). This suggests that many commonly consumed legumes are poor sources of iron whether legumes other than soy may have an "offsetting" enhancing effect on heme-iron absorption cannot be predicted. 

Seligman, P. A., Caskey, J. H., Prazier, J. L., Zucker, P. M., Podell, E. R., and Allen, R. H. (1983). Measurements of iron absorption from prenatal multivitamin-mineral supplements. Obstet. Gynecol. 61, 356-362. 

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. 

The individual reactions affected by iron stress can be considered as regulated biochemical pathways, although regulation by iron is not understood. The mechanism of iron absorption and transport involves the release of hydrogen ions by the root, which lowers the pH of the root zone. This favors Fe3+ solubility and reduction of Fe3 to Fe2+. Reductants are released by roots or accumulate in roots of plants that are under iron stress. These "reductants, along with Fe3+ reduction by the root, reduce Fe3+ to Fe2+, and Fe2+ can enter the root. Ferrous iron has been detected throughout the protoxylem of the young lateral roots. The Fe2+ is probably kept reduced by the reductant in the root, and it may or may not have entered the root by a carrier mechanism. The root-absorbed Fe2+ is believed to be oxidized to Fe3, chelated by citrate, and transported in the metaxylem to the tops of the plant for use. We assume Fe2+ is oxidized as it enters the metaxylem because there is no measureable Fe2+ there (13), and Fe3+ citrate is transported in the xylem exudate (30, 31,32). 

Under in vivo conditions, the hydroxyl radical probably possesses a mean free path of less than 10 A, due to its extreme reactivity. Consequently, the production of hydroxyl radicals is undesirable and there are a number of protective measures adopted by cells to protect against its formation foremost amongst these is the tight control of iron absorption, transport and storage within multicellular organisms. 

Since the weighing of such small sample quantities can influence the analytical error, it is desirable to avoid the need to weigh the sample. One channel is occupied by the reference absorption measurement, for example with pure iron or low-alloy steels an iron hollow-cathode lamp is used and the measurement takes place at 372 nm. In this case, only one further element can be determined. The carrying out of the method is simple (Fig. 6). Tests of this compact sample technique with standard materials, of which there are at present only a few containing certified trace elements, show satisfactory agreement (Fig. 7). As regards traces, the chips of standard materials are very homogeneous. 

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... 

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... 

A study for examining the feasibility of using stable iron isotopes to measure iron absorption by human subjects was carried out as part of a 12 week study of the effects of exercise on riboflavin requirements (20). The study was carried out in the Francis Johnson-Charlotte Young Human Nutrition Unit... 

Variability in Absorption Estimates In this study, the occurrence of a negative absorption value for one subject and the absence of a significant vitamin C effect raise some questions about the accuracy of the method However, the expected changes in absorption due to dietary treatments may be masked by the analytical variations associated with absorption measurements and biological variabilities of iron absorption Analytical variations can be introduced at several stages of the analytical procedures incomplete fecal collection, inhomogeneous samples, iron contamination, incomplete colorimetric reaction, non-quantitative recovery after chemical ashing, and variations in isotopic measurements due to ion statistics, memory effects, instrument drift, etc Some of these are not as serious as others, for example, contamination with natural iron woiold not affect the estimate of tracer concentrations provided it occurs before the total iron content is measured ... 

Precision The precision of the absorption value depends upon tfe precision of P, and E measurements This method for isotopic enrichment measurements by mass spectrometry has a precision of 2% as does the measurement of F by atomic absorption This precision is adequate for absorption and bioavailability studies with zinc and copper (Table I) since zinc and copper absorption are in the range of 30-70% Only fairly large changes in iron absorption can be discerned because non-heme iron absorption is typically less than 10% This may not be a serious problem in bioavailability studies since it is doubtful that very small changes in iron absorption from single foods are biologically significant ... 

The paramount aim of treatment is to reach a negative iron balance. This must be achieved before complications due to the chronic iron overload have their effect on the predisposed organs. The extreme uncontrolled increase in iron absorption is treatable by adjuvant measures, albeit with limited success. 

In a series of studies conducted on human volunteers in the Division of Human Nutrition and Biology at the Institute of Nutrition of Central America and Panama, we employed the radiocobalt absorption test in the context of iron absorption tests. We used a modification of a 6-h cobalt excretion test to estimate absorption (44). Approximately 2.5 fiCi of cobalt-60 mixed with 4.74 mg (20 fimol) of cobalt chloride hexahy-drate was given in 100 mL of water after an overnight fast. The subjects remained fasting for 2 h postingestion and then consumed a standard breakfast. A liter or more of water was consumed during the final 4 h of the study. All urine produced during the 6 h was collected the excreted radioactivity was measured in a well-type y-counter. 

The observation that iron absorption is reduced when gastric acid secretion is compromised (14) suggests that gastrointestinal pH does influence food iron availability. The explanation for this appears to be related to the importance of acid for the release of iron from food. Bezwoda et al. (15) measured the capacity to solubilize iron in bread of gastric juice from normal and iron deficient subjects. Gastric juices with pH values above 2 had limited capacity to solubilize bread iron while below pH 2, solubilization of iron increased linearly with decreasing pH. 

Effect of Heat Processing on Bioavailability of Added Iron. Several studies in Table III measured directly the effect of heat processing on added iron. These studies compared processed foods to a control group of identical unprocessed food. Studies in Table 111 utilizing unprocessed controls include 15, 19, and 23. Other studies did not employ an unprocessed control, but used a reference dose to enable comparisons from study to study. Reference doses of ferrous sulfate (most animal assays) or ferrous ascorbate (most human tests) were frequently used. Preparation of ferrous ascorbate, usually a 2 1 molar ascorbic acid iron solution, has been detailed by Layrisse et al. (25). These controls enabled measurement of variation in iron absorption from subject to subject, important in view of greater absorption of an iron deficient versus an iron replete subject. When a reference dose was fed as a radiolabeled salt (55Fe), and on alternate times the test diet was fed with a different radiolabel (59Fe), errors due to variation in subject absorption were eliminated, as each subject served as its own control. The different availabilities of various iron sources from baked enriched rolls were established in this manner (17). 

Radioisotope techniques have allowed precise measurement of dietary iron absorption. Initial studies utilized test meals of individual food which had been intrinsically labeled with radios active iron prior to harvesting (1,2). Utilization of these single food meals allowed a rank order to be established among the tested foods. Subjects could serve as their own controls when an identical reference dose was given to each subject. 

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