Stable Iron Isotope Research Articles

The effect of pH on stable iron isotope exchange and fractionation between aqueous Fe(II) and goethite

Enriched Fe isotope tracer studies demonstrate that aqueous Fe(II) undergoes electron transfer and atom exchange with goethite. Such processes influence contaminant fate and trace-element mobility, and result in stable Fe isotope fractionation in both biological and abiological processes. To date, the majority of experimental studies of aqueous Fe(II) and Fe oxide interactions have been done at circumneutral pH. The effect of pH variations on the rate and extent of Fe isotope exchange between aqueous Fe(II) and iron oxide minerals, as well as the natural mass-dependent fractionation between these species, has not been adequately explored. Here, the three-isotope method (57Fe–56Fe–54Fe), using an enriched 57Fe tracer, was used to investigate the effect of pH (between 2.5 and 7.5) on the rate and extent of isotopic exchange. 56Fe/54Fe ratios were used to determine the natural, mass-dependent stable isotope fractionation, between aqueous Fe(II) and goethite. Three Fe(II) solutions differing in their initial 56Fe/54Fe ratios were used to approach isotopic equilibrium from multiple directions. The 57Fe-enriched tracer data indicate that the extent of isotopic exchange between Fe(II)aq and goethite was positively correlated with pH, where the least amount of exchange occurred at the lowest pH. Similarly, initial kinetic isotope fractionations were influenced by pH; at low pH, minimal kinetic isotope effects were observed relative to large effects at high pH, suggesting a relation between the extent of sorbed Fe(II) and kinetic isotope effects. Continued exchange over time at high pH, however, erases the initial kinetic isotope effects, and the system fundamentally reached isotopic equilibrium by the end of the experiment. Our results show that the interplay between kinetic and equilibrium effects may prevent confident extrapolation to infer equilibrium fractionation factors when only small amounts of Fe exchange occur.

Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom

The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼ 40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

Duodenal Absorption and Tissue Utilization of Dietary Heme and Nonheme Iron Differ in Rats

Background:Dietary heme contributes to iron intake, yet regulation of heme absorption and tissue utilization of absorbed heme remains undefined. Objectives:In a rat model of iron overload, we used stable iron isotopes to examine heme- and nonheme-iron absorption in relation to liver hepcidin and to compare relative utilization of absorbed heme and nonheme iron by erythroid (RBC) and iron storage tissues (liver and spleen). Methods:Twelve male Sprague-Dawley rats were randomly assigned to groups for injections of either saline or iron dextran (16 or 48 mg Fe over 2 wk). After iron loading, rats were administered oral stable iron in the forms of 57Fe-ferrous sulfate and 58Fe-labeled hemoglobin. Expression of liver hepcidin and duodenal iron transporters and tissue stable iron enrichment was determined 10 d postdosing. Results:High iron loading increased hepatic hepcidin by 3-fold and reduced duodenal expression of divalent metal transporter 1 (DMT1) by 76%. Nonheme-iron absorption was 2.5 times higher than heme-iron absorption (P = 0.0008). Absorption of both forms of iron was inversely correlated with hepatic hepcidin expression (heme-iron absorption: r = −0.77, P = 0.003; nonheme-iron absorption: r = −0.80, P = 0.002), but hepcidin had a stronger impact on nonheme-iron absorption (P = 0.04). Significantly more 57Fe was recovered in RBCs (P = 0.02), and more 58Fe was recovered in the spleen (P = 0.01). Conclusions:Elevated hepcidin significantly decreased heme- and nonheme-iron absorption but had a greater impact on nonheme-iron absorption. Differential tissue utilization of heme vs. nonheme iron was evident between erythroid and iron storage tissues, suggesting that some heme may be exported into the circulation in a form different from that of nonheme iron.

Phytic Acid Concentration Influences Iron Bioavailability from Biofortified Beans in Rwandese Women with Low Iron Status

Background:The common bean is a staple crop in many African and Latin American countries and is the focus of biofortification initiatives. Bean iron concentration has been doubled by selective plant breeding, but the additional iron is reported to be of low bioavailability, most likely due to high phytic acid (PA) concentrations. Objective:The present study evaluated the impact of PA on iron bioavailability from iron-biofortified beans. Methods:Iron absorption, based on erythrocyte incorporation of stable iron isotopes, was measured in 22 Rwandese women who consumed multiple, composite bean meals with potatoes or rice in a crossover design. Iron absorption from meals containing biofortified beans (8.8 mg Fe, 1320 mg PA/100 g) and control beans (5.4 mg Fe, 980 mg PA/100 g) was measured with beans containing either their native PA concentration or with beans that were ∼50% dephytinized or >95% dephytinized. Results:The iron concentration of the cooked composite meals with biofortified beans was 54% higher than in the same meals with control beans. With native PA concentrations, fractional iron absorption from the control bean meals was 9.2%, 30% higher than that from the biofortified bean meals (P < 0.001). The quantity of iron absorbed from the biofortified bean meals (406 μg) was 19% higher (P < 0.05) than that from the control bean meals. With ∼50% and >95% dephytinization, the quantity of iron absorbed from the biofortified bean meals increased to 599 and 746 μg, respectively, which was 37% (P < 0.005) and 51% (P < 0.0001) higher than from the control bean meals. Conclusions:PA strongly decreases iron bioavailability from iron-biofortified beans, and a high PA concentration is an important impediment to the optimal effectiveness of bean iron biofortification. Plant breeders should focus on lowering the PA concentration of high-iron beans. This trial was registered at clinicaltrials.gov as NCT01521273.

Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts

Mineralogical variations in the Earth's mantle and the relative proportions of peridotitic versus enriched and potentially crustally-derived pyroxenitic domains within the mantle have important implications for mantle dynamics, magma generation, and the recycling of surface material back into the mantle. Here we present iron (Fe) stable isotope data (δ57Fe, deviation in 57Fe/54Fe from the IRMM-014 standard in parts per thousand) for peridotite and garnet–pyroxenite xenoliths from Oahu, Hawaii and explore Fe isotopes as tracer of both peridotitic and pyroxenitic components in the source regions of oceanic basalts. The pyroxenites have δ57Fe values that are heavy (0.10 to 0.27‰) relative to values for mid-ocean ridge and ocean island basalts (MORB; OIB; δFe57∼0.16‰) and the primitive mantle (PM; δFe57∼0.04‰). Pyroxenite δ57Fe values are positively correlated with bulk pyroxenite titanium and heavy rare earth element (REE) abundances, which can be interpreted in terms of stable isotope fractionation during magmatic differentiation and pyroxene cumulate formation. In contrast, the peridotites have light δ57Fe values (−0.34 to 0.14‰) that correlate negatively with degree of melt depletion and radiogenic hafnium isotopes, with the most depleted samples possessing the most radiogenic Hf isotope compositions and lightest δ57Fe values. While these correlations are broadly consistent with a scenario of Fe isotope fractionation during partial melting, where isotopically heavy Fe is extracted into the melt phase, leaving behind low-δ57Fe peridotite residues, the extent of isotopic variation is far greater than predicted by partial melting models. One possibility is derivation of the samples from a heterogeneous source containing both light-δ57Fe (relative to PM) and heavy-δ57Fe components. While pyroxenite is a viable explanation for the heavy-δ57Fe component, the origin of the depleted light-δ57Fe component is more difficult to explain, as melting models predict that even large (>30%) degrees of melt extraction do not generate strongly fractionated residues. Multiple phases of melt extraction or other processes, such as metasomatism, melt percolation or the assimilation of xenocrystic olivine with light δ57Fe values may need to be invoked to explain these light δ57Fe values; a caveat to this is that these processes must either preserve, or generate correlations between δ57Fe and Hf isotopes. Published variations in δ57Fe in mantle melting products, such as MORB and OIB, are also greater than predicted by melting models assuming derivation from δ57Fe-homogeneous mantle. For example, OIB from the Society and Cook-Austral islands, which have radiogenic Pb and Sr isotope compositions indicative of recycled components such as subduction modified, low-Pb oceanic crust and terrigenous sediments have heavy mean δ57Fe values (∼0.21‰) significantly distinct to those of other OIB and MORB, which could explained by the presence of heavy-δ57Fe pyroxenite cumulate or pyroxenitic melt components, whereas large degree partial melts, such as komatiites and boninites, display light Fe-isotopic compositions which may reflect sampling of refractory, light-δ57Fe mantle components. Iron stable isotopes may therefore provide a powerful new means of fingerprinting mineralogical variations within the Earth's mantle and identifying the mineralogy of depleted and enriched components within the source regions of volcanic rocks.

A Higher Proportion of Iron-Rich Leafy Vegetables in a Typical Burkinabe Maize Meal Does Not Increase the Amount of Iron Absorbed in Young Women

Food-to-food fortification can be a promising approach to improve the low dietary iron intake and bioavailability from monotonous diets based on a small number of staple plant foods. In Burkina Faso, the common diet consists of a thick, cereal-based paste consumed with sauces composed of mainly green leaves, such as amaranth and jute leaves. Increasing the quantity of leaves in the sauces substantially increases their iron concentration. To evaluate whether increasing the quantity of leaves in sauces would provide additional bioavailable iron, an iron absorption study in 18 young women was conducted in Zurich, Switzerland. Burkinabe composite test meals consisting of the maize paste tô accompanied by an iron-improved amaranth sauce, an iron-improved jute sauce, or a traditional amaranth sauce were provided as multiple meals twice a day for 2 consecutive days. Iron absorption was measured as erythrocyte incorporation of stable iron isotopes. Mean fractional iron absorption from maize paste consumed with an iron-improved amaranth sauce (4.9%) did not differ from the same meal consumed with an iron-improved jute sauce (4.9%; P = 0.9), resulting in a similar quantity of total iron absorbed (679 vs. 578 μg; P = 0.3). Mean fractional iron absorption from maize paste accompanied by a traditional amaranth sauce (7.4%) was significantly higher than that from the other 2 meal types (P < 0.05), but the quantity of total iron absorbed was similar (591 μg; P = 0.4 and 0.7, respectively). A food-to-food fortification approach based on an increase in leafy vegetables does not provide additional bioavailable iron, presumably due to the high phenolic compound concentration of the leaves tested. Alternative measures, such as adding iron absorption enhancers to the sauces, need to be investigated to improve iron nutrition from Burkinabe maize meals.

Ocean chemistry: Fingerprints of a trace nutrient.

Lack of dissolved iron in the sea limits biological productivity and the uptake of carbon dioxide. The sources of dissolved iron in the North Atlantic Ocean have been identified from isotopic variations of this trace nutrient. See Letter p.212 Iron availability limits phytoplankton growth throughout the oceans, acting as a key influence on the global carbon cycle and the oceanic response to changing climate. But large uncertainties remain as to the relative importance of the various sources of iron, including windblown dust and hydrothermal vents. This paper presents a high-resolution transect of seawater dissolved stable iron isotope ratios and iron concentrations in the North Atlantic Ocean. Saharan dust aerosol emerges as the dominant source of dissolved iron along the section, with sediments and hydrothermal vents also significant. Changes in these sources through time may have wide-ranging implications for the global carbon cycle.

Quantification of dissolved iron sources to the North Atlantic Ocean

Dissolved iron is an essential micronutrient for marine phytoplankton, and its availability controls patterns of primary productivity and carbon cycling throughout the oceans. The relative importance of different sources of iron to the oceans is not well known, however, and flux estimates from atmospheric dust, hydrothermal vents and oceanic sediments vary by orders of magnitude. Here we present a high-resolution transect of dissolved stable iron isotope ratios (δ(56)Fe) and iron concentrations ([Fe]) along a section of the North Atlantic Ocean. The different iron sources can be identified by their unique δ(56)Fe signatures, which persist throughout the water column. This allows us to calculate the relative contribution from dust, hydrothermal venting and reductive and non-reductive sedimentary release to the dissolved phase. We find that Saharan dust aerosol is the dominant source of dissolved iron along the section, contributing 71-87per cent of dissolved iron. Additional sources of iron are non-reductive release from oxygenated sediments on the North American margin (10-19per cent), reductive sedimentary dissolution on the African margin (1-4per cent) and hydrothermal venting at the Mid-Atlantic Ridge (2-6per cent). Our data also indicate that hydrothermal vents in the North Atlantic are a source of isotopically light iron, which travels thousands of kilometres from vent sites, potentially influencing surface productivity. Changes in the relative importance of the different iron sources through time may affect interactions between the carbon cycle and climate.

Magma redox and structural controls on iron isotope variations in Earth's mantle and crust

The heavy iron isotopic composition of Earth's crust relative to chondrites has been explained by vaporization during the Moon-forming impact, equilibrium partitioning between metal and silicate at core–mantle-boundary conditions, or partial melting and magma differentiation. The latter view is supported by the observed difference in the iron isotopic compositions of MORBS and peridotites. However, the precise controls on iron isotope variations in igneous rocks remain unknown. Here, we show that equilibrium iron isotope fractionation is mainly controlled by redox (Fe3+/Fetot ratio) and structural (e.g., polymerization) conditions in magmas. We measured, for the first time, the mean force constants of iron bonds in silicate glasses by synchrotron Nuclear Resonant Inelastic X-ray Scattering (NRIXS, also known as Nuclear Resonance Vibrational Spectroscopy – NRVS, or Nuclear Inelastic Scattering – NIS). The same samples were studied by conventional Mössbauer and X-ray Absorption Near Edge Structure (XANES) spectroscopy. The NRIXS results reveal a +0.2 to +0.4‰ equilibrium fractionation on 56Fe/54Fe ratio between Fe2+ and Fe3+ end-members in basalt, andesite, and dacite glasses at magmatic temperatures. These first measurements can already explain ∼1/3 of the iron isotopic shift measured in MORBs relative to their source. Further work will be required to investigate how pressure, temperature, and structural differences between melts and glasses affect equilibrium fractionation factors. In addition, large fractionation is also found between rhyolitic glass and commonly occurring oxide and silicate minerals. This fractionation reflects mainly changes in the coordination environment of Fe2+ in rhyolites relative to less silicic magmas and mantle minerals, as also seen by XANES. We provide a new calibration of XANES features vs. Fe3+/Fetot ratio determinations by Mössbauer to estimate Fe3+/Fetot ratio in situ in glasses of basaltic, andesitic, dacitic, and rhyolitic compositions. Modeling of magma differentiation using rhyolite-MELTS shows that iron structural changes in silicic magmas can explain the heavy iron isotopic compositions of granitoids and rhyolites. This study demonstrates that iron stable isotopes can help reveal planetary redox conditions and igneous processes. Other heterovalent elements such as Ti, V, Eu, Cr, Ce, or U may show similar isotopic variations in bulk rocks and individual minerals, which could be used to establish past and present redox condition in the mantles of Earth and other planets.

Measurement of the 54,57Fe(n,γ) Cross Section in the Resolved Resonance Region at CERN n_TOF

54Fe and 57Fe are stable iron isotopes, which play an important role in the nucleosynthesis of the slow neutron capture process (s process). In addition, these nuclei are present in many structural materials, and therefore, the knowledge of their neutron capture cross sections is of importance for reactor design studies. This contribution summarizes the results of the (n, γ) cross sections of these two isotopes in the resolved resonance region. The experiment was carried out at the CERN n_TOF facility using the Pulse-Height Weighting Technique in combination with an experimental set-up optmized for reducing neutron induced backgrounds.

An iron stable isotope comparison between human erythrocytes and plasma

We present precise iron stable isotope ratios measured by multicollector-ICP mass spectrometry (MC-ICP-MS) of human red blood cells (erythrocytes) and blood plasma from 12 healthy male adults taken during a clinical study. The accurate determination of stable isotope ratios in plasma first required substantial method development work, as minor iron amounts in plasma had to be separated from a large organic matrix prior to mass-spectrometric analysis to avoid spectroscopic interferences and shifts in the mass spectrometer's mass-bias. The (56)Fe/(54)Fe ratio in erythrocytes, expressed as permil difference from the "IRMM-014" iron reference standard (δ(56/54)Fe), ranges from -3.1‰ to -2.2‰, a range typical for male Caucasian adults. The individual subject erythrocyte iron isotope composition can be regarded as uniform over the 21 days investigated, as variations (±0.059 to ±0.15‰) are mostly within the analytical precision of reference materials. In plasma, δ(56/54)Fe values measured in two different laboratories range from -3.0‰ to -2.0‰, and are on average 0.24‰ higher than those in erythrocytes. However, this difference is barely resolvable within one standard deviation of the differences (0.22‰). Taking into account the possible contamination due to hemolysis (iron concentrations are only 0.4 to 2 ppm in plasma compared to approx. 480 ppm in erythrocytes), we model the pure plasma δ(56/54)Fe to be on average 0.4‰ higher than that in erythrocytes. Hence, the plasma iron isotope signature lies between that of the liver and that of erythrocytes. This difference can be explained by redox processes involved during cycling of iron between transferrin and ferritin.

The Iron Stable Isotope Fingerprint of the Human Diet

The stable isotopes of iron disclose the metabolic pathways of iron within the human food chain. We have measured with precise multicollector ICP-MS the iron concentrations and stable isotope composition of 60 food products that are representative of the average German diet. We find that vegetables fall within the range typical of strategy I plants (-0.1 to -1.4‰ in δ(56)Fe), crop products and processed crop foods into the range typical of strategy II plants (-0.6 to +0.4‰), and animal products into the (54)Fe-enriched range known for animal tissue and blood (-1.1 to -2.7‰). Weighting these isotope compositions by the average iron dietary sources, we find a representative composition of European vegetarian diet of -0.45‰, whereas that of omnivores is -0.82‰. For human blood, known to be enriched in light iron isotopes, we find fractionation factors for iron absorption of -2.0 and -2.3‰ for vegetarians (female and male, respectively) and -1.3 and -1.5‰ for omnivores (female and male, respectively). Knowing these fractionation factors is a prerequisite for using stable iron isotope ratios in blood as monitors of intestinal iron uptake regulation.

Dephytinisation with Intrinsic Wheat Phytase and Iron Fortification Significantly Increase Iron Absorption from Fonio (Digitaria exilis) Meals in West African Women

Low iron and high phytic acid content make fonio based meals a poor source of bioavailable iron. Phytic acid degradation in fonio porridge using whole grain cereals as phytase source and effect on iron bioavailability when added to iron fortified fonio meals were investigated. Grains, nuts and seeds collected in Mali markets were screened for phytic acid and phytase activity. We performed an iron absorption study in Beninese women (n = 16), using non-dephytinised fonio porridge (FFP) and dephytinised fonio porridge (FWFP; 75% fonio-25% wheat), each fortified with 57Fe or 58Fe labeled FeSO4. Iron absorption was quantified by measuring the erythrocyte incorporation of stable iron isotopes. Phytic acid varied from 0.39 (bambara nut) to 4.26 g/100 g DM (pumpkin seed), with oilseeds values higher than grains and nuts. Phytase activity ranged from 0.17±1.61 (fonio) to 2.9±1.3 phytase unit (PU) per g (whole wheat). Phytic acid was almost completely degraded in FWFP after 60 min of incubation (pH≈5.0, 50°C). Phytate∶iron molar ratios decreased from 23.7∶1 in FFP to 2.7∶1 in FWFP. Iron fortification further reduced phytate∶iron molar ratio to 1.9∶1 in FFP and 0.3∶1 in FWFP, respectively. Geometric mean (95% CI) iron absorption significantly increased from 2.6% (0.8–7.8) in FFP to 8.3% (3.8–17.9) in FWFP (P<0.0001). Dephytinisation of fonio porridge with intrinsic wheat phytase increased fractional iron absorption 3.2 times, suggesting it could be a possible strategy to decrease PA in cereal-based porridges.

Total Iron Absorption by Young Women from Iron-Biofortified Pearl Millet Composite Meals Is Double That from Regular Millet Meals but Less Than That from Post-Harvest Iron-Fortified Millet Meals

Iron biofortification of pearl millet (Pennisetum glaucum) is a promising approach to combat iron deficiency (ID) in the millet-consuming communities of developing countries. To evaluate the potential of iron-biofortified millet to provide additional bioavailable iron compared with regular millet and post-harvest iron-fortified millet, an iron absorption study was conducted in 20 Beninese women with marginal iron status. Composite test meals consisting of millet paste based on regular-iron, iron-biofortified, or post-harvest iron-fortified pearl millet flour accompanied by a leafy vegetable sauce or an okra sauce were fed as multiple meals for 5 d. Iron absorption was measured as erythrocyte incorporation of stable iron isotopes. Fractional iron absorption from test meals based on regular-iron millet (7.5%) did not differ from iron-biofortified millet meals (7.5%; P = 1.0), resulting in a higher quantity of total iron absorbed from the meals based on iron-biofortified millet (1125 vs. 527 μg; P < 0.0001). Fractional iron absorption from post-harvest iron-fortified millet meals (10.4%) was higher than from regular-iron and iron-biofortified millet meals (P < 0.05 and P < 0.01, respectively), resulting in a higher quantity of total iron absorbed from the post-harvest iron-fortified millet meals (1500 μg; P < 0.0001 and P < 0.05, respectively). Results indicate that consumption of iron-biofortified millet would double the amount of iron absorbed and, although fractional absorption of iron from biofortification is less than that from fortification, iron-biofortified millet should be highly effective in combatting ID in millet-consuming populations.

Genetic Reduction of Phytate in Common Bean (Phaseolus vulgaris L.) Seeds Increases Iron Absorption in Young Women1–4

Iron bioavailability from common beans is negatively influenced by phytic acid (PA) and polyphenols (PPs). Newly developed low-PA (lpa) beans with 90% less PA and variable PPs might improve iron bioavailability. The aim of this study was to evaluate the influence of lpa beans on iron bioavailability in women (n = 20). We compared iron absorption from 4 different beans using a paired, double meal, crossover design. Iron absorption was measured as erythrocyte incorporation of stable iron isotopes (Fe57, Fe58) from 2 lpa bean lines, one high in PPs (means ± SDs; PA = 124 ± 10 mg/100 g; PPs = 462 ± 25 mg/100 g) and one low in PPs (PA = 70 ± 10 mg/100 g; PPs = 54 ± 2 mg/100 g). The other 2 beans used were their parents with a normal PA concentration, one high in PPs (PA = 1030 ± 30 mg/100 g; PPs = 676 ± 19 mg/100 g) and one low in PPs (PA = 1360 ± 10 mg/100 g; PPs = 58 ± 1 mg/100 g). Fractional iron absorption from the lpa bean high in PPs was 6.1% (95% CI: 2.6, 14.7), which was 60 and 130% higher compared with the parent high in PPs (P < 0.001) and low in PPs (P < 0.001), respectively. The total amount of iron absorbed per test meal from the lpa bean high in PPs (372 μg; 95% CI: 160, 890) was 60 and 163% higher compared with the parent high in PPs (P < 0.001) and low in PPs (P < 0.001), respectively. Fractional iron absorption from the lpa line low in PPs (4%; 95% CI: 1.8, 8.7) was 50% higher and the total amount of iron absorbed per test meal (261 μg; 95% CI: 120, 570) was 85% higher than iron from the parent low in PPs (P < 0.001). There was no difference between the lpa beans high or low in PPs or between the parents high or low in PPs. A 90% reduction in PA leads to an increase in bioavailable iron from beans, independent of the PP concentration. The lpa mutation could be a key tool for improving iron bioavailability from beans.

Increasing dietary protein acutely augments intestinal iron transporter expression and significantly increases iron absorption in rats

Iron (Fe) deficiency is endemic worldwide. Little data are available regarding acute effects of dietary protein on intestinal Fe absorption. The current study evaluated the short-term effects of increasing dietary protein on Fe absorption and expression of genes involved in Fe homeostasis. Sprague Dawley rats (24, female) were randomly assigned to custom-formulated isocaloric diets containing 40, 20 (control), or 5% protein (as percentage of total kilocalories) for 7 d. Whole-body Fe balance studies demonstrated that Fe retention was greater in the 40% group than in the 5% group (30.8 vs. 7.3%; P<0.01). In a separate study utilizing stable iron isotopes, the 40% group absorbed 30% of ingested Fe, while the 20% group absorbed 18% (P=0.005). Whole-genome profiling revealed that increasing dietary protein from 5 to 40% increased duodenal transcript expression of divalent metal transporter 1 (DMT1) 3.2-fold, duodenal cytochrome b (Dcytb) 1.8-fold, and transferrin receptor (TfR) 1.8-fold. Consistent with these findings, DMT1 transcript expression was 4-fold higher in RNA prepared from duodenal mucosa in the 40% group compared to the 20% group (P<0.001). These data suggest that increasing dietary protein increases intestinal Fe absorption in part by up-regulating DMT1, Dcytb, and TfR.

Stable iron isotope tracing reveals significant brain iron uptake in adult rats

Iron deposits in the brain are a common hallmark of Alzheimer's disease and Parkinson's disease. This has spurred the hypothesis that iron may play a functional role in the pathogenesis of neurodegenerative disorders through free radical damage. Previous short-term studies using radiotracers suggested that brain iron uptake is small as compared to other tissues in adult rodents. This has led to the assumption that brain iron uptake must also be marginal in humans after brain development is complete. In this study we applied a novel approach to determine directly the fraction of iron that was transferred over time from diet to brain and other organs in adult rats. A known amount of a stable iron isotope ((57)Fe) was fed with drinking water to adult rats over 4 months. Uptake of the tracer iron and final iron content in tissues were assessed by Negative Thermal Ionization Mass Spectrometry (NTI-MS). We found that only a very small amount of dietary iron entered the brain (0.000537 ± 0.000076%). This amount, however, is considerable relative to the total brain iron content (9.19 ± 0.71%), which was lower but comparable to percentage uptake in other tissues. Whereas it remains unclear whether excessive dietary iron intake is a risk factor in neurodegenerative diseases or whether high systemic iron correlates with iron deposits in the brain, our study suggests that uptake of dietary iron is much higher than previously thought. This finding challenges current beliefs and points to a possible role of iron nutrition in the pathogenesis of neurodegenerative disorders.

Phytic Acid-to-Iron Molar Ratio Rather than Polyphenol Concentration Determines Iron Bioavailability in Whole-Cowpea Meal among Young Women ,

Limited data exist on iron absorption from NaFeEDTA and FeSO4 in legume-based flours. The current study compared iron absorption from NaFeEDTA and FeSO4 as fortificants within and between red and white varieties of cowpea with different concentrations of polyphenols (PP) but similar phytic acid (PA)-to-iron molar ratios. We performed a paired crossover study in young women (n = 16). Red-cowpea (high-PP) and white-cowpea (low-PP) test meals (Tubani) were each fortified with (57Fe)-labeled NaFeEDTA or (58Fe)-labeled FeSO4 and were randomly administered. Iron absorption was measured as erythrocyte incorporation of stable iron isotopes. Per serving, the mean (±SD) PP concentrations of the white- and red-cowpea–based meals were 74 ± 3.6 and 158 ± 1.8 mg, respectively, and the molar ratio of PA to iron was 3.0 and 3.3. Iron bioavailabilities from red and white cowpeas were 1.4 and 1.7%, respectively, in NaFeEDTA-fortified meals and 0.89 and 1.2%, respectively, in FeSO4-fortified meals. Compared with FeSO4, fortification with NaFeEDTA increased the amount of iron absorbed from either of the cowpea meals by 0.05 to 0.08 mg (P < 0.05). Irrespective of the fortificant used, there was no significant difference in the amount of iron absorbed from the 2 varieties of cowpea. The results suggest that NaFeEDTA is more bioavailable in legume-based flours compared with FeSO4. In cowpea-based flours, the major determinant of low iron absorption may be the high molar ratio of PA to iron and not variations in PP concentration.

Nutritional iron supplementation studies based on enriched 57Fe, added to milk in rats, and isotope pattern deconvolution‐ICP‐MS analysis

Enriched stable iron isotopes in combination with isotope pattern deconvolution and ICP-MS have been used to study the absorption and bioavailability of iron from supplemented formula milk administrated to lactating rats. The use of two enriched stable isotope tracers, one as the metabolic tracer (here (57) Fe) and the other ((54) Fe) as quantitation tracer, is shown to provide quantitative data about endogenous and exogenous (supplemented) total Fe distribution in rat feces, urine, red blood cells (RBCs), serum, liver, and kidney. The proposed analytical methodology was validated using reference materials (serum, urine, and liver) spiked with both (54) Fe and (57) Fe. Quantitative information about iron absorption/bioavailability and/or metabolism can be obtained from the amounts of endogenous and exogenous iron found in the tissues and fluids analyzed, and about its kinetic after 2 weeks of iron supplementation. The obtained results are discussed in terms of iron exchanged and its half-life in lactating rats and the observed iron levels in serum, RBCs, liver, and kidney comparing nonsupplemented rats and maternal feed rats.

The flux of iron and iron isotopes from San Pedro Basin sediments

Iron is an important nutrient in the ocean, but the different sources and sinks of iron are not well constrained. Here, we use measurements of Fe concentration and Fe stable isotope ratios to evaluate the importance of reducing continental margins as a source of Fe to the open ocean. Dissolved iron concentration ([Fe]) and iron stable isotope ratios (δ56Fe) were measured in the San Pedro and Santa Barbara basins. Dissolved δ56Fe ranges from −1.82‰ to 0.00‰ in the San Pedro Basin and from −3.45‰ to −0.29‰ in the Santa Barbara Basin, and in both basins the lowest δ56Fe values and highest Fe concentrations are found at the bottom of the basin reflecting the input of isotopically light Fe from reducing sediment porewaters. In the San Pedro Basin, we are also able to fingerprint an advective source of Fe from shallow continental shelves next to the basin and the atmospheric deposition of Fe into surface waters. A one-dimensional model of the Fe isotope cycle has been constructed for the deep silled San Pedro Basin. By fitting model output to data, values of several important iron cycle parameters are predicted including a flux of Fe from sediment porewaters into the water column of 0.32–1.14μmolm−2d−1, a first-order dissolved Fe precipitation rate constant of 0.0018–0.0053d−1, a flux δ56Fe of −2.4‰, and an isotope effect for Fe precipitation of Δδ56Feparticulate-dissolved=−0.8‰. Applying our model-predicted Fe cycle parameters to the global ocean suggests that continental margins contribute 4–12% of world ocean dissolved Fe and make the ocean’s Fe lighter by −0.08‰ to −0.26‰. The dramatically negative δ56Fe signature seen in the water column of the San Pedro and Santa Barbara basins demonstrate the utility of Fe isotopes as a tracer for continental margin Fe input from reducing sediments to the oceans, while the isotopic fractionation observed during loss of Fe from the dissolved phase suggests that this signature will be modified by subsequent reactions. Our modeling provides an initial framework for testing how these signals are transmitted into the open ocean.