Contributions of heme and nonheme iron to human nutrition

Heme transport exhibits polarity in Caco-2 cells: evidence for an active and membrane protein-mediated process

Heme prosthetic groups are vital for all living organisms, but they can also promote cellular injury by generating reactive oxygen species. Therefore, intestinal heme absorption and distribution should be carefully regulated. Although a human intestine brush-border heme receptor/transporter has been suggested, the mechanism by which heme crosses the apical membrane is unknown. After it enters the cell, heme is degraded by heme oxygenase-1 (HO-1), and iron is released. We hypothesized that heme transport is actively regulated in Caco-2 cells. Cells exposed to hemin from the basolateral side demonstrated a higher HO-1 induction than cells exposed to hemin from the apical surface. Hemin secretion was more rapid than absorption, and net secretion occurred against a concentration gradient. Treatment of the apical membrane with trypsin increased hemin absorption by threefold, but basolateral treatment with trypsin had no effect on hemin secretion. Neither apical nor basolateral trypsin changed the paracellular pathway. We conclude that heme is acquired and transported in both absorptive and secretory directions in polarized Caco-2 cells. Secretion is via an active metabolic/transport process. Trypsin applied to the apical surface increased hemin absorption, suggesting that protease activity can uncover a process for heme uptake that is otherwise quiescent. These processes may be involved in preventing iron overload in humans.

Identification of a human heme exporter that is essential for erythropoiesis

FLVCR, a member of the major facilitator superfamily of transporter proteins, is the cell surface receptor for feline leukemia virus, subgroup C. Retroviral interference with FLVCR display results in a loss of erythroid progenitors (colony-forming units-erythroid, CFU-E) and severe anemia in cats. In this report, we demonstrate that human FLVCR exports cytoplasmic heme and hypothesize that human FLVCR is required on developing erythroid cells to protect them from heme toxicity. Inhibition of FLVCR in K562 cells decreases heme export, impairs their erythroid maturation and leads to apoptosis. FLVCR is upregulated on CFU-E, indicating that heme export is important in primary cells at this stage. Studies of FLVCR expression in cell lines suggest this exporter also impacts heme trafficking in intestine and liver. To our knowledge, this is the first description of a mammalian heme transporter.

Effects of irradiated phytic acid on antioxidation and color stability in meat models

Lipid oxidation and color stability of meats treated with irradiated phytic acid were investigated during storage for 2 weeks at 4 degrees C. The phytic acid in deionized distilled water (DDW) was degraded by irradiation at 10 and 20 kGy, and the irradiated phytic acid showed a strong antiradical activity. For measuring the antioxidant effects of irradiated phytic acid in food models, beef and pork were prepared with DDW (control), irradiated (10 and 20 kGy) or non-irradiated phytic acid, and ascorbic acid as a model system. Irradiated phytic acid significantly inhibited the lipid oxidation in meats compared to the control and ascorbic acid treated samples during storage (P < 0.05). The redness of the meats treated with phytic acid had a higher value than did the control and ascorbic acid treated samples, but a significant difference was not observed in the samples treated with phytic acid regardless of irradiation treatment. Irradiated phytic acid was also effective in inhibiting the loss of heme iron and metmyoglobin formation during storage. Results indicated that irradiation might be helpful for improving the antioxidant activity of phytic acid in meats.

Heme iron, zinc, alcohol consumption, and colon cancer: Iowa Women's Health Study

We examined associations among colon cancer incidence and dietary intake of heme iron, a possible prooxidant, zinc, a possible antioxidant, and alcohol, a disruptor of iron homeostasis. During 15 years of follow-up, 34 708 postmenopausal women, aged 55-69 years at baseline who completed a food-frequency questionnaire for the Iowa Women's Health Study, were followed for incident colon cancer. After adjusting for each micronutrient, the relative risks for proximal colon cancer increased more than twofold across categories of heme iron intake (P(trend) =.01) and the corresponding relative risks decreased more than 50% across categories for zinc intake (P(trend) =.01). The positive association with heme iron and the inverse association with zinc intake were stronger among women who consumed alcohol than among those who did not. Zinc intake was also associated with a decreased risk of distal colon cancer (P(trend) =.03), regardless of alcohol or heme iron consumption. Our results suggest that intake of dietary heme iron is associated with an increased risk of proximal colon cancer, especially among women who drink, but that intake of dietary zinc is associated with a decreased risk of both proximal and distal colon cancer.

Comparisons of vegetarian and beef-containing diets on hematological indexes and iron stores during a period of resistive training in older men

OBJECTIVE

To test the hypothesis that older men who consumed a vegetarian (lacto-ovo) diet would develop a lower iron status compared with older men who consumed a beef-containing diet during a period of resistive training (RT).

DESIGN

Experimental, repeated measures study.

SUBJECTS

Twenty-one healthy men aged 59 to 78 years, with a BMI range of 24 to 33 kg/m(2), completed the study.

INTERVENTION

All men consumed a vegetarian diet for 2 weeks (baseline). After this, the men were randomly assigned to one of two dietary groups. Eleven men consumed a beef-containing diet, and 10 men continued to consume a vegetarian diet for 12 weeks. During this time all subjects participated in RT three days per week, designated as RT1 to RT12.

MAIN OUTCOME MEASURES

Serum ferritin and serum iron concentrations, transferrin saturation, transferrin receptor, total iron binding capacity, and selected hematological variables, as well as selected nutrient intakes and estimated iron bioavailability from three-day diet records, were determined at baseline, RT5, and RT12.

STATISTICAL ANALYSES

A general linear model repeated-measures ANOVA was used to examine the effects of group, time, and group x time interactions for iron status and dietary data.

RESULTS

Total iron intake was not different between the two groups; however, the beef group had a three to four times greater intake of bioavailable iron (P<.01) than the vegetarian group. Serum iron, total iron binding capacity, transferrin saturation, and transferrin receptor were not significantly different between the beef and vegetarian groups, or changed over time with RT. Serum ferritin decreased over time in both the beef and vegetarian groups during RT (P<.01). Re-introduction of beef into the diets of the beef group increased hemoglobin concentration and hematocrit compared with the vegetarian group during the 12 weeks of RT (group x time, P<.05). These changes were within clinically normal limits.

APPLICATIONS/CONCLUSIONS

Older men who consume a beef-containing, higher-bioavailable-iron diet, compared with a vegetarian, lower-bioavailable-iron diet, have an increased hematological profile during a 12-week period of RT. Older men who consume either a beef-containing or a vegetarian diet maintain a hematological profile within clinically normal limits during 12 weeks of RT.

Estimated absorbable iron intakes in quebec adults

In the last 30 years, the prevalence of low dietary iron intake has increased, partly because North Americans have been encouraged to choose lower fat foods, including leaner meats. As a result, there has been a decrease in red meat consumption. The objective of this study was to estimate absorbable iron intakes of a representative sample of the Quebec adult population and to compare these results with the new North American recommendations for iron. Dietary intakes were obtained by 24-hour recall and absorbable iron intakes were estimated using Monsen & coworker's model. While 18.9 % and 2.5% of women and men respectively had an iron intake below the estimated average requirement (EAR), 66.2 % and 25.6% had an estimated available iron intake below the recommended level for absorbed iron. Beef consumption had the strongest association with estimated absorbable iron, followed by vegetables. Subjects with a high education level consumed significantly less meat, beef and heme iron than subjects whose education level was low or moderate. These results indicate that there is a need for improving iron intake in the Quebec adult population.

High specific activity heme-Fe and its application for studying heme-Fe metabolism in Caco-2 cell monolayers

Heme-Fe is an important source of dietary iron in humans. Caco-2 cells have been used extensively to study human iron absorption with an emphasis on factors affecting nonheme iron absorption. Therefore, we examined several factors known to affect heme iron absorption. Cells grown in bicameral chambers were incubated with high specific activity [59Fe]heme alone or with 1% globin, BSA, or fatty acid-free BSA (BSA-FA) to examine the effect of protein source on absorption. Heme iron absorption was enhanced by globin and inhibited by BSA and BSA-FA. Absorption of heme iron in cells pretreated for 7 days with serum-free medium containing 1, 25, 50, or 100 microM Fe was higher in the 1-microM-Fe pretreatment group than in all other groups (P < 0.05), showing an effect of iron status. Increased heme concentrations resulted in decreased percent absorbed but increased total heme iron absorption and increased transport rate across the basolateral membrane. Finally, cells treated with 10 microM CdCl2, which induces heme oxygenase, demonstrated higher absorption of [59Fe]heme than control cells (P < 0.05). Our results from Caco-2 cells are in agreement with human studies and make this a promising model for examining intestinal heme iron absorption.

Pathways of iron absorption

Iron is vital for all living organisms but excess iron can be lethal because it facilitates free radical formation. Thus iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where meat is a significant part of the diet, most body iron is derived from dietary heme because heme binds few of the dietary chelators that bind inorganic iron. Uptake of heme into enterocytes occurs as a metalloporphyrin in an endosomal process. Intracellular iron is released from heme by heme oxygenase to enter plasma as inorganic iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin pathway (IMP) which is unshared with other nutritional metals. Ferrous iron uptake is facilitated by a DMT-1 pathway which is shared with manganese. In the iron deficient gut, large quantities of both mobilferrin and DMT-1 are found in goblet cells and intraluminal mucins suggesting that they are secreted with mucin into the intestinal lumen to bind iron to facilitate uptake by the cells. In the cytoplasm, IMP and DMT associate in a large protein complex called paraferritin which serves as a ferrireductase. Paraferritin solublizes iron binding proteins and reduces iron to make iron available for production of iron containing proteins such as heme. Iron uptake by intestinal absorptive cells is regulated by the iron concentration within the cell. Except in hemochromatosis it remains in equilibrium with total body stores via transferrin receptors on the basolateral membrane of absorptive cells. Increased intracellular iron either up-regulates or satiates iron binding proteins on regulatory proteins to alter their location in the intestinal mucosa.

Organic acids influence iron uptake in the human epithelial cell line Caco-2

It has previously been suggested that organic acids enhance iron absorption. We have studied the effect of nine organic acids on the absorption of Fe(II) and Fe(III) in the human epithelial cell line Caco-2. The effect obtained was dose-dependent, and the greatest increase (43-fold) was observed for tartaric acid (4 mmol/L) on Fe(III) (10 micromol/L). Tartaric, malic, succinic, and fumaric acids enhanced Fe(II) and Fe(III) uptake. Citric and oxalic acid, on the other hand, inhibited Fe(II) uptake but enhanced Fe(III) uptake. Propionic and acetic acid increased the Fe(II) uptake, but had no effect on Fe(III) uptake. Our results show a correlation between absorption pattern and chemical structure; e.g. hydroxyl groups, in addition to carboxyls, were connected with a positive influence. The results may be important for elucidating factors affecting iron bioavailability in the small intestine and for the development of foods with improved iron bioavailability.

Iron-overload induces oxidative DNA damage in the human colon carcinoma cell line HT29 clone 19A

Dietary iron may contribute to colon cancer risk via production of reactive oxygen species (ROS). The aim of the study was to determine whether physiological ferric/ferrous iron induces oxidative DNA damage in human colon cells. Therefore, differentiated human colon tumour cells (HT29 clone 19A) were incubated with ferric-nitrilotriacetate (Fe-NTA) or with haemoglobin and DNA breaks and oxidised bases were determined by microgelelectrophoresis. The effects of Fe-NTA were measured with additional H(2)O(2) (75microM) and quercetin (25-100microM) treatment. Analytic detection of iron in cell cultures, treated with 250microM Fe-NTA for 15 min to 24h, showed that 48.02+/-5.14 to 68.31+/-2.11% were rapidly absorbed and then detectable in the cellular fraction. Fe-NTA (250-1000microM) induced DNA breaks and oxidised bases, which were enhanced by subsequent H(2)O(2) exposure. Simultaneous incubation of HT29 clone 19A cells with Fe-NTA and H(2)O(2) for 15 min, 37 degrees C did not change the effect of H(2)O(2) alone. The impact of Fe-NTA and H(2)O(2)-induced oxidative damage is reduced by the antioxidant quercetin (75-67% of H(2)O(2)-control). Haemoglobin was as effective as Fe-NTA in inducing DNA damage. From these results we can conclude that iron is taken up by human colon cells and participates in the induction of oxidative DNA damage. Thus, iron or its capacity to catalyse ROS-formation, is an important colon cancer risk factor. Inhibition of damage by quercetin reflects the potential of antioxidative compounds to influence this risk factor. Quantitative data on the genotoxic impact of ferrous iron (e.g. from red meat) relative to the concentrations of antioxidants (from plant foods) in the gut are now needed to determine the optimal balance of food intake that will reduce exposure to this type of colon cancer risk factor.

Influence of the extent of hemoglobin hydrolysis on the digestive absorption of heme iron. An in vitro study

This study was designed to assess the interactions of heme with peptides produced by enzyme hydrolysis of hemoglobin, and their relationship with heme iron absorption. Bovine hemoglobin was hydrolyzed by pepsin or by subtilisin, which differ in their hydrolysis processes. The hydrolysis rate ranged from 0 (native hemoglobin) to 15%. Heme solubility and heme-peptides interactions were compared to iron absorption by the Ussing chamber model, at intestinal pH (7.5). Increasing hemoglobin hydrolysis enhanced iron absorption; the highest value was reached between 8 and 11% hydrolysis, whatever the enzyme used. Comparing the products of hydrolysis of the two enzymes showed that heme iron absorption depends not only on its solubility, but relies mainly on the balance between the strength of heme-peptides and the polymerization rate of heme.

Cellular location of proteins related to iron absorption and transport

K562 erythroleukemia cells and IEC6 rat cells were examined using confocal microscopy and antibodies raised against DMT-1 (Nramp-2, DCT-1), transferrin receptor (CD71), beta(3) integrin (CD61), mobilferrin (calreticulin), and Hephaestin. The cellular location of each of these proteins was identified by immunofluorescence in both saponin-permeabilized and non-permeabilized cells. Fluorescent reactivity was observed on or near the cell surface of each of these proteins, suggesting that they might participate in surface membrane transport of iron. Fluorescence was observed in the region of the cytoplasm with each antibody to include beta(3) integrin and transferrin receptor. It was pronounced in cells incubated with mobilferrin, Hephaestin, and DMT-1 antibodies. Speckled nuclear fluorescence was observed in cells incubated with anti-DMT-1. While these observations are descriptive, they demonstrate that there are significant concentrations of DMT-1, mobilferrin, and Hephaestin in the cytoplasmic region of cells. This suggests that there may be intracellular roles for these proteins in addition to their serving to transit iron across the cell surface membrane.

Histidine content of low-molecular-weight beef proteins influences nonheme iron bioavailability in Caco-2 cells

The objective of this study was to isolate and characterize beef muscle proteins that enhance nonheme iron bioavailability. Beef sirloin was cooked, lyophilized and reconstituted with water before in vitro digestion. After centrifugation, the digest supernatant was sequentially ultrafiltered using 10- and 1-kDa molecular weight cut-off membranes. Nonheme iron bioavailability was assessed by Caco-2 cell monolayer (59)Fe uptake using an extrinsic labeling method. All ultrafiltration fractions significantly (P < 0.001) increased iron solubility at pH 6.0, compared with the blank. However, iron uptake was significantly (P < 0.001) greater than the blank only in the presence of the 1-kDa retentate (1KR). Therefore, the 1KR was chosen for further analysis. Immobilized metal affinity chromatography (IMAC) of the 1KR yielded four fractions, i.e., three distinct fractions (F1, F3, F4) and one fraction (F2) comprised of a few closely associated peaks. All four IMAC fractions resulted in significantly (P < 0.001) greater (two- to fivefold) iron solubility at pH 6.0, compared with the blank. Iron uptake with F2 and F4 was significantly greater than the blank (P < 0.001 and P < 0.05, respectively). Gel electrophoresis and matrix-assisted laser desorption/ionization analysis illustrated that F1-F4 contained many peptides ranging from 1- to 7-kDa. Amino acid composition analysis revealed that histidine concentration increased progressively from F1 to F4, corresponding to a general, but not parallel increase in iron solubility and uptake. Our results suggest that the enhancement of nonheme iron absorption by beef may be due to peptides produced during gastrointestinal digestion and that histidine content may be important.

The effect of cysteine and 2,4-dinitrophenol on heme and nonheme absorption in a rat intestinal model

Previous studies have showed that purified heme iron forms insoluble polymers that are poorly absorbed. The presence of peptides and of amino acids maintaining heme iron in a soluble form could improve its bioavailability. The digestive uptake and transfer of a concentrated hydrolysate of heme peptides (HPH) and of iron gluconate (Gluc) at 100 µM were compared in vitro in a Ussing chamber. The effects of an enhancing amino acid (L-cysteine) on the uptake and transfer of both forms were assessed. An inhibitor of the oxidative phosphorylation (2,4-dinitrophenol; DNP) was used to differentiate the active and passive mechanisms of the absorption. The mucosal uptake (%Tot) and enterocyte transfer (%S) of the two sources of iron did not differ. DNP significantly reduced %Tot and %S of both forms. Cysteine significantly enhanced %Tot and %S of HPH and Gluc, partly corrected the inhibition exerted by DNP on %Tot of HPH and %S of both forms, and fully restored %Tot of Gluc. In presence of peptides produced by globin hydrolysis, the absorption of hemoglobin iron was efficient; it was mostly energy dependent and, therefore, should have occurred by a regulated transcellular pathway. Cysteine enhanced the passive uptake of iron and the passive processes involved in the enterocyte transfer of the common pool made of both sources (heme and nonheme) of iron. These results showed that heme iron can be purified and concentrated without impairing its digestive absorption, provided it remains in presence of peptides or amino acids.

Iron status and exercise

The prevalence of iron deficiency anemia is likely to be higher in athletic populations and groups, especially in younger female athletes, than in healthy sedentary individuals. In anemic individuals, iron deficiency often not only decreases athletic performance but also impairs immune function and leads to other physiologic dysfunction. Although it is likely that dietary choices explain much of a negative iron balance, evidence also exists for increased rates of red cell iron and whole-body iron turnover. Other explanations of decreased absorption and increased sweat or urine losses are unlikely. The young female athlete may want to consider use of low-dose iron supplements under medical and dietary supervision to prevent a decline in iron status during training.

Iron absorption and transport-an update

Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intracellularly, iron is released from heme by heme oxygenase so that iron leaves the enterocyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from enterocytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Enterocytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake.

Use of heme iron concentrate in the fortification of weaning foods

The purpose of this study was to evaluate the technological feasibility of fortifying homogenized weaning food with a porcine heme concentrate. The stability of iron and the organoleptic qualities of two infant weaning foods (a commercial homogenized nonfortified weaning food, NFWF, and the same food fortified with 0.5% of porcine heme concentrate, FWF) were tested throughout 8 months of storage at room temperature and at 37 degrees C. Heme iron decreased with storage time; however, the proportion of this highly available iron was considerably higher in FWF than in NFWF. The addition of heme iron changed significantly the color of the weaning food measured instrumentally, although high temperatures and length of storage time, did not modify Lab, chroma, and hue angle values in both samples. Organoleptic attributes presented a marked stability even in NFWF stored at room temperature and 37 degrees C.

Caco-2 cells can be used to assess human iron bioavailability from a semipurified meal

A Caco-2 cell model with extrinsic radioiron was used to evaluate the effect of dietary factors on nonheme iron bioavailability from a semipurified meal. Study 1 was conducted to evaluate the effect of enhancers (ascorbic acid) and inhibitors (bran, phytate and tea) on iron bioavailability when added to semipurified meal containing egg albumen as a protein source. The effect of various proteins [bovine serum albumin (BSA), casein, beef and soy] on iron bioavailability was evaluated in Study 2 by substituting the above protein sources for egg albumen. Protein solubilization following in vitro digestion for individual test meals was not significantly different from the control. On the other hand, nonheme iron solubilization (0.8+/-0.0 to 5.9+/-0.3 vs. 4.9+/-0.8 mg/L) varied significantly. The total iron uptake for each meal was calculated based on the percentage of radioiron taken up and transported by Caco-2 cells and the amount of nonheme iron present in uptake solutions. Iron uptake ratios represent test/control values. With the exception of BSA and ascorbic acid, the effect of dietary factors was similar to that found in humans reported in the literature. A significant correlation (r = 0.97; P<0.0001) was found between the published human absorption data and the iron uptake by the Caco-2 cells. The results of our study indicate the usefulness of Caco-2 cells in assessing human iron absorption and the feasibility of this cell model in studying iron bioavailability from various food combinations, otherwise not easily performed in humans.

Estimation of nonheme-iron bioavailability from meal composition

BACKGROUND

Considerable data are available on the individual effects of dietary factors on nonheme-iron absorption, but their combined effect when they are present in the same meal is not known.

OBJECTIVE

Our objective was to predict the bioavailability of iron from complex meals that are consumed commonly in the United States on the basis of the contents of factors that are known to promote or inhibit food iron absorption.

DESIGN

Radioisotopic measurements of nonheme-iron absorption from 25 meals were made in 86 volunteer subjects by using extrinsic radioiron labeling. The meal contents of nonheme iron, calcium, ascorbic acid, polyphenols, and phytic acid were determined by biochemical analysis; energy and protein contents were estimated from food-composition tables. Animal tissue content was based on weight or was obtained from the manufacturer.

RESULTS

After adjusting iron absorption for differences in iron status, the significant biochemical predictors of iron absorption as determined by multiple regression analysis were the contents of animal tissue (P = 0.0001), phytic acid (P = 0.0001), and ascorbic acid (P = 0. 0441). Collectively, these 3 variables accounted for 16.4% of the variation in absorption. On the basis of the multiple regression analysis, we developed the following equation to estimate iron absorption: Ln absorption, % (adjusted to serum ferritin concentration of 30 microg/L) = 1.9786 + (0.0123 x animal tissue in g) - (0.0034 x phytic acid in mg) + (0.0065 x ascorbic acid in mg).

CONCLUSION

For the 25 meals evaluated, only the contents of animal tissue, phytic acid, and ascorbic acid were useful for estimating nonheme-iron absorption.

Current methods for estimating dietary iron bioavailability do not work in China

Three current equations for estimating iron bioavailability were evaluated, and adjustments were proposed that would allow us to most effectively study iron bioavailability in China. Dietary intake data were obtained from 24-h dietary recalls taken over three consecutive days as part of the third Chinese National Nutrition Survey. Hemoglobin status was measured for 42,606 Chinese adults aged 18-60 y. The mean iron intake was 24.4 mg per capita per day, which was 177% of the Chinese RDA (209% of U.S. RDA). About 18% of the sample was classified as being anemic, indicating a large iron deficiency anemia and iron bioavailability problem in China. A number of methods proposed by World Health Organization and U.S. scholars were examined for adjusting iron bioavailability. Even the methods that consider several iron enhancers and inhibitors did not work adequately for the Chinese diet. The statistical assessment of the fit between iron bioavailability and hemoglobin status provided direction for adjusting the best of these predictive equations. We propose a new predictive approach for iron bioavailability which is more predictive of Chinese iron status. Consideration of additional dietary elements such as rice and bean consumption patterns are important. Our findings provide insight into additional factors which may influence iron bioavailability as well as possible improved methods for estimating the combined effect of multiple dietary factors on iron bioavailability, particularly in a vegetarian diet.

[Evaluating the impact of a hematinic iron-rich nutritional supplement]

A quasi-experimental epidemiological study was performed to evaluate the effect of a nutritional supplement made from bovine blood on human blood parameters. Healthy women who were neither pregnant nor breast-feeding were allocated to two groups: study (n=32) and control (n=17). Women in the study group received 0.5 mg of iron per day, while controls received placebo. Food intake frequency and side effects related to ferrous sulfate were recorded. A 24-hour dietary recall was performed every two weeks, and blood samples were collected to determine biochemical parameters. Compared to controls, the study group showed higher serum iron and iron retention capacity, lower calorie, protein, and vitamin C intake, and lower consumption of dairy products and fruit. Adjusted mean blood parameters were calculated using ANOVA. The output showed increased serum iron (p=0.009) and decreased iron retention capacity (p=0.031) at the end of the study. The results favor use of the product to treat iron deficiency anemia.

Iron absorption and transport

Iron is vital for living organisms because it is essential for multiple metabolic processes to include oxygen transport, DNA synthesis, and electron transport. However, iron must be bound to proteins to prevent tissue damage from free radical formation. Thus, its concentrations in body organs must be regulated carefully. Intestinal absorption is the primary mechanism regulating iron concentrations in the body. Three pathways for intestinal iron uptake have been proposed and reported. These are the mobilferrin-integrin pathway, the divalent cation transporter 1 (DCT-1) [or natural resistance-associated macrophage protein (Nramp2)] pathway, and a separate pathway for uptake of heme by absorptive cells. Each of these pathways are incompletely described. However, studies with blocking antibodies, observations in rodents with disorders of iron metabolism, and studies in tissue culture cells suggest that the DCT-1 pathway is dominant in embryonic cells and is involved with cellular uptake of ferrous iron, whereas the mobilferrin-integrin pathway facilitates absorption of dietary inorganic ferric iron. Thus, there are separate pathways for cellular uptake of ferric and ferrous inorganic iron. Body iron can enter intestinal cells from plasma via basolateral membranes containing the classical transferrin receptor pathway with a high affinity for holotransferrin. This keeps the absorptive cell informed of the state of iron repletion of the host. Intestinal mucosal cell iron seems to exit the cell via a distinct apotransferrin receptor and a newly described protein named hephaestin. Unlike the absorptive surface of intestinal cells, most other cells possess transferrin receptors on their surfaces and the vast majority of iron entering these cells is transferrin associated. There seem to be 2 distinct pathways by which transferrin iron enters nonintestinal cells. In the classical clathrin-coated pitendosome pathway, iron accompanies transferrin into the cell to enter a vesicle, which releases the iron to the cytosol with acidification (high affinity, low capacity). Under physiological conditions, a second transferrin associated pathway (low affinity, high capacity) exists which has been named the transferrin receptor independent pathway (TRIP). How the TRIP delivers iron to cells is incompletely described. In addition, tissue culture studies show that nonintestinal cells can accept iron from soluble iron salts. This occurs via the mobilferrin-integrin and probably the DCT-1 pathways. Cellular uptake of iron from iron salts probably occurs in iron overloading disorders and may be responsible for free radical damage when the iron binding capacity of plasma is exceeded. Radioiron entering the cell via the heme and transferrin associated pathways can be found in isolates of mobilferrin/paraferritin and hemoglobin. This interaction probably occurs to permit NADPH dependent ferrireduction so iron can be used for synthesis of heme proteins. Production of heme from iron delivered via these routes indicates functional specificity for the pathways.

On the methods for studying the mechanisms and bioavailability of iron

Studies of the molecular mechanisms involved in the absorption and bioavailability of iron are important to attempts made worldwide to control the high incidence of iron-associated disorders. The ultimate objective of these studies is to develop methods that are relevant to iron bioavailability and interactions in humans. However, a comprehensive understanding of the chemical and physiologic mechanisms that influence iron bioavailability is necessary to achieve this goal. Initial studies using in vitro and animal models offer the potential for flexibility and manipulation of experimental variables that could provide valuable information toward the understanding and improvement of food iron bioavailability.

Available iron and zinc in major lean meat cuts and their contribution to the recommended trace element supply in Switzerland

The objective of the present study was to analyze iron and zinc in major lean meat cuts in order to estimate the contribution of the average lean meat consumption in Switzerland (1995) for these trace elements. Iron, heme iron and zinc contents were analyzed in following muscles: pork (longissimus dorsi muscle and shoulder), beef (longissimus dorsi muscle and shoulder), veal (longissimus dorsi muscle) and chicken (breast and thigh). Beef and pork shoulder were the best sources of iron, heme iron and zinc. Pork and veal longissimus dorsi muscle and chicken were relatively poor in these trace elements. With an average daily lean meat consumption of 105 g, iron and zinc intake were about 1.1 mg/d and 3.8 mg/d, respectively. Recommendations for daily iron intake were met to 11% (men) and 7% (women) and for zinc to 25% (men) and 32% (women). Applying a modified Monsen model, the requirements for absorbed iron were met in the range of 10-30% and 7-20% for men and women, respectively. Taking into account a zinc absorption rate from meat of about 20-36%, the daily requirements for absorbed zinc were covered to 32-56%. In conclusion, the average amount of lean meat as consumed in Switzerland was high enough to be an important source of available iron and zinc, particularly for people with low iron and zinc status.

Adjustment of iron intake for dietary enhancers and inhibitors in population studies: bioavailable iron in rural and urban residing Russian women and children

Although determining iron intakes is essential in assessing adequacy of iron in the diet, estimating iron availability may be more useful for evaluating whether iron requirements are met. Our objectives were to describe the dietary information, analytical steps, and computer algorithms needed for iron bioavailability adjustments and to demonstrate the effects of various dietary factors on calculated iron absorption. Our study was based on 9890 women and children participating in the Russian Longitudinal Monitoring Survey. Between August 1992 and February 1993, two 24-h recalls were collected from each participant, and total, heme and nonheme iron intakes were calculated. Nonheme iron availability was adjusted for meat, fish and poultry and vitamin C consumed in the same meal and then further adjusted for tea and phytates. We found mean total iron intakes to be comparable to those of women of reproductive age in the United States and lower than those of United States children. When these intakes were adjusted for enhancers and inhibitors of absorption, the iron bioavailability in these vulnerable Russian groups was extremely low. Mean bioavailable iron as well as the 25th-75th percentile ranges of intake were below the bottom of the range of requirements, indicating that iron adequacy in this population may be considerably less than expected based on total iron intakes alone. Furthermore, rural and urban food availability had a significant effect on iron bioavailability. Future research on dietary iron adequacy should be based on estimates of available iron by collecting meal-level dietary data and using detailed information on mixed dishes and phytates.

Iron metabolism: a comprehensive review

Despite its abundance in the earth's crust, iron deficiency is a serious health issue in many parts of the world. Although fundamental observations about iron metabolism and the significance of iron nutriture were first noted some time ago, the molecular mechanisms involved in iron metabolism are just now being defined.

The influence of different protein sources on phytate inhibition of nonheme-iron absorption in humans

The inhibiting effect of phytate on nonheme-iron absorption from different protein sources was examined in human subjects using extrinsic radioiron labeling. A drink containing maltodextrose and corn oil was used as a control meal to which was added sufficient sodium phytate to provide 300 mg phytic acid and/or various protein sources. The proteins were selected to cover a broad range of effects on bioavailability and included egg white, meat, and phytate-free soy protein. When sodium phytate alone was added, there was a pronounced 83-90% reduction in mean absorption in separate studies with a composite average decline of 86%. Despite a wide range in absorption from meals containing the three protein sources, a remarkably similar relative inhibition was observed when sodium phytate was added. No significant difference in the inhibiting effect of phytate could be detected with additions ranging from the equivalent of 50-300 mg phytic acid to a meal containing egg white as the protein source. Our studies found no evidence that the inhibiting effect of phytate depends on the protein composition of the meal.

Overview and mechanisms of iron regulation

There are two major disturbances of iron balance: iron deficiency and iron overload. Iron-deficiency anemia is a major problem in developing countries and affects between 500 million and 600 million people worldwide. While iron overload is much less prevalent, it has a number of major pathologic sequelae and there have been recent suggestions that even modest increases in the body's iron stores may have pathologic associations. To understand the ways in which iron balance can be disturbed, it is necessary to have an understanding of how losses from the body are matched by the absorption of iron in different dietary settings as well as the limits of this regulatory control.