Absorption of nonheme iron

Iron Absorption. A very important effect of ascorbic acid is the enhancement of absorption of nonheme iron from foods. Ascorbic acid also enhances the reduction of ferric iron to ferrous iron. This is important both in increasing iron absorption and in its function in many hydroxylation reactions (140,141). In addition, ascorbic acid is involved in iron metaboHsm. It serves to transfer iron to the Hver and to incorporate it into ferritin. 

Monsen, E. R., and Cook, J. D. (1976). Food iron absorption in human subjects. IV. The effects of calcium and phosphate salts on the absorption of nonheme iron. Am.. Clin. Nutr. 29, 1142-1148. 

Roughead, Z.K., Zito, C. A., and Flunt, J. R. (2002). Initial uptake and absorption of nonheme iron and absorption of heme iron in humans are unaffected by the addition of calcium as cheese to a meal with high iron bioavailability. Am. ]. Clin. Nutr. 76, 419 25. 

Figure 1. The percent absorption of nonheme iron by individuals with iron body stores of 0 ( ), 250 (— —), 500 (— — ), and 1000...

In vitro experiments (38) showed that iron could form soluble chelates with ascorbic acid at the pH of the normal intestinal lumen. Soluble ascorbic acid-iron chelates formed at an acidic pH remained in solution even after the alkalinization of the medium (39). Intraintestinal installation of the pH-adjusted chelates into the rat enhanced the absorption of iron. The authors also suggest that the ascorbic acid normally present in mammalian bile has a physiologically important role in the absorption of nonheme iron from the diet. Whether the whole ascorbic acid-iron chelate is taken up intact into the mucosal cell under these conditions has not been established. Iron is, at the same time, more soluble, reduced, and more absorbable at intestinal pH in the presence of ascorbic acid those factors, with or without direct mucosal uptake of ascorbic acid-iron complexes, explain the contribution of ascorbic acid to the enhancement of iron availability. 

The 500 mgs iron store Reference Individual is assumed to absorb 23% of ingested heme iron (estimated at 40% of meat, fish or poultry iron) and 3-8% of ingested nonheme iron (plant iron plus remaining meat, fish, poultry iron). The quantity of enhancing factors consumed at a specific meal, i.e. mgs ascorbic acid plus gms cooked meat/fish/poultry, determines % absorption of nonheme iron ... 

Reviewing the studies in which nonheme iron absorption has been assessed at various levels of ascorbic acid in test meals composed of either single food items or food mixtures it appears that, within any individual study, additional increments of ascorbic acid consistently increased absorption of nonheme iron (Table 1). Considering the wide differences in experimental conditions, the various studies incorporating 12,5 to 1000 mg ascorbic acid indicate clearly the enhancing effect that ascorbic acid has on nonheme iron absorption. 

The Effect of Ascorbic Acid on Absorption of Nonheme Iron in Humans... 

The enhancing factors considered for the absorption of nonheme iron are ascorbic acid and meat, fish and poultry. The... 

Other factors in the diet have also been found to modify the effect of phosphorus on iron and zinc metabolism. One such factor is ascorbic acid. Peters, et al. (35) observed that when human subjects were fed a solution of iron chloride, they absorbed very little iron if ascorbic acid was not present in the solution (Table IV). In fact, if ascorbic acid was not present, absorption of iron was so low it was difficult to tell whether dietary factors, such as phosphorus, affected the absorption of iron. However, ascorbic acid may also counteract the effect of dietary phosphorus on the absorption of nonheme iron. Investigators have demonstrated that the addition of ascorbic acid to a diet counteracted the effect of the phosphoprotein in egg yolk on iron absorption (17. 18). 

The nonheme fraction represents by far the largest amount of dietary iron ingested by the economically underprivileged. In a vegetarian diet, nonheme iron is absorbed very poorly because of the inhibitory action of a variety of dietary components, particularly phosphates. Ascorbic add and meat facilitate the absorption of nonheme iron. Ascorbate forms complexes with and/or reduces ferric to ferrous iron. Meat facilitates the absorption of iron by stimulating production of gastric... 

FIGURE 53-4 Effect of iron status on the absorption of nonheme iron in food. The percentages of iron absorbed from diets of low, medium, and high bioavailability in individuals with iron stores of 0, 250, 500, and 1000 mg are... 

Mate has been observed to significantly inhibit absorption of nonheme iron in rats (Gutnisky et al. 1992). 

Absorption of nonheme iron was reduced in healthy women who ingested a phenolic-rich extract of rosemary with a test meal (Samman et al. 2001). 

Probing Metalloproteins Electronic absorption spectroscopy of copper proteins, 226, 1 electronic absorption spectroscopy of nonheme iron proteins, 226, 33 cobalt as probe and label of proteins, 226, 52 biochemical and spectroscopic probes of mercury(ii) coordination environments in proteins, 226, 71 low-temperature optical spectroscopy metalloprotein structure and dynamics, 226, 97 nanosecond transient absorption spectroscopy, 226, 119 nanosecond time-resolved absorption and polarization dichroism spectroscopies, 226, 147 real-time spectroscopic techniques for probing conformational dynamics of heme proteins, 226, 177 variable-temperature magnetic circular dichroism, 226, 199 linear dichroism, 226, 232 infrared spectroscopy, 226, 259 Fourier transform infrared spectroscopy, 226, 289 infrared circular dichroism, 226, 306 Raman and resonance Raman spectroscopy, 226, 319 protein structure from ultraviolet resonance Raman spectroscopy, 226, 374 single-crystal micro-Raman spectroscopy, 226, 397 nanosecond time-resolved resonance Raman spectroscopy, 226, 409 techniques for obtaining resonance Raman spectra of metalloproteins, 226, 431 Raman optical activity, 226, 470 surface-enhanced resonance Raman scattering, 226, 482 luminescence... 

The so called extrinsic labeling technique has been used extensively to label foods and meals for iron absorption studies. An inorganic radioiron salt and biosynthetically radioiron-labeled hemoglobin are mixed with food to label the nonheme and heme iron pools in the food respectively. It has been confirmed that absorption of extrinsic tracers is similar to the absorption of intrinsic iron in the food (9, 10). 

Averill, B., and Vincent, JB (1993). Electronic absorption spectroscopy of nonheme iron proteins. 

The interactions between different foods have sparked some interest. For exam-pie, if rice is consumed with orange juice, the orange juice can enhance the absorption of the iron in the rice. This effect results from the chelation of the iron by the ascorbate in the juice and the increased absorbability of the iron from the complex, On the other hand, if rice is consumed with tea, the tannins in the tea can reduce the absorption of the iron in the rice because the iron in the iron-tannin complex is not readily available. In general, including meat in the diet can increase the availability of iron (iom other foods. The mechanism of this effect is not dear. The availability of heme iron is not much influenced by other components of the food known to influence availability of nonheme iron. These components include ascorbate, tannins, phytate, phosphates, and fibers. 

To some extent, the body s needs for iron can be controlled at the point of absorption. Human subjects can vary iheir absorption of heme iron between 20 and 50%, and of nonheme iron between 1 and 40%, according to the body s needs. With iron... 

This reference dose of ferrous sulfate ascorbic acid was absorbed at a higher rate in iron deficient subjects than in iron replete subjects a ratio of the absorption of the test meal to the reference dose allowed comparisons to be made between individual subjects. As these early studies were limited to study of single food items an effort was made to extend the technique by developing designs utilizing extrinslcally tagged test meals (3,4). Utilization of these techniques has given evidence that dietary iron forms two separate pools in the gut, one a pool of heme iron and the other a pool of nonheme iron. The predominate source of iron in human diets is in the form of nonheme iron (5),... 

Extensive research on the absorption of iron from various types of meals has allowed guidelines to be developed by which the amount of dietary iron available for absorption may be estimated. Iron is the first trace mineral to be thus treated and thus serves as a model for other nutrients (19). The model for estimating bioavailable iron is based on the concept that iron forms a) a pool of heme iron which is readily available to humans and is uneffected by other dietary components and b) a pool of nonheme iron which is of low bioavailability unless enhancing factors are present concommitantly (20). 

Absorption of heme iron is estimated to be 23% for the reference individual. Heme iron may be estimated at 40% of the iron in meat, fish and poultry. Nonheme iron would be calculated as the remaining 60% of meat, fish and poultry iron plus the iron in vegetables, grain and other foods. The available nonheme iron and heme iron from each meal and snack can be summed to calculate the total day s available iron. 

The Importance of iron in nutritional status has been recognized for centuries. In an effort to dissolve iron and make it more available numerous therapies have been devised including such rarities as syrups of sherry in which iron wire has been soaked for 30 days, slices of apple into which iron nails have been imbedded, and solutions of vinegar into which iron filings have been placed weeks earlier. The interaction of ascorbic acid and iron has been recognized more recently (30). It may be that the most useful and readily found therapy for iron deficiency will be dietary ascorbic acid which has the capability of increasing the rate of nonheme iron absorption several fold. 

Monoferric phytate is the major fraction of iron in wheat bran, and is a highly bioavailable form of dietary iron in contrast to insoluble di- or tetra-ferric phytate. Monoferric phytate equilibrates with the miscible nonheme iron pool of a meal in extrinsic label iron absorption tests. Whole wheat bran depressed absorption by humans of nonheme iron in a meal. Dephytinized wheat bran also inhibited nonheme iron absorption by humans and the inhibition could not be clearly attributed to either the insoluble or soluble fractions of the dephytinized bran. Adult men who consumed 36 g of wheat bran per day had positive iron balances. Iron balance was not increased when dephytinized bran was consumed. The form of ferric phytate must be known to properly explain the effect of phytic acid on iron absorption. The overall meal composition must be considered to evaluate the effect of wheat bran on iron nutrition of humans. 

Human studies. Simpson et al. (14) measured absorption by humans of free monoferric phytate from meals of both high and low bioavailability. Absorption was measured by the extrinsic tag method (15). Iron (2 mg) was added to the meals in each pair of absorption tests as either monoferric phytate or FeClj, each compound was labeled with either 55 Fe or 59Fe. The absorption ratio of monoferric phytate to FeClj was not significantly different from unity (Table III). The free biological form of iron in wheat bran, monoferric phytate, was no less well absorbed than the dietary nonheme iron in the meals. Absorption of both compounds was several fold greater from the standard (STD) than from the semisynthetic (SS) meal reflecting the relative bioavailability of nonheme iron from the two types of meals. 

Table V. Dephytinized Wheat Bran and Absorption of Nonheme Dietary Iron by Humans ...

It is difficult to determine the reasons for decreased absorption of vegetable iron sources however, some researchers speculate that the ferritin administered with vegetable sources is incompletely miscible with a nonheme iron pool or that it actually forms a separate iron pool (3,5). 

The biochemical architecture of photosynthetic bacteria is not as complex as that of green plants. For example, photosynthetic bacteria have only one photosystem, while green plants have two. The reaction center protein from several species of photosynthetic bacteria can be isolated from the photosynthetic membrane. Reaction centers from the species Rhodopseudomonas sphaeroides have been extensively studied. Although minor details will change from one species to another, the important features are nearly identical. The reaction center protein has a molecular weight of about 70,000 daltons. Within the reaction center protein extracted from the carotenoidless mutant strain R26 of the species R. sphaeroides, are found four molecules of bacteriochlorophyll a, two molecules of bacteriopheophytin a, one atom of nonheme iron, and, depending on the isolation procedure used, one or two molecules of ubiquinone. The absorption spectrum of the isolated reaction center has been well characterized. It is shown in Fig. 4. Based on in vitro absorption spectra, the bands at 870, 800, and 600 nm have been assigned to the bacteriochlorophyll a molecule. Bands at 760 and 530 nm have been attributed to the bacteriopheophytin a. 

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