Animal, iron absorption

The iron uptake processes of animals appear to differ from those of microorganisms for example, siderophores have not been identified in animals. Iron absorption from the gut is not understood in detail, though it is clear that the nature of the diet and the nutritional and iron status of the animal are important influences on iron uptake... 

HFE has been shown to be located in cells in the crypts of the small intestine, the site of iron absorption. There is evidence that it associates with P2 niicroglobu-lin, an association that may be necessary for its stability, intracellular processing, and cell surface expression. The complex interacts with the transferrin receptor (TfR) how this leads to excessive storage of iron when HFE is altered by mutation is under close smdy. The mouse homolog of HFE has been knocked out, resulting in a potentially useful animal model of hemochromatosis. 

Time Dependence. As the initial iron deficiency (by milk feeding) stimulates iron absorption (53,86-90), which in turn may affect negatively manganese absorption (as described here), the body iron state must also be taken into account. It is therefore useful to establish data about the time factor, i.e. how long the animals can be treated with iron supplemented milk before an alteration in manganese transport is observed and also, how long it takes for manganese transport to return to normal once iron treatment has ceased. 

Animal Studies. Studies with experimental animals have demonstrated enhancement of iron absorption in the presence of ascorbic acid. Brown and Bother (3) failed to observe an enhancement of iron uptake after 30 min when 20 fig of iron-59-labeled ferrous sulfate and 100 mg of ascorbic acid were injected by stomach tube into 200-g rats, but Van Campen (4) found that 17.6 mg of ascorbic acid promoted the absorption of a 100-/Ag dose of iron-59-labeled ferric chloride given by stomach tube to 6-week-old rats. 

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). 

One way in which the diets varied was in regard to the levels of iron and zinc in the diets. Buttner Muhler (41) demonstrated that dietary phosphorus levels could only affect iron absorption and hence the amount of iron stored in the liver, if animals were fed diets that contained more than minimal amounts of iron. Similarly Magee Fu (44) demonstrated with rats that dietary phosphorus levels only affected storage of zinc in the liver if the diets fed the rats contained zinc. Vohra Kratzer (32) observed a similar phenomenon in turkeys. 

It does not seem that the human intestinal tract possesses this enzyme. If phytic acid does influence the iron absorption, a parallel is difficult to draw between rats and humans. As in humans the enzyme has not been demonstrated in pigs and, for this reason, pigs were chosen for experimental animals. 

Many factors have been identified as influencing the absorption of iron. In addition to changes within the host which affect iron absorption and the form of the iron salt, various dietary constituents which may increase or decrease iron bioavailability have also been studied. As diets become more plant product oriented and less iron is provided by animal products, the occurrence of these other dietary factors is also likely to change. Factors which have been implicated include the following amount of heme iron, ascorbic acid level, dietary protein,... 

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

Several amino acids are speculated to be effective in increasing iron absorption and can be divided into three categories. These are 1) amino acids which act as buffering agents in the Intestine and delay the increase of the pH towards neutrality where iron is oxidized and forms insoluble precipitates 2) amino acids which form iron-amine chelates that act to enhance iron absorption and 3) amino acids which act to stimulate iron transport systems within the animal (3). A wide variety of soy products ranging from soy meal to soy protein isolates were found to have a strong inhibitory effect on... 

Most signs of scurvy can be related to inadequate or abnormal collagen synthesis. Ascorbate enhances prolyl and lysyl hydroxylase activities (Chapter 25). Collagen formed in scorbutic patents is low in hydroxyproline and poorly cross-linked, resulting in skin lesions, bone fractures, and rupture of capillaries and other blood vessels. The absolute amount of collagen made in scorbutic animals may also decrease independently of the hydroxylation defect. The anemia of scurvy may result from a defect in iron absorption or folate metabolism. 

It is well known that ferrous iron is absorbed from the intestinal tract of animals much more rapidly than ferric iron and that ascorbic acid can reduce ferric to ferrous iron. Additional proofs of the facilitation of ferric iron absorption by ascorbic acid have been reported (Cll, B29, W5). The same action in a cell-free system was demonstrated with the stimulation by ascorbic add of the incorporation of protein-bound ferric iron into protoporphyrin (L20). This enzyme reaction is known to require a reducing agent, and as these workers demonstrated, glutadiione functions equally as well as ascorbic add in this regard. 

Several elements, including iron (Davis et al. 1992a), phosphorus (Wedekind et al. 1991), and calcium (Wilgus and Patton 1939) are known to decrease manganese absorption in adults and animals. Iron-poor diets result in increased manganese absorption in humans (Mena et al. 1969) and in rats (Pollack et al. 

A multiple-step mechanism could also control iron absorption through the intestine. For example, it has been proposed that the bone marrow controls the secretion of a specific humoral factor responsible for regulating the intestinal absorption of iron. If such a hormone exists, it is unlikely to be erythropoietin. Erythropoietin administration to animals has no effect on iron absorption. Conversely, intestinal absorption and the iron stores influence the rate of erythropoiesis. Iron loading of anemic dogs considerably stimulates hemoglobin synthesis. 

The availability of mineral elements is commonly high in young animals fed on milk and milk products but declines as the diet changes to solid foods. An additional complication is that the absorption, and hence apparent availability, of some mineral elements is under homeostatic control (determined by the animal s need for them). Iron absorption, discussed in Chapter 8, is the clearest example of this effect, but in ruminants the efficiency of calcium absorption also appears to be dependent on the animal s requirements. 

It is well known and confirmed that heme-bound iron from animal-based foods is absorbed differently from iron in plant-based foods [111]. This requires intrinsic labeling of the diet/test meal if iron absorption from meat needs to be assessed. A separate pathway of iron absorption has been suggested for ferritin-bound iron [112], but it appears to be of minor relevance to iron nutrition. Ferritin is the body s most important iron storage protein and can trap iron in a hollow protein sphere, but it is effectively released during digestion [113]. Therefore, it should enter the same common chemical pool from which all other non-heme iron is absorbed. 

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