Bioavailability iron, factors affecting

Much of the current research has centered upon the role of phytic acid on zinc and iron bioavailability (110-124). Work performed at the authors institution with several different types of soy foods suggests that phytic acid is a major factor affecting availability of zinc from foods derived from the legume (110-114). In addition, it appears that endogenous zinc in high-phytate foods may be a limiting factor in optimal utilization of these foods for man. We have found that fortification of soy foods (under proper conditions) with zinc, iron, magnesium, or calcium results in excellent... 

Iron deficiency was not a major factor affecting bioavailable Cu and Zn (Table 6, Fig. 5). Copper and Zn uptake by Fe-deficient plants was reduced because of the 25-50% decrease in biomass that occurred relative to Fe-sufficient plants (Table 3). Despite substantial decreases in biomass, Fe deficiency resulted in similar or greater amounts of Cu and Zn being absorbed by plants, relative to Fe-sufficient plants. This may be a consequence of enhanced exudation of phy-tosiderophores as a response to Fe deficiency (Fig. 2), as shown for wheat grown in a Cu-contaminated, calcareous soil (Chaignon et al., 2002b). 

Besides the effect of dietary calcium level, the amount of iron, and perhaps the level of other metals, may affect zinc bioavailability. Solomons and Jacob (13) have shown in human subjects that increasing the iron/zinc ratio from 0 1 to 3 1 in solutions containing 25 mg of zinc and corresponding amounts of iron as ferrous sulfate produced a progressive decrease in the plasma zinc response. They further reported that the chemical form of iron was an important determinant of the interaction. Solomons (14) extensively reviewed both inhibitory factors and enhancers of zinc bioavailability found in foods. 

This chapter mentions some iron chemistry Important to its bioavailability and the changes which may be induced by food processing. The reader may refer to the chapter by Spiro and Saltman (1) for a discussion of inorganic iron chemistry. This author has critically reviewed the iron sources used for food enrichment earlier (2). A good review of the chemistry of iron in myoglobin has been published by Livingston and Brown (3). Forth and Rummel (4) have made available an extensive review on iron absorption and factors which affect iron absorption. 

Iron within a food matrix provides an extremely reactive vehicle for complexation with a great number of chemical compounds. In fact, it is this very reactivity which makes some of the most bioavailable forms of iron so objectionable to the food processor since the chemical reactions which occur can drastically affect quality. Conversely, the most functionally suitable forms of iron are often not very bioavailable. This nutritional/functional campatability, although not a subject of focus here, should be mentioned since any fortification program must consider this campatability factor or it will be doomed to failure. This problem has been discussed in some detail by Lee and Clydesdale and Zoller et al ... 

The differences in bioavailability found with certain complexes from study to study are frustrating at best. It seems difficult to isolate the exact affect of a given ligand and the Particular complex it might form. For instance, Derman et al (i2/ recently found that iron absorption from maize and sorghum beer was more than twelve-fold greater than from a gruel made from the constituents used to prepare the beer. The authors postulated that at least three factors are responsible for this l) the re-... 

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,... 

The results of dietary zinc analysis need to be considered in terms of the availability of the zinc in the food for intestinal absorption. The zinc content of whole meals and the total daily zinc intake are not sufficient information on their own, without knowledge of factors which inhibit or promote intestinal absorption (O Dell, 1984). Free ionic zinc probably does not exist in the intestinal tract, zinc being bound to molecular species such as protein, amino acids, phytic acid, citrate and others. The bioavailability of the metal is determined by the nature of these zinc binding ligands. When the zinc complex is insolubie as in Zn-phytate, the uptake from diet is poor, whereas zinc-protein or zinc-amino acid complexes are more easily dissociated and are a good source of available zinc. Other dietary components affect zinc absorption such as the amount of iron, calcium and phosphate. 

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