Phytate protein complex

It has generally been presumed and observed (36,42,43), that purified sodium phytate had deleterious effects in humans similar to those observed in animal studies while the effects of indigenous phytic acid have been questioned. Recent results (23,44 Table II) indicate that even with purified sodium phytate, deleterious effects are not always observed. The form of the phytic acid-mineral-protein complex present (12) may be the determinant factor as to whether a deleterious effect is observed. 

The complexity of food effects on zinc absorption is illustrated by the studies of Sandstrom et al.(25,37,38) in which phytate, protein, calcium, zinc and other factors appear to have variable effects on zinc absorption. Although It appears certain that food interferes with zinc absorption, the effects of individual food substances are unknown and difficult to predict. Regardless of individual effects, zinc administration with food will complicate efforts to measure the effects of other variables on intestinal absorption. 

Almost all the evidence showing that phytate decreases zinc absorption in man and animals is based on pure phytate added to the diet. The effect of natural phytate is variable (18). It has, however, been reported that phytate in bran affected zinc bioavailability in the same way as sodium phytate (19). Dietary fibre in the rural Iranian diet was considered to be the main cause of zinc deficiency in Iran (20). However, the addition of 26 g of fibre from various sources to the American diet did not have any significant effect on the zinc requirements of male adults (21). Similarly, Indian men consuming a diet containing only 10.8 mg of zinc were reported to be in balance in spite of a dietary fibre intake of 50 g per day (22). Moreover, the presence of fibre and phytate in soy flour did not affect the bioavailability of zinc added as zinc carbonate, to the diet of rats (17), although others (23) have reported that the bioavailability of zinc in breakfast cereals depends mainly on their phytate-zinc molar ratio. Our results indicate that there is some, as yet, undetermined difference in the phytate or the fibre of cereals which affects the bioavailability of zinc. It may be some component of dietary fibre (24) or the intrinsic differences in the protein-phytate-mineral complex (10). 

Iron absorption takes place predominantly in the duodenum where the acid environment enhances solubility, but also throughout the gut, allowing sustained-release preparations to be used. Most iron in food is present as ferric hydroxide, ferric-protein complexes or haem-protein complexes. Ferrous (Fe " ) iron is more readily absorbed than ferric (Fe ). Thus the simultaneous ingestion of a reducing agent, such as ascorbic acid, increases the amount of the ferrous form ascorbic acid 50 mg increases iron absorption from a meal by 2-3 times. Food reduces iron absorption due to inhibition by phytates, taimates and phosphates. 

However, tamarind seeds have low levels of phytic acid comparable that of lima bean (Egbe and Akinyele, 1990, cited in 4)). Phytic acid decreases bioavailability of certain minerals and may interfere with the ntihzation of proteins dne to the formation of phytate-protein and phytate-mineial-protein complexes and also inhibits the digestive enzymes (Reddy et al, 1982, cited in 4)). The phytate conld, however, be snbstantially eliminated by processing methods snch as soaking and antoclaving (Reddy et al, 1982, cited in 4)). 

In corn, phytate is contained primarily in the germ in a water soluble form. Though it seems unlikely that such a strong acid would be free, complexation with potassium, for example, would render water solubility without being identifiable current methodology. Also complexation with proteins which were either water soluble or whose isoelectric point were above or below the pH of water would render solubility. In legumes, phytate has been shown to be associated with protein (9, 10). 

This association is greatest at the isoelectric point of the protein and being readily dissociated at pH s above or below the isoelectric point ( iJ.) Phytate association in cereal grains is less well defined but is contained in significant concentrations both in the bran and germ (12). There appears to be at least one ferric ion associated in the otherwise soluble phytate complex in wheat bran (13). Phytate in sesame seed appears to be the most unique and least soluble of all seeds. In this case magnesium appears to be the predominant cation (9 ) ... 

J). Bread components other than phytate were examined for their ability to bind metals. Fiber, protein and starch of wheat formed stable complexes with zinc and calcium, and later iron was found to share this behavior. The metals combined with protein or wheat starch, however, were released during digestion with peptidases and amylases (2,1)5). By contrast dietary fiber, being resistant to digestive secretions, retained bound metal intact. Removal of phytate, which had in the past been held to be the main source of metal complexation by bread, did not decrease but tended to enhance the binding of the metal (J2.). Further doubt about the role of... 

Calcium absorption is reduced by high pH complex-ing agents such as oxalate, phytate, free fatty acids, and phosphate and shortened transit times. These factors are probably of clinical importance only when associated with vitamin D deficiency, marginal calcium intake, or malabsorption disorders. Absorption is also reduced by increased intake of protein, fat, and plant fiber increasing age stress chronic alcoholism immobilization (e.g., prolonged hospitalization) and drugs such as tetracycline, thyroid extract, diuretics, and aluminum-containing antacids. 

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. 

The inositol hexa- and pentakisphos-phates are prevalent in soils compared to lower-order esters, probably because stability in the soil is linked to the number of phosphate groups. The most widespread stereoisomer is myo-inositol (Dalai, 1977). Inositol phosphates are more resistant to mineralization than the other fractions of the soil organic phosphorus and, therefore, are probably poorly available to plants (Williams and Anderson, 1968). They are present in soils in highly complex forms associated with clay minerals, fulvic and humic acids (Anderson and Arlidge, 1962), proteins and some metallic ions (Rojo et al., 1990). The various forms of inositol phosphates are often imprecisely referred to as phytic acid, which is reserved exclusively for the free acid form of myo-inositol hexakisphosphate. Salt forms of myo-inositol hexakisphosphate, also known as phytates, are very stable and consequently accumu-... 

Phytic Acid. Recent reviews (67,68,69) summarized the literature covering the relationship between phytic acid and mineral bioavailability in soy protein products. The formation of phytate-proteln-mineral complexes (particularly zinc chelates in flours, concentrates, and Isolates prepared from mature soybeans) may be responsible for reduced mineral availability. However, the iron in Fe-labeled mature soybeans is more available to iron-deficient rats than the iron in green-immature soybeans, even though mature soybeans contain three times more phytic acid (70). The factor(s) responsible for this difference in bioavallablllty has not been identified. 

The iron present in animal foods is more readily available than that of plant products. Not only is haem iron more easily absorbed than inorganic iron but, in addition, the amino acids derived from the proteins form soluble complexes with the inorganic iron. In contrast the iron in cereals and vegetables is bound with proteins, phytates, other phosphates, oxalates and carbonates as insoluble ferric complexes and is poorly absorbed. 

Zinc—Meat, poultry, fish, eggs, and dairy products are good sources of zinc. The animal proteins are generally much better sources of dietary zinc than plants because the phytate present in many plant sources complexes the zinc and makes it unavailable. 

The issue of bioavailability from food sources and the interactions between food groups and copper availability remains a critical question. Lonnerdal et al. demonstrated that heat treatment of cows milk formula decreases the copper bioavailability. Transitional complexes form in the milk upon heating that have a similar configuration to copper and thereby directly inhibit copper absorption. High doses of zinc also reduce copper bioavailability, as does combined iron and zinc supplementation. The dilemma is how to prepare an infant formula containing adequate copper, iron, and zinc that will meet the RDA for copper. Other nutrients dramatically affect copper absorption from foods. Soy protein-based diets promote less copper retention in tissues than lactalbumin-based diets. However, it is unclear if this effect is solely due to the soy protein composition or to the higher zinc in these soy-based formulas. In animals, phytate causes a drop in serum copper but human stable isotope studies reveal no... 

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