Phytate-containing cereals

Phytate (myo-inositol hexaphosphate Fig. 15.3, structure 33) is found in many food species and can be considered as a phytochemical. Its role in the plant is primarily as a phosphate store in seeds, but it is found in other tissues as well, for example, tubers (Harland et al., 2004). Phytate and its hydrolysis products are anti-nutrients that chelate metal ions and thus reduce their bioavailability (Persson et al., 1998 House, 1999). This is particularly a problem with cereal grains, but pre-processing can improve mineral absorption from these foods (Agte and Joshi, 1997). There is some concern that high phytate foods could also contain higher levels of toxic heavy metals caused by natural accumulation. Plants also contain phytate-degrading enzymes that can also influence metal ion bioavailability (Viveros et al., 2000). 

The free iron in the food enters the intestinal mucus from which it is absorbed by the epithelial cells via a transporter protein. This absorption is decreased by tea, coffee and phytate (inositol hexaphosphate) present in cereal fibre. Iron combined in haem is absorbed directly by epithehal cells after being released from haem-containing protein. The free iron is released in these cells by the enzyme haem oxidase. The free Fe is then bound to paraferritin and released into the blood where it is bound by ferritin. The three reactions are as follows. 

Feed peas, like cereal grains, are low in Ca but contain a slightly higher level of P (about 4g/kg). They contain about 12g/kg phytate, similar to that in soybeans (Reddy et al., 1982). The levels of trace minerals and vitamins in peas are similar to those found in cereal grains. 

Wheat bran and whole wheat cereals quantitatively contain high amounts of manganese and are often listed as particularly valuable sources of manganese. However, zinc also is contained in appreciable amounts in wheat bran and whole wheat products but is poorly absorbed by the human from these sources. This has been attributed to either the phytate or the fiber contents of these products or a combination of these two dietary factors. These same factors may also affect the absorption of manganese. 

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 ) ... 

From these results, it is evident that supplementation of the diet containing mixed infant cereal resulted in a body weight gain better than and a femur zinc value comparable to that observed before in weanling rats fed a diet containing 12 pg/g zinc from zinc sulphate (7). The zinc supplementation reduced the phytate/zinc molar ratio of the diet from 10 to 5. 

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). 

Most of our knowledge about factors affect zinc absorption ( , 10, 18) comes from studies on animals. The most extensive studies are made on the affect of phytic acid, which is present in vegetables and unrefined cereal products. Decreased zinc absorption has been found both when substantial amounts of sodium-phytate are added to the diet and when the diets are based on phytic-containing food, such as soy beans. The explanation of this is that a complex between Zn and phytic acid is made, resulting in a decreased bioavailability. 

Iron deficiency anemia occurs mainly in infants, children, and fertile women. For this reason, a variety of foods, including infant formula and infant cereals, is fortified with iron. Ferrous sulfate is a form of iron that is most readily absorbed by the gut, but when added to dry cereals it can promote their spoilage and rancidity. For this reason, dry cereals are fortified with elemental iron particles, ferric pyrophosphate, or ferrous fumarate (Davidsson et ah, 1997). Ascorbic add may also be added to the cereal to enhance iron absorption. To view some of the numbers, infant cereals may contain 75 mg iron/kg cereal (1.3 mmol iron/kg), 1 mmol phytic acid/kg, and 2.6 mmol ascorbic acid/kg (Davidsson et cd., 1997). Although phytic acid impairs iron absorption, the added ascorbate serves to prevent this effect. An alternate method for preventing phytate from impairing iron absorption is to treat the food with the enzyme phytase. A parent interested in enhancing a child s iron absorption can easily feed a child some orange juice, but it would not be practical to pretreat the child s cereal with phytase. A typical availability of ferrous sulfate in infants is about 3-5% (with no ascorbate), and 6-10% (with ascorbate). Ascorbate is effective when present in a twofold molar excess over the iron. 

Binding of nutritionally important cations to dietary fiber has recently been introduced as a possible cause for poor utilization by humans of iron from diets high in vegetables and unrefined cereals. Reinhold et al. (J2) and Ismail-Beigi et al. (33) have demonstrated that the fiber component of wheat bread and bran binds iron even after the phytate has been extracted by dilute acid. In our study, the water-insoluble fraction (containing most of the neutral detergent fiber of the bran) of enzymatically dephytinized wheat bran did not inhibit, whereas... 

Larsson, M. and Sandberg, A.-S. 1991. Phytate reduction in bread containing oat flour, oat bran or rye bran. J. Cereal Sci. 14, 141-149. 

Nuts, seeds, and grains are not considered as sources of organic acids. Other than amino acids and fatty acids, they contain phenolic acids and phytic acid. Phytic acid and its salts, phytats, are regarded as the primary storage form of both phosphate and inositol in seeds, mostly within the hulls. The phytic acid content of cereals (whole grain) varies from 0.5% to 2.0% [21],... 

Milk, cereal grains and fishmeal products containing bone are good sources of phosphorus the content in hays and straws is generally very low. Considerable attention has been paid to the availability of phosphorus. Much of the element present in cereal grains is in the form of phytates, which are salts of phytic acid, a phosphoric acid derivative ... 

Certain foods such as unrefined cereals and sugar cane juice contain organic phosphates, e.g. phytate (inositol hexaphosphate), which reduce enamel solubility, apparently by reacting with calcium phosphate salts on the enamel surface. The view that this type of compound has a caries-inhibiting effect is supported by observations that dietary supplements of calcium glycerophosphate reduce caries in rats and monkeys. At the same time, the calcium-binding properties of phytate may adversely affect the absorption of calcium from the intestine (page 143). 

Cereals contain about 1% of phytate [myoinositol (1,2,3,4,5,6) hexakisphosphate], which binds about 70% of the phosphorus in the grain. Since it occurs mainly in the aleurone layer, the content of phytate in flour depends on the extent of grinding (Table 15.20). A part of it is hydrolyzed in stages to myo-inositol during dough making. 

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