Cereal products phytate

Without the results from our second study, a definitive conclusicai is not easy. If results of our 1st study are applicable to a range of dietary zinc intakes, a phytate to zinc molar ratio of 10 to 12 is not a hinderance to bioutilization of zinc by humans and the level of intake of zinc becomes a limiting factor in determining adequacy. We know from our analysis of self-chosen diets, and estimation by others, that most people consume diets that are less than 10 in phytate/zinc molar ratio. Vegetarians, particularly those who eat a lot of cereal products, will very likely exceed the value of 10. There is wide variation between individuals and from day to day by each individual. The role of adaptation or response of homestatic control mechanisms require further clarification but it is our feeling at present that humans can consume a wider molar ratio of phytate to zinc than heretofore thought. 

A number of studies have shown that zinc, as compared to other minerals, is poorly utilized from phytate-contalning foods. The relative BV of zinc in a number of phytate-contalning foods was recently examined by Franz et al ( ). Each product was prepared as for human consumption and Incorporated into a semi-purifled diet fed to rats. Whole corn and brown rice had a low relative BV (0.58 or less compared to 1.00 for zinc in zinc sulfate) while wholewheat flour and unleavened wholewheat bread had medium values (0.63-0.74). Refined cereal products such as white flour, leavened and unleavened white bread and white rice (all low in phytate) had high relative BV (0.89-1.08) as did leavened wholewheat bread... 

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. 

Calcium forms stable insoluble salt with oxalic acid (see Section 10.2.3.2). In plant cells with higher concentrations of oxalic add, caldum oxalate can be actually present in the form of crystals. Some plants have been shown to bind metals in mixed complexes. For example, chromium can be bound in an oxalate-malate complex, and nickel and zinc can form a dtrate malate complex. Citric add has been proven to be a low molecular weight zinc ligand in human milk, and in casein micelles it binds calcium. It is also used as a food additive (acidulant, synergist to antioxidants and sequestrant), so great attention has been paid to the formation of its complexes with metal ions. The addition to cereal products leads to increased solubihty of naturally present iron, due to its release from phytic acid salts (phytates). 

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). 

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. 

The phytate and neutral detergent fibre in cereal grains and soy products, which make up infant cereals or breakfast cereals, have unknown intrinsic differences which have varying effects on zinc bioavailability. Consequently, the concept of a common phytate/zinc molar ratio below which there is no adverse effect on the bioavailability of zinc cannot be supported from our results. 

Phytate-bound iron may or may not constitute available forms of iron to the human as discussed in several other chapters of this book. Earlier work sugests that phytates inhibit iron absorption. Since phytates and oxalates are provided by cereal/ plant products, an Increase in the plant components of diet is likely to increase the intake of these inhibitors. 

Cereals, pulses and soya bean products are particularly rich in phytate. The [phytate] [zinc] molar ratio has been used as an index of the bioavailability of zinc from different foods. Diets where the ratio is 20 1 or more have been suggested as a cause of human... 

Liu, Z. H., H. Y. Wang, X. E. Wang, G. P. Zhang, P. D. Chen, and D. J. Liu. 2006. Phytase activity, phytate, iron, and zinc contents in wheat pearling fractions and their variation across production locations. Journal of Cereal Science 45 319-326. 

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 ... 

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. 

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