Phytic acids, mineral availability

Little agreement has been reached as to which dietary components or which food processes physiologially affect mineral availability. Many plant foods contain phytic acid, oxalic acid or other dietary fiber components that can be shown to chelate minerals. The effect of these dietary substances upon the final bioavailability of the mineral in question will depend upon the digestibility of the chelate (106). 

Phytic acid, although restricted to a more narrow range of food products, mainly grains, complexes a broader spectrum of minerals than does oxalic acid. Decreased availability of P is probably the most widely recognized result of excessive intakes of phytic acid, yet Ca. Cu, Zn, Fe. and Mn are also complexed and rendered unavailable hy this compound. High intakes of both calcium and vitamin D help to offset ihe deleterious effects of oxalates. 

Although sesame seed is not known to contain any protease inhibitors or other ANFs, high levels of oxalic and phytic acids may have adverse effects on palatability (Ravindran and Blair, 1992) and on availability of minerals and protein (Aheme and Kennelly, 1985). Decortication of seeds almost completely removes oxalates, but it has little effect on phytate (Ravindran and Blair, 1992). Complete decortication is difficult because of the small size of the seeds. 

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. 

Torre M, Rodriguez AR, Saura-Calixto F. 1991. Effects of dietary fiber and phytic acid on mineral availability. Crit Rev Food Sci Nutr 30(2) 1-22. 

The fact that phytic acid P is not completely available to the body, even in absence of calcium in the diet, was established by Lowe and Steenbock (1936). Increased Ca in the diet depresses phytic acid digestion considerably in the rat. Harrison and Mellanby (1939) observed that the mineralization of bones in puppies suffered as the result of the addition of phytic acid to the diet, and the explanation that phytic acid depresses the... 

Cereals are considered a very poor source of calcium (Sema-Saldivar 1993). However, some food processes such as nixtamalization for tortilla making (Chapter 9) and cooking in the presence of ashes (i.e., alkaline to. Chapter 16) increase its concentration (Sema-Saldivar et al. 1990, 1991, 1992). Phosphorus is the mineral present in the highest amounts unfortunately, its bioavailability is questionable because most is bound to phytic add (Lasztity and Lasztity 1990). Phytates also bind other cations such as iron and zinc, considerably lowering their availability. The phytic acid... 

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