Abstract
Phytin is the Ca2+-Mg2+-K+salt of phytic acid that is created and deposited in the aleurone layer and/or germ of grains and legumes.Its high presence in feedstuffs for fowl and swine diets results in it being a universal and significant impediment to optimum performance.Phytin impairs gastrointestinal recovery of a wide array of nutrients,the effect varying with the nutrient concerned.On exposure to low pH during gastric digestion,phytin dissociates into phytic acid and solubilized Ca2+.Even at low gastric pH,phytic acid is negatively charged which forms the basis of its anti-nutritive behavior.Pepsinogen has extensive basic amino acids on its activation peptide that are presented as cations at low pH which are targeted by pepsin for activation.Partially crystalized Ca2+near the enzyme's active site further stabilizes its newly formed structure.Thus,phytic acid appears to interfere with gastric digestion by several mechanisms;interfering with pepsinogen activation by binding to the polypeptide's basic amino acids;coordinating free Ca2+,destabilizing pepsin;binding some dietary proteins directly,further compro-mising gastric proteolysis.Upon digesta attaining neutrality in the duodenum,Ca2+and other cations re-bind with accessible anions,phytic acid being a significant contender.Phytate not only binds free cations but can also strip them from enzymes(e.g.Ca2+,Zn2+)which reduces their structural resistance to autolysis and ability as co-factors(e.g.Zn2+)to increase enzyme activity.Goblet cells initially employ Ca2+as an electronic shield between mucin layers enabling granule formation and cell storage.After mucin granule release,Ca2+is progressively displaced by Na+to free the viscous mucins enabling its translocation.Mucin entangles with the glycocalyx of adjacent enterocytes thereby constructing the unstirred water layer(USWL).Excessive removal of Ca2+from mucin by phytic acid increases its fluidity facilitating its loss from the USWL with its associated Na+.This partly explains increased mucin and Na+losses noted with high phytate diets.This review suggests that phytic acid binding of Ca2+and less so Zn2+is the basis for the diversity in nutrient losses encountered and that such losses are in proportion to dietary phytate content.
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