a1 Poultry Research Foundation, The University of Sydney, 425 Werombi Road, Camden, NSW 2570, Australia
a2 Australian Synchrotron, 800 Blackburn Road, Clayton, VIC 3168, Australia
a3 Institute for Food Nutrition and Human Health, Massey University, Palmerston North 4442, New Zealand
Protein–phytate interactions are fundamental to the detrimental impact of phytate on protein/amino acid availability. The inclusion of exogenous phytase in pig and poultry diets degrades phytate to more innocuous esters and attenuates these negative influences. The objective of the present review is to reappraise the underlying mechanisms of these interactions and reassess their implications in pig and poultry nutrition. Protein digestion appears to be impeded by phytate in the following manner. Binary protein–phytate complexes are formed at pH levels less than the isoelectric point of proteins and complexed proteins are refractory to pepsin digestion. Once the protein isoelectric points are exceeded binary complexes dissociate; however, the isoelectric point of proteins in cereal grains may be sufficiently high to permit these complexes to persist in the small intestine. Ternary protein–phytate complexes are formed at pH levels above the isoelectric point of proteins where a cationic bridge links the protein and phytate moieties. The molecular weights of protein and polypeptides in small-intestinal digesta may be sufficient to allow phytate to bind nutritionally important amounts of protein in ternary complexes. Thus binary and ternary complexes may impede protein digestion and amino acid absorption in the small intestine. Alternatively, phytate may interact with protein indirectly. Myo-inositol hexaphosphate possesses six phosphate anionic moieties (HPO4 2–) that have strong kosmotropic effects and can stabilise proteins by interacting with the surrounding water medium. Phytate increases mucin secretions into the gut, which increases endogenous amino acid flows as the protein component of mucin remains largely undigested. Phytate promotes the transition of Na+ into the small-intestinal lumen and this suggests that phytate may interfere with glucose and amino acid absorption by compromising Na+-dependent transport systems and the activity of the Na pump (Na+-K+-ATPase). Starch digestion may be depressed by phytate interacting with proteins that are closely associated with starch in the endosperm of cereal grains. While elucidation is required, the impacts of dietary phytate and exogenous phytase on the site, rate and synchrony of glucose and amino acid intestinal uptakes may be of importance to efficient protein deposition. Somewhat paradoxically, the responses to phytase in the majority of amino acid digestibility assays in pigs and poultry are equivocal. A brief consideration of the probable reasons for these inconclusive outcomes is included in this reappraisal.
Abbreviations: AID, apparent ileal digestibility; FTU, phytase units; IP6, myo-inositol hexaphosphate; phytate-P, phytate-bound phosphorus