Consequences of calcium interactions with phytate and phytase for poultry and pigs

Despite increasing practical experience and cascades of scientific reports on exogenous microbial phytases, several issues associated with their use remain unresolved because of the ambiguous and, at times, conflicting data that has been generated. One possible cause of these inconsistent outcomes is dietary calcium (Ca) levels, which are mainly derived from limestone. Thus the purpose of this review is to examine Ca interactions with dietary phytate and phytases, particularly exogenous, microbial phytases, and their consequences for poultry and pigs. The polyanionic phytate molecule has a tremendous capacity to chelate cations and form insoluble Ca-phytate complexes, which are refractory to phytase activity. Thus Ca-phytate complex formation along the gastrointestinal tract, where one phytate (IP₆) molecule binds up to five Ca atoms, assumes importance and approximately one third of dietary Ca may be bound to phytate in digesta. Consequently, phytate limits the availability of both P and Ca as a result of insoluble Ca-phytate complex formation, the extent of which is driven by gut pH and molar ratios of the two components. It is accepted that Ca-phytate complexes are mainly formed in the small intestine where they have a substantial negative influence on the efficacy of mucosal phytase. However, exogenous phytases are mainly active in more proximal segments of the gut and lower pH levels, so their efficacy should not be influenced by Ca-phytate complexes in the small intestine. There is, however, data to indicate that Ca and phytate interactions occur under acidic conditions with the formation of soluble and insoluble Ca-phytate species, which could negatively impact on exogenous phytase efficacy. Also, Ca will tend to elevate gut pH because of limestone's very high acid binding capacity, which will favour Ca-phytate interactions and may influence the activity of exogenous phytases depending on their pH activity spectrum. The de novo formation of binary protein-phytate complexes that are refractory to pepsin hydrolysis may be fundamental to the negative impact of phytate on the digestibility of protein/amino acids. However, high dietary Ca levels may disrupt protein-phytate complex formation by interacting with both phytate and protein even at acidic pH levels, thereby influencing the outcomes of phytase amino acid digestibility assays. Finally, it is increasingly necessary to define the Ca and nonphytate-P requirements of pigs and poultry offered phytase-supplemented diets.