Mechanisms of intestinal phosphorus absorption and availability of dietary phosphorus in pigs

Phosphorus (P) is an essential component of many organic and inorganic compounds in vertebrates such as pigs. Therefore, adequate dietary P supply is important to meet daily requirements in order to maintain P homeostasis. Under normal circumstances regulation of P homeostasis occurrs by controlling the absorption rate of inorganic phosphate (Pi) in the upper small intestines and by renal Pi excretion. These processes are mainly mediated by parathyroid hormone (PTH) and calcitriol (1,25-dihydroxycholecalciferol, 1,25-(OH)2D3). If, for example, the Pi level in plasma decreases, renal calcitriol production is stimulated and higher amounts of the hormone are released into the circulation. Calcitriol increases Pi absorption from the intestinal tract by stimulation of a secondary active, sodium-coupled Pi-cotransport system in the upper small intestines. In addition, calcitriol is involved in the mobilization of bone and soft tissue P. Simultaneously, hypercalcemia develops, which can be induced by either increased intestinal Ca absorption and/or Ca mobilization from bone. Hypophosphatemia and hypercalcemia suppress PTH release from the parathyroid glands and thus minimize urinary Pi losses. The concerted action of increased/decreased circulating calcitriol/PTH on the intestinal tract, bone and kidneys normalizes Pi levels in plasma. With respect to adequate P supply in animal nutrition, it must be considered that utilization of dietary P not only depends on absorption capacity of the pig intestinal tract but also on differences in availability of dietary P between ingredients. In feedstuffs of plant origin most of the P is bound as phytate-P and can only be absorbed after enzymatic breakdown of phytic acid by phytases. Intrinsic phytase activity differs between plant materials such as wheat, wheat bran, barley and triticale with higher activities than found in maize and legume seeds subjected to thermal treatments. Supplementation of microbial phytase increased P digestibility more pronounced in those feedstuffs which showed very limited intrinsic phytase activity. At present, a digestibility of about 70% seems to be the upper level for digestibility of P from plant material. From the environmental point of view, an increased digestibility resulting from phytase supplementation offers the possibility to reduce the supplementation of phosphates and the concentration of total P in the diet. Therefore, the amount of P being excreted by the pig can be remarkably reduced. However, the first step for minimizing faecal P excretion should be to supply P in accordance with the animal's requirement.