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 oc cures by controlling the absorption rate of inorganic phosphate (P-i) in the upper small intestines and by renal P-i excretion. These proces ses are mainly mediated by parathyroid hormone (PTH) and calcitriol (1 ,25-dihydroxycholecalciferol, 1,25-(OH)(2)D-3). If, for example, the P -i level in plasma decreases, renal calcitriol production is stimulate d and higher amounts of the hormone are released into the circulation. Calcitriol increases Pi absorption from the intestinal tract by stimu lation of a secondary active, sodium-coupled P-i-cotransport system in the upper small intestines. In addition, calcitriol is involved in th e mobilization of bone and soft tissue P. Simultaneously, hypercalcemi a develops, which can be induced by either increased intestinal Ca abs orption and/or Ca mobilization from bone. Hypophosphatemia and hyperca lcemia suppress PTH release from the parathyroid glands and thus minim ize urinary P-i losses. The concerted action of increased/decreased ci rculating calcitriol/PTH on the intestinal tract, bone and kidneys nor malizes P-i levels in plasma. With respect to adequate P supply in ani mal nutrition, it must be considered that utilization of dietary P not only depends on absorption capacity of the pig intestinal tract but a lso on differences in availability of dietary P between ingredients. I n feedstuffs of plant origin most of the P is bound as phytate-P and c an only be absorbed after enzymatic breakdown of phytic acid by phytas es. Intrinsic phytase activity differs between plant materials such as wheal, wheat bran, barley and triticale with higher activities than f ound in maize and legume seeds subjected to thermal treatments. Supple mentation of microbial phytase increased P digestibility more pronounc ed in those feedstuffs which showed very limited intrinsic phytase act ivity. At present, a digestibility of about 70% seems to be the upper level for digestibility of P from plant material. From the environment al point of view, an increased digestibility resulting from phytase su pplementation offers the possibility to reduce the supplementation of phosphates and the concentration of total P in the diet. Therefore, th e amount of P being excreted by the pig can be remarkably reduced. How ever, the first step for minimizing faecal P excretion should be to su pply P in accordance with the animal's requirement.