CALCIUM AND PHOSPHORUS-METABOLISM IN GROWING PIGS .2. SIMULTANEOUS RADIO-CALCIUM AND RADIO-PHOSPHORUS KINETICS

Kinetic studies of Ca and P were performed with pigs at 35 and 65 kg live weight (LW) fed graded levels of Ca and P. Endogenous excretion and bone parameters were estimated. Accretion of Ca and P was estimated by the Bauer-Carlsson-Lindquist (BCL) formulation and by the accumulated radioactivity in bones. Accretion of Ca was also estimated by a power function. Bone resorption was estimated as the difference between bone accretion and balance. Endogenous output of Ca and P was positively related to intake (P < 0.001 and P < 0.07, respectively). Neither were proportional to LW, hence, expressions of endogenous losses relative to LW may be misleading. Parameters of bone metabolism were described equally well by the three methods. However, level differences were obvious between methods. BCL formulation values of P accretion were notably higher than those obtained by bone radioactivity. The former method expresses whole body accretion, while the latter accounts only for bone accretion. Bone accretion of both Ca and P was constant and independent of intake level. Accretion relative to bone ash declined with LW (P < 0.05), conversely, daily accretion increased with LW (P < 0.05) due to the larger bone mass. Bone resorption of Ca and P was strongly and negatively related to mineral intake and accounted nearly exclusively for increases in dietary intake and intestinal absorption.