Mechanisms of intestinal calcium absorption and availability of dietary calcium in pigs.

Calcium absorption from the intestine may be active via the transcellular route or passive via the paracellular route, but the active component is necessary for the maintenance of calcium homeostasis. The active transport depends on vitamin D and is more restricted to the proximal part of the small intestine. Calcitriol, the active metabolite of vitamin D, induces the synthesis of calbindin, a Ca(2+)-binding protein which seems to be involved in transcellular Ca(2+)-transport, increases the number of Ca(2+)-transport components in the apical membrane, but is without direct influence on Ca(2+)-pumps of the basolateral membrane. A phytate-phosphorus containing diet not only reduces the availability of phosphorus but also of calcium. However, with sufficient supply of vitamin D, this may be compensated by the hormonal regulation of calcium homeostasis. Addition of lactose to the diet increases the apparent digestibility of calcium in the postileal segment of the gastrointestinal tract by an so far unknown mechanism.