Contributions of cellular and paracellular pathways to transepithelial intestinal calcium transport.

The small intestine may supply adequate levels of nutritionally required calcium in the absence of stimulated (1,25(OH)2-D3) cell-mediated mechanism of absorption if dietary calcium levels are sufficiently high. This same route may also account for net loss of systemic calcium due to asymmetrical paracellular calcium secretion in the absence of sufficient dietary calcium. Fine adjustments in net calcium balance may occur in the large intestine where inefficiencies of paracellular leaks are significantly reduced and larger calcium gradients can be sustained.