Intestinal calcium absorption. Interplay of paracellular and cellular pathways.

Intestinal absorption of calcium as characterized by studies in the rat follows both mediated mechanisms through enterocytes as well as fluxes between epithelial cells. Data summarized in this review suggests that the extracellular or paracellular route in distal small intestine plays a dominant role in calcium absorption at luminal calcium concentrations above 1 mM; proximal small intestine and cecum/colon show a much greater dependence on cellular translocation processes. The wide spectrum of paracellular:cellular transport ratios found in the intestine suggests a well-adapted mechanism to minimize energy expenditure for calcium absorption in the presence of adequate dietary calcium with capacity for efficient calcium scavenging when dietary supplies are limited. No evidence to date supports a role for vitamin D in the control of paracellular calcium absorption. Other mechanisms regulating paracellular calcium absorption will undoubtedly become a significant new area for future inquiry.