Intestinal calcium absorption: differences in transport between duodenum and ileum

The interaction of calcium with sugar, sodium, and water absorption was studied in rats fed laboratory chow and in vitamin D-deficient and -repleted rats with the use of intestinal loops in vivo. Calcium absorption in the duodenum was enhanced by D-glucose only in the vitamin D-deficient state. In both vitamin D-deficient and -repleted ileum, calcium absorption decreased when NaCl was replaced by mannitol; however, it remained unchanged when NaCl was replaced by choline chloride or urea. Calcium absorption was enhanced by actively transported sugars and by increasing net water flow generated by differences in osmotic pressure and urea. Under all experimental conditions there was a strong association between net water and calcium absorption in the ileum. These findings suggest that calcium absorption is enhanced by solvent drag, although the role of sodium cannot be excluded entirely.