Influence of glucose, fructose, and water movement on calcium absorption in the jejunum.

The first experiment demonstrated that glucose stimulated calcium absorption. Possible mechanisms include: (a) glucose stimulation of active calcium absorption, (b) glucose stimulation of water absorption with enhanced calcium absorption by solvent drag, or (c) glucose stimulation of water absorption caused an increased luminal calcium concentration with resultant increased active and/or passive calcium absorption. In a second experiment, neither glucose, fructose of water absorption stimulated calcium absorption when luminal calcium concentration was maintained at a constant level. These results suggest: (a) glucose and fructose indirectly enhance calcium absorption via an effect on water movement, (b) water absorption enhances calcium absorption by virtue of concentrating unabsorbed calcium within the intestinal lumen, not by solvent drag, and (c) bulk water movement and passive calcium ion movement take place via separate channels.