Intestinal calcium absorption: mechanisms and applications.

Calcium absorption from the intestine involves two sets of events. One, a saturable transcellular process is regulated by vitamin D via its molecular product, the calcium-binding protein (CaBP, MW = 8800). This transcellular movement is largely confined to the proximal portion of the intestine. The second process is nonsaturable, occurs throughout the length of the intestine and is paracellular. Evidence in support of these statements is discussed, with emphasis on kinetic considerations. It is proposed that CaBP acts as a ferry, amplifying the intracellular movement of calcium by a factor of about 60, thereby enabling transcellular calcium transport to reach the measured values of Vm = 22 mumol/h per gram (wet) duodenum, with Km = 3.9 mM. The transcellular process is subject to down-regulation and is influenced by functional or nutritional factors such as age or calcium intake. The nonsaturable process, on the other hand, is not directly influenced by these or related events. Vitamin D therapy alters active calcium transport, but may lead to undesirable effects at other target organs, e.g., kidney or bone. An increase in calcium intake is the simplest method for increasing the amount absorbed. Future research may show whether paracellular pathway alterations are a practical approach to changing the amount of calcium absorbed by the nonsaturable process.