Calcium moves to the basolateral area of the cell by way of microvesicular transport. Calcium is rapidly and reversibly bound to the calmodulin-actin-myosin I complex. Because the luminal concentration of calcium is usually much higher than the intracellular concentration of calcium, a large concentration gradient favors the passive movement of calcium. Calcium moves down its concentration gradient through a calcium channel into the apical section of the microvillae. However, 1,25(OH) 2D mainly controls the active absorption of calcium.
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The paracellular route is indirectly influenced by calcitriol because it is capable of altering the structure of intracellular tight junctions by activation of protein kinase C, making the tight junction more permeable to the movement of calcium. The paracellular pathway is passive, and it is the predominant route of calcium absorption when the lumen concentration of calcium is high.
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There are two routes for the absorption of calcium across the intestinal epithelium: the paracellular pathway ( i.e., between the cells) and the transcellular route ( i.e., through the cell) ( Figure 2A). Calcium (A), phosphate (B), and magnesium (C) balance is a complex process involving bone, intestinal absorption of dietary calcium, phosphate, and magnesium, and renal excretion of calcium, phosphate, and magnesium. Each of these intestinal segments has a high absorptive capacity for calcium, with their relative calcium absorption being dependent on the length of each respective intestinal segment and the transit time of the food bolus ( 3).Ĭalcium, phosphate, and magnesium flux between body compartments. Calcium is absorbed almost exclusively within the duodenum, jejunum, and ileum. Although serum calcium levels can be maintained in the normal range by bone resorption, dietary intake is the only source by which the body can replenish stores of calcium in bone. Therefore, a net absorption of calcium is approximately 200 mg/d (20%) ( 3). Approximately 400 mg of the usual 1000 mg dietary calcium intake is absorbed by the intestine, and calcium loss by way of intestinal secretions is approximately 200 mg/d.
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When 1 g of calcium is ingested in the diet, approximately 800 mg is excreted in the feces and 200 mg in the urine. This amount will vary depending on the amount of dairy product consumed. In healthy adults, approximately 800–1000 mg of calcium should be ingested daily. Calcium balance is tightly regulated by the concerted action of calcium absorption in the intestine, reabsorption in the kidney, and exchange from bone, which are all under the control of the calciotropic hormones that are released upon a demand for calcium ( Figure 1A).