Vitamin D and calcium absorption in the small intestine
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Vitamin D and Calcium Absorption in the Small Intestine
Role of Vitamin D in Intestinal Calcium Absorption
Vitamin D is essential for increasing calcium absorption in the small intestine, which is critical for bone health and overall calcium homeostasis 4678. The active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)₂D), binds to the vitamin D receptor (VDR) in intestinal cells, triggering gene transcription that enhances calcium uptake 456. When dietary calcium is low, vitamin D-dependent absorption becomes especially important, while at high calcium intakes, passive absorption through paracellular diffusion predominates 46.
Mechanisms of Calcium Absorption: Transcellular and Paracellular Pathways
Calcium is absorbed in the small intestine by two main mechanisms: an active, vitamin D-dependent transcellular pathway and a passive, vitamin D-independent paracellular pathway 5610. The transcellular pathway involves three steps: calcium entry into the enterocyte via epithelial calcium channels, intracellular transport by calcium-binding proteins (such as calbindin), and extrusion into the bloodstream by a calcium pump 610. Vitamin D stimulates the synthesis of these channels, pumps, and binding proteins, thereby increasing active calcium absorption 610.
Recent research also shows that vitamin D can enhance paracellular calcium diffusion, suggesting its influence is not limited to the active pathway 5610. The paracellular route, which occurs through tight junctions between cells, becomes more significant when dietary calcium is abundant 46.
Calcium-Binding Protein and Its Correlation with Vitamin D
The production of calcium-binding protein (CaBP) in the small intestine is closely linked to vitamin D activity. After vitamin D administration, CaBP synthesis increases, which directly correlates with enhanced calcium absorption 13. The amount of CaBP present rises within hours of vitamin D exposure, and this increase matches the timing of improved calcium uptake, supporting the idea that CaBP is crucial for vitamin D-mediated calcium transport 13.
Regional Differences in Calcium Absorption in the Small Intestine
Most dietary calcium absorption occurs in the ileum, the lower segment of the small intestine, due to its longer transit time compared to the duodenum and jejunum . While the duodenum is the primary site for active, vitamin D-dependent absorption, the ileum also exhibits vitamin D-dependent active transport, and passive diffusion is significant throughout the small intestine . About 90% of total calcium absorption takes place in the small intestine, with the colon accounting for the remaining 10% .
Regulation and Adaptation of Calcium Absorption
The efficiency of intestinal calcium absorption adapts to physiological needs and dietary availability, largely orchestrated by the vitamin D endocrine system . Factors such as age, pregnancy, menopause, and disease states can modify vitamin D-regulated calcium absorption 46. In vitamin D deficiency, both the production of CaBP and the rate of calcium absorption are significantly reduced, leading to impaired calcium homeostasis 37.
Modulation by Other Hormones
Other hormones, such as calcitonin, can influence vitamin D-induced calcium absorption. For example, calcitonin can acutely decrease calcium absorption in vitamin D-treated intestines, indicating a complex hormonal regulation of this process .
Conclusion
Vitamin D is a critical regulator of calcium absorption in the small intestine, acting primarily through the stimulation of active, transcellular transport mechanisms involving calcium-binding proteins and transporters. It also enhances passive, paracellular absorption under certain conditions. The process is highly adaptable, responding to dietary calcium levels and physiological demands, and is influenced by other hormones such as calcitonin. Deficiency in vitamin D leads to reduced calcium absorption and can compromise bone health and calcium balance 13456789+2 MORE.
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