Calcium is absorbed through two primary mechanisms in the small intestine: active transport (transcellular) and passive diffusion (paracellular).
Active Transport (Transcellular)
- This process primarily occurs in the duodenum and jejunum, the upper parts of the small intestine, where calcium demand is higher.
- It involves transporting calcium through the intestinal cells.
- Calcitriol (active form of Vitamin D) plays a crucial role by stimulating the production of proteins involved in this active transport.
- Specifically, calcitriol binds to the Vitamin D receptor (VDR) in intestinal cells, triggering the synthesis of proteins like:
- Calcium channels (e.g., TRPV6): Allow calcium to enter the intestinal cells from the gut lumen.
- Calbindin D9k: A calcium-binding protein that shuttles calcium across the intestinal cell's cytoplasm.
- Plasma membrane Ca2+-ATPase (PMCA) and Na+/Ca2+ exchanger (NCX1): These pumps actively transport calcium out of the intestinal cell and into the bloodstream.
Passive Diffusion (Paracellular)
- This process happens all along the small intestine, but it's particularly important in the ileum, the lower part of the small intestine, when calcium concentration in the intestinal lumen is high.
- It involves calcium moving between intestinal cells, following a concentration gradient (from an area of high concentration to an area of low concentration).
- Passive diffusion doesn't require energy or specific transport proteins.
- The effectiveness of paracellular absorption depends on:
- Calcium concentration in the intestinal lumen: Higher concentrations favor passive diffusion.
- Integrity of tight junctions: The junctions between intestinal cells influence permeability.
- Age: The paracellular pathway becomes less efficient with increasing age.
Factors Affecting Calcium Absorption
Several factors can influence calcium absorption, including:
- Vitamin D status: Adequate Vitamin D is essential for active transport.
- Age: Absorption efficiency decreases with age.
- Dietary factors: Other nutrients, such as phosphate, can affect calcium absorption.
- Gastric acidity: Stomach acid helps solubilize calcium, making it easier to absorb.
- Intestinal health: Conditions affecting the small intestine can impair absorption.
In summary, calcium absorption relies on both active and passive transport mechanisms. Active transport, regulated by Vitamin D, is crucial when calcium intake is low, while passive diffusion plays a greater role when calcium concentrations in the gut are high.
