How does the gastrointestinal tract absorb and excrete magnesium?
Only about a quarter, or 100 mg, of the ingested magnesium is absorbed by the GI tract and the rest is excreted. Much of the absorption takes place through passive paracellular transport, driven by an electrochemical gradient and solvent drag. There is a second active transport system mediated by TPRM 6 and 7 channels (TRPM stands for “transmembrane receptor potential subfamily melastatin”). The active magnesium transport becomes more active with low magnesium status and is downregulated with magnesium excess. This active transport system is more active in the terminal ileum and proximal colon.
The kidneys are able to dynamically adjust the reabsorption of magnesium to respond to changes in magnesium levels. In the presence of low magnesium, fractional reabsorption of magnesium can rise to 99.5%. With magnesium excess, the fractional excretion of magnesium can fall to 30%.