Magnesium absorption and metabolism
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Magnesium Absorption: Intestinal Pathways and Influencing Factors
Magnesium absorption in the body occurs mainly through the intestines, using both passive paracellular and active transcellular mechanisms. The passive paracellular route allows magnesium to move between cells, primarily in the small intestine and colon, and is facilitated by proteins called claudins (such as claudin-2, -12, -16, and -19) 123569. The active transcellular pathway involves magnesium entering cells through channels like TRPM6 and TRPM7, especially in the colon and distal convoluted tubule (DCT) of the kidney 1369. The efficiency of absorption depends on the dose: at low intakes, a higher percentage of magnesium is absorbed, while at higher intakes, the fractional absorption decreases, following a curvilinear relationship 46.
Several dietary and physiological factors influence magnesium absorption. High intake of other minerals, certain fibers (like cellulose and lignin), phytate, and oxalate can reduce absorption, while proteins, medium-chain triglycerides, and some carbohydrates (such as inulin and resistant starch) can enhance it 610. Prebiotic fibers, in particular, have been shown to improve magnesium absorption and retention, with effects more pronounced than for calcium . The form of magnesium (organic vs. inorganic salts) has a minor effect on bioavailability compared to the dose and the individual's magnesium status .
Vitamin D can increase magnesium absorption, especially at pharmacological doses, but a significant portion of absorption occurs independently of vitamin D. Interactions with calcium and phosphate also affect magnesium absorption, though the exact mechanisms are not fully understood 57.
Magnesium Metabolism: Distribution, Regulation, and Excretion
Magnesium is the second most common intracellular cation and is essential for over 300 enzymatic reactions, energy metabolism, muscle contraction, and neurotransmission 1238. The body maintains magnesium balance through a dynamic interplay of intestinal absorption, exchange with bone, and renal excretion 1238. After absorption, magnesium is distributed in the extracellular and intracellular fluids, as well as in subcellular organelles .
The kidneys play a central role in regulating serum magnesium levels. About 20% of filtered magnesium is reabsorbed in the proximal tubule, 60% in the thick ascending limb (TAL) of the loop of Henle, and 5-10% in the DCT 123. The TAL uses passive paracellular transport, while the DCT uses active transcellular transport via TRPM6/7 channels, which are regulated by hormones such as insulin and epidermal growth factor 13. Basolateral magnesium extrusion from cells is sodium-dependent and involves proteins like CNNM2 and SLC41A3 .
Hormonal factors, including parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3, can influence magnesium metabolism by affecting renal excretion and bone resorption 78. Acid-base status, sex hormones, and certain immunosuppressive drugs also regulate magnesium transport proteins .
Clinical and Dietary Considerations in Magnesium Homeostasis
Disturbances in magnesium absorption or renal reabsorption can lead to hypomagnesemia, with common causes including alcoholism, type 2 diabetes, and the use of certain medications like proton-pump inhibitors and diuretics 39. Genetic mutations affecting magnesium channels or transporters can also disrupt homeostasis 13. There is no simple, widely available method to assess magnesium status; serum magnesium and the magnesium tolerance test are most commonly used, though measurement of ionized magnesium may become more accessible 26.
In summary, magnesium absorption and metabolism are complex processes influenced by intestinal transport mechanisms, dietary factors, hormonal regulation, and renal handling. Understanding these pathways is essential for managing magnesium balance and addressing related health issues.
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