Mg, the second intracellular cation, is an essential nutrient that plays a role in most metabolic pathways as well as in the ionic equilibrium of the cell membranes. It is absorbed in the intestine, it is used to build bones, it maintains the plasma, extra-and intracellular pools, and it is excreted via the urine. The mineralized tissues contain 60-65% of all body Mg (±25 g) and, of the remaining 35-40%, ±30% is found in muscle and ±7% inside the eukaryotic cells whereas plasma and extracellular fluids contain only 1% of total body content. In plasma total Mg consists of 3 fractions (i.e., about 30% as protein-bound, 13% as salts or chelates, and about 55% as free Mg ions). The normal plasma concentrations of total and ionized Mg are 0.7-0.9 and 0.4-0.45 mM, respectively. The intracellular Mg plays major roles in controlling the activity of a wide range of coenzymes (especially ATP) and enzymes (±300 enzymes are Mg dependent) as well as transport or regulatory proteins. Mg deficiency is associated with neuromuscular excitability, muscular discomfort changes in phosphocalcic metabolism, and potassium deficit. Various hormones (e.g., parathyroid hormone, calcitonin, growth hormone, aldosterone, and vitamin D) affect Mg metabolism, but there is no evidence of a specific Mg-regulating hormone. Mg homeostasis (especially Mg plasma concentration) results from a balance between gastrointestinal uptake and renal filtration-reabsorption and excretion processes.
The Mg pools in the body are in balance when absorption equates with losses. However and despite extensive research on Mg absorption, the exact site and mechanism of Mg absorption are still largely unknown.70 The major site of absorption is the distal small intestine (jejunum and ileum) but colonic absorption is also significant. Two main absorption processes exist, namely an active and a passive (or simple diffusion) absorption process, but the second process is probably much more important than the former. The active transport operates essentially at low Mg concentration. The passive absorption involves paracellular moves between the mucosal cells and accounts for most Mg absorption when Mg intake is adequate or high.70 It is essentially controlled by osmolarity in the gut lumen and in the extracellular fluids, but also by the intraluminal pH, by the presence of specific compounds (e.g., SCFAs), and by the concentration of Mg in the gut lumen. It is independent of age or vitamin D intake. Usually, approximately 30-50% of Mg in food is absorbed but it can be higher or lower, depending on food composition, especially its daily intake (i.e., fractional absorption is ±70% and ±12% when intake is ±25 and ±1000 mg/d, respectively). In particular, it has been shown that increasing the amount of dietary proteins raises the apparent Mg absorption.
The fecal Mg pool is composed of the nonabsorbed dietary Mg. Indeed, once Mg has reached the bloodstream, the main excretion route is the kidney. But, as the filtration-reabsorption process functions close to saturation, the Mg that is absorbed in excess is easily excreted in the urine.
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