Magnesium is the fourth most abundant cation in the body and the major intracellular divalent cation. The amount of magnesium in an adult human is ˜2000 mEq (24 g), of which 60% is in the skeleton and 40% is intracellular. Less than 1% is found in the extracellular fluid compartment.1,2 and 3 Magnesium is essential for the function of many enzyme systems, including those that use adenosine triphosphate.4,5 Magnesium can be absorbed along the entire gastrointestinal tract, including both the small and large intestine, but it is absorbed most efficiently in the jejunum and ileum, through both a saturable transport system and passive diffusion.6,7 Fractional intestinal magnesium absorption appears to depend on the amount of magnesium in the diet; with an average magnesium intake, ˜40% is absorbed.6,7 The major dietary sources of magnesium include vegetables and meats, but magnesium is ubiquitous in food. Some studies have suggested that 1,25-dihydroxyvitamin D [1,25(OH)₂D] enhances intestinal magnesium absorption,8 although this may be secondary to the effect of this vitamin on calcium transport.9 The regulation of magnesium homeostasis occurs primarily in the kidney.10 Micropuncture studies have demonstrated that magnesium is absorbed along the proximal convoluted tubule (5% to 15%), the thick ascending limb of the loop of Henle (50% to 60%), and the distal tubule (10%).11 Although a true tubular maximum for magnesium has not been demonstrated in isolated micropuncture studies of the nephron, the whole kidney does demonstrate a maximal capacity to reabsorb magnesium.12,13 When the filtered magnesium load exceeds that amount filtered under normal circumstances (serum ultrafilterable magnesium concentration of ˜1.3 mEq/L), the excess is excreted. Under conditions of magnesium deprivation, however, when the filtered load is below 1.3 mEq/L, magnesium is virtually completely reabsorbed. Despite efficient regulation of magnesium metabolism, no factor or hormone has yet been described that primarily regulates this system.