NORMAL MAGNESIUM BALANCE
Magnesium intake averages 20–30 mEq/d (240– 370 mg/d) in adults. Of that amount, only 30–40% is absorbed, mainly in the distal small bowel. Renal excretion is the primary route for elimination, averaging 6–12 mEq/d. Magnesium reabsorption by the kidneys is very efficient. Twenty-five per-cent of filtered magnesium is reabsorbed in the proximal tubule, whereas 50–60% is reabsorbed in the thick ascending limb of the loop of Henle. Fac-tors known to increase magnesium reabsorption in the kidneys include hypomagnesemia, PTH, hypo-calcemia, ECF depletion, and metabolic alkalosis. Factors known to increase renal excretion include hypermagnesemia, acute volume expansion, hyper-aldosteronism, hypercalcemia, ketoacidosis, diuret-ics, phosphate depletion, and alcohol ingestion.
Plasma [Mg2+] is closely regulated between 1.7 and 2.1 mEq/L (0.7–1 mmol/L or 1.7–2.4 mg/dL) through interaction of the gastrointestinal tract (absorption), bone (storage), and the kidneys (excre-tion). Approximately 50–60% of plasma magnesium is unbound and diffusible.
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