Magnesium Metabolism and Physiology
The total body stores of magnesium are between 21 and 28 g in the average 70 kg adult. Normal serum magnesium usually has a range of 1.7 to 2.5 mg/dL. Most of the body's magnesium is in the skeletal bone mass, which accounts for more than 50% of the body's stores. The remainder is located in soft tissue, of which only 0.3% is located extracellularly. The common nutritional sources of magnesium are green leafy vegetables, legumes, nuts, and animal protein.[6]
Of the total magnesium consumed, approximately 30% to 50% is absorbed, mainly from the upper small intestine. The level of absorption of magnesium varies, depending on endogenous magnesium status. Magnesium is excreted via the kidneys. When magnesium stores are normal, excretion usually equates with absorption. There is a circadian excretory rhythm, with the maximal excretion occurring at night. Approximately one third of serum magnesium is bound to albumin and therefore is not filterable at the glomerulus. A total of 20% of serum magnesium is filtered by the kidneys, from which 50% to 60% is reabsorbed by the ascending loop of Henle, in contrast to other major electrolytes, which are reabsorbed principally at the proximal loop of Henle.
Extracellular magnesium in serum is 33% protein bound, 12% complexed to anions, and 55% in the free ionized form. At the cellular level, magnesium appears to influence the properties of various cell membranes; this process is thought to occur by means of calcium channels and ion transport mechanisms. Calcium flux is inhibited by magnesium from sarcolemmal membranes, through competition for binding sites on actin and via changes in the adenylate cyclase-cyclic AMP system. The next known physiologic role of magnesium involving cell membranes pertains specifically to its interrelationship with the sodium-potassium-ATPase pump. At the cellular level, magnesium also serves as a cofactor for many intracellular enzymes that generate energy via hydrolysis of ATP. It is also involved in DNA transcription and protein synthesis. Magnesium is responsible for the maintenance of transmembrane gradients of sodium and potassium. Patients with refractory hypokalemia will often not respond to potassium supplementation until magnesium deficiency is corrected.[6,7,8] As a result, magnesium deficiency should be considered whenever severe potassium deficiency is encountered.
From this short review, it is apparent that magnesium plays many roles in energy metabolism: as an enzyme cofactor, in electrolyte balance, and in the maintenance of the properties of various cell membranes. From this background, magnesium deficiency is being considered as an important mediator in various medical conditions.
South Med J. 2001;94(12) © 2001 Lippincott Williams & Wilkins
Cite this: Magnesium: Its Proven and Potential Clinical Significance - Medscape - Dec 01, 2001.
