Magnesium — The forgotten electrolyte

Abstract

Magnes ium is fourth most plentiful cation in h u m a n body and second most abundan t in t racel lu lar element. The adult h u m a n body contains 21 to 28 gm or about 2,000 mEq of magnesium. Approximately ha l f total body magnes ium is contained in bone with rest equally divided between muscular and nonmuscula r soft tissue. 1 The average daily intake of magnes ium of an adult on a normal diet is 25 mEq, much of which is contained in chlorophyll portion of green vegetables. 2 Magnesium serves as an activator in an array of enzyme systems that are vital to in t race l lu lar metabolism. Most prominent among these are those that hydrolyze and t r a n s f e r phospha te groups t h a t f igure in syn t he s i s of adenosine triphospate (ATP). ATP is necessary for glucose utilization, synthesis of fat, protein, nucleic acids and co-enzymes, as well as for muscle contraction and main tenance of sodium-potassium pump. Interference with magnes ium metabolism can affect all of these functions. 2 Despite ubiqui tousness of magnesium, deficiency states have not been fully recognized and described unti l recently, 3-7 largely because of inabil i ty to measure ion accurately. As a result, evaluat ion of pat ient with abnormal fluid and electrolyte losses, of ei ther na tu ra l or iatrogenic cause, has often excluded magnes ium, hence the forgotten electrolyte. The advent of atomic absorption spectrometry and other new techniques has permit ted rapid and accurate me a su r e me n t of magnes ium. 1 Normal serum magnes ium levels range from 1.6 to 2.1 mEq/liter, about one third of which is protein bound with other two thirds present as free cat ion2 He~ce, p~ote in avai labi l i ty influences serum levels. 2 The kidney is principal organ in homeostasis of magnes ium ion. The diffusible fraction of blood magnes ium is filtered at glomerulus. Approximately 75% of total serum concentrat ion is filtered daily, greater than 90% is reabsorbed. 2 Thus, only 3% to 5% of filtered Mg: + + is excreted in urine. In presence of deficiency, there is maximal tubu la r reabsorption of this ion. However, there remains an obligatory magnes ium loss even in presence of severe hypomagnesemia, s If normal renal function is present, about 50% of adminis tered magnes ium will be lost even when marked deficiency of ion exists, s The level of serum magnes ium does not always accurately reflect total body