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Overview of Disorders of Magnesium Concentration

by James L. Lewis, III, MD

Mg is the 4th most plentiful cation in the body. A 70-kg adult has about 2000 mEq of Mg. About 50% is sequestered in bone and is not readily exchangeable with Mg in other compartments. The ECF contains only about 1% of total body Mg. The remainder resides in the intracellular compartment. Normal serum Mg concentration ranges from 1.4 to 2.1 mEq/L (0.70 to 1.05 mmol/L).

The maintenance of serum Mg concentration is largely a function of dietary intake and effective renal and intestinal conservation. Within 7 days of initiation of a Mg-deficient diet, renal and stool Mg excretion each fall to about 1 mEq/day (0.5 mmol/day).

About 70% of serum Mg is ultrafiltered (filtered through minute pores) by the kidney; the remainder is bound to protein. Protein binding of Mg is pH dependent. Serum Mg concentration is not closely related to either total body Mg or intracellular Mg content. However, severe serum hypomagnesemia may reflect diminished total body Mg.

Many enzymes are activated by or are dependent on Mg. Mg is required by all enzymatic processes involving ATP and by many of the enzymes involved in nucleic acid metabolism. Mg is required for thiamine pyrophosphate cofactor activity and appears to stabilize the structure of macromolecules such as DNA and RNA. Mg is also related to Ca and K metabolism in an intimate but poorly understood way.

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