Magnesium is the 4th most plentiful cation in the body. A 70-kg adult has about 2000 mEq (1000 mmol) of magnesium. About 50% is sequestered in bone and is not readily exchangeable with magnesium in other compartments. The ECF contains only about 1% of total body magnesium. The remainder resides in the intracellular compartment. Normal serum magnesium concentration ranges from 1.8 to 2.6 mg/dL (0.74 to 1.07 mmol/L).
The maintenance of serum magnesium concentration is largely a function of dietary intake and effective renal and intestinal conservation. Within 7 days of initiation of a magnesium-deficient diet, renal and stool magnesium excretion each fall to about 12.5 mg/day (0.5 mmol/day).
About 70% of serum magnesium is filtered by the kidneys; the remainder is bound to protein. Protein binding of magnesium is pH dependent.
Serum magnesium concentration is not closely related to either total body magnesium or intracellular magnesium content. However, severe serum hypomagnesemia may reflect diminished total body magnesium. Hypermagnesemia is most often caused by renal failure.
Many enzymes are activated by or are dependent on magnesium. Magnesium is required by all enzymatic processes involving ATP ( adenosine triphosphate) and by many of the enzymes involved in nucleic acid metabolism. Magnesium is required for thiamine pyrophosphate cofactor activity and appears to stabilize the structure of macromolecules such as DNA and RNA.
Magnesium is also related to calcium and potassium metabolism in an intimate but poorly understood way.