Magnesium salts

Uses

Magnesium salts are used in the following ways:

• as antacids (magnesium trisilicate, magnesium hydroxide) usually in combination with an aluminium salt; the combination of magnesium hydroxide and aluminium hydroxide is called co-magaldrox;

• as laxatives (magnesium oxide);

• in the treatment of eclampsia (magnesium sulfate);

• to treat cardiac dysrhythmias (magnesium sulfate);

• in urological irrigation solutions, used for dissolution of urinary calculi (magnesium citrate).

Magnesium deficiency

Several epidemiological studies have shown an association between calcium and magnesium and coronary heart disease mortality and morbidity. Western diets often have a shortage of magnesium, and daily intake of magnesium does not reach the current recommended daily allowance in many subjects. Therefore, magnesium deficiency is common in industrialized countries. The recommended dietary allowance of magnesium is 420 mg/day for men and 320 mg/day for women.

Residents in soft-water areas have lower concentrations of magnesium in heart muscle and coronary arteries than do residents in hard-water areas [ ]. This suggests that the content of magnesium in the diet is inadequately low. The findings of a positive correlation between cardiovascular mortality and the estimates of the calcium:magnesium ratio in the diet of various countries thus suggest that a high calcium:magnesium ratio in the diet may be harmful.

The relation between the risk of acute myocardial infarction and the content of calcium, magnesium, and chromium in local groundwater in Finnish rural areas has been examined [ ]. Data on 14 495 men aged 35–74 years with their first acute myocardial infarction in the years 1983, 1988, or 1993 were pooled. Geochemical data consisted of 4300 measurements of each element in the local groundwater. The median concentrations of calcium, magnesium, and chromium in well water were 12 mg/l, 2.6 mg/l, and 0.27 μg/l respectively; the calcium:magnesium ratio was 5.39. Each 1 mg/l increment in magnesium concentration reduced the risk of acute myocardial infarction by 4.9%, whereas a one unit increment in the calcium:magnesium ratio increased the risk by 3.1%. Calcium and chromium did not have any statistically significant effect on the incidence and spatial variation of acute myocardial infarction. This result is compatible with previously reported assumptions about a protective role of hard water against cardiovascular disease [ ]. A positive correlation between cardiovascular disease and the estimated calcium:magnesium ratio in the diet has also been suggested in several countries [ ].

The underlying mechanisms explaining the effect of calcium and magnesium on myosin ATPase are different. Magnesium is essential to maintain the enzymatic activity of myosin in cardiac muscle contraction. Calcium has a role conducting signals and regulating functions. Magnesium deficiency may reduce ATPase activity, leading to increases in intracellular calcium and vasoconstriction. Magnesium is closely involved in maintaining cellular ionic balance through its association with calcium, sodium, and potassium, and may influence the binding of other cations, such as calcium, that may have antagonistic or synergistic effects, depending on their concentrations. Finally, magnesium deficiency can encourage atherosclerosis and platelet aggregation.

General adverse effects and adverse reactions

Acute toxicity can arise from overdosage or intestinal or renal disease. The main symptoms of magnesium toxicity are neuromuscular (paralysis) and cardiac (electrocardiographic changes, hypotension, bradycardia, heart block). In more severe cases there will be nervous system or respiratory depression, hypomotility of the bowel, muscle paralysis, and hyporeflexia.

Magnesium sulfate in bolus doses of 2–4 g over up to 5 minutes causes feelings of warmth, flushing, and sometimes sweating [ ]. Transient nausea has also been reported [< Wesley 1129 >,<Gurfinkel 35 >].

Systematic reviews

In a meta-analysis of the use of intravenous magnesium sulfate to treat atrial fibrillation in 515 patients in 10 studies, magnesium caused transient minor symptoms (flushing, tingling, and dizziness) in 17% [ ], Although intravenous magnesium was less effective than either calcium channel blockers or amiodarone (21% versus 59%; OR = 0.19, 95% CI = 0.09, 0.44) it was also less likely to cause significant bradycardia or atrioventricular block (0% versus 9.2%; OR = 0.13, 95% CI = 0.02, 0.76); there was significant hypotension in only six patients.