The effect of magnesium supplementation in increasing doses on the control of type 2 diabetes

Hypomagnesemia occurs in 25-38% of patients with type 2 diabetes. Several studies have suggested an association between magnesium (Mg) depletion and insulin resistance and/or reduction of insulin secretion in these cases. Our purpose was to evaluate if Mg supplementation (as magnesium oxide [MgO]) would improve metabolic control in patients with type 2 diabetes.

We studied 128 patients with type 2 diabetes (32 men, 96 women, aged 30-69 years), treated by diet or diet plus oral antidiabetic drugs, in the Bahia Federal University Hospital, Brazil. Patients at risk for hypomagnesemia or with reduced renal function were excluded. This study was a clinical randomized double-blind placebo-controlled trial. Patients received either placebo, 20.7 mmol MgO, or 41.4 mmol MgO daily (elementary Mg) for 30 days. Mg concentrations were measured in plasma, in mononuclear cells, and in 24-h urine samples. Fasting blood glucose, HbA1, and fructosamine were used as parameters of metabolic control.

Of the patients, 47.7% had low plasma Mg, and 31.1% had low intramononuclear Mg levels. Intracellular Mg in patients with diabetes was significantly lower than in the normal population (62 blood donors; 1.4 +/- 0.6 vs. 1.7 +/- 0.6 micrograms/mg of total proteins). No correlation was found between plasma and intracellular Mg concentrations (r = -0.179; P = 0.15) or between Mg concentrations and glycemic control (r = -0.165; P = 0.12). Intracellular Mg levels were lower in patients with peripheral neuropathy than in those without (1.2 +/- 0.5 vs. 1.5 +/- 0.6 micrograms/mg). Similar findings were observed in patients with coronary disease (1.0 +/- 0.5 vs. 1.5 +/- 0.6 micrograms/mg). In the placebo and in the 20.7 mmol Mg groups, neither a change in plasma and intracellular levels nor an improvement in glycemic control were observed. Replacement with 41.4 mmol Mg tended to increase plasma, cellular, and urine Mg and caused a significant fall (4.1 +/- 0.8 to 3.8 +/- 0.7 mmol/l) in fructosamine (normal, 1.87-2.87 mmol/l).

Mg depletion is common in poorly controlled patients with type 2 diabetes, especially in those with neuropathy or coronary disease. More prolonged use of Mg in doses that are higher than usual is needed to establish its routine or selective administration in patients with type 2 diabetes to improve control or prevent chronic complications.