Research appearing in the Archives of Internal Medicine (2008 March 10:168(5):469-65) looked at magnesium levels in 26,000 Finnish men between the ages of 50 and 69 years. Researchers kept track of the magnesium intake of the group for 14 years. The men with the highest magnesium intake had a 15% lower risk of stroke (infarction) than those with the lowest magnesium intake.
It makes sense, because of the connection between magnesium levels and blood pressure. Research appearing in Nutrition, Metabolism and Cardiovascular Diseases (April 7, 2009) looked at the effect magnesium supplementation had on blood pressure. The study was conducted to see if magnesium supplementation helped improve insulin sensitivity and blood pressure in 155 overweight adults. The subjects were randomly divided into two groups and received either 300 milligrams of magnesium per day or a placebo. The magnesium had no effect on insulin insensitivity, but it did have an effect on blood pressure. The subjects with hypertension who received magnesium supplementation had significant reduction in blood pressure at the end of 12 weeks. Systolic (the bigger number) blood pressure was reduced by an average of 5.6 points (mm of mercury) and diastolic (the smaller number) blood pressure was reduced by an average of 2 points. This effect was noticed only in patients with high blood pressure at the start of the study; the subjects with normal blood pressure did not experience a change in blood pressure.