In the search for answers that contribute to the metabolic control of patients with diabetes and the primary prevention of the disease, we performed a review of the evidence from cohort studies on the relationship between serum and/or magnesium intake with the risk of developing type 2 diabetes as well as of clinical trials on the efficacy of oral magnesium salts on reducing glycemia.An electronic search using the databases MEDLINE, EMBASE, and Cochrane Controlled Trials Register, updated to September 30, 2013, was performed.A total of seven cohort studies (24,388 persons/year) show unequivocally that magnesium intake is associated with decreased risk of developing type 2 diabetes; two studies (13,076 persons/year) indicate that low magnesium intake is not associated with the risk of diabetes; one study (8,735 persons/year) shows that hypomagnesemia is associated with the development of impaired glucose metabolism. A total of 11 randomized controlled trials were identified; five show the effectiveness of oral magnesium salts in reducing glycemia in high-risk subjects and six studies carried out in patients with type 2 diabetes show inconsistent results.Magnesium intake in the customary diet of subjects of the general population and the high-risk groups and/or oral magnesium supplementation is recommended for the prevention of diabetes. The efficacy of oral magnesium supplementation in the reduction of glucose levels in type 2 diabetic patients is inconsistent.