Methods for blood processing in the ARIC Study have been described (10). Participants were asked to fast for 12 hours before their morning clinic appointments. The measurement of serum magnesium was based on the procedure of Gindler and Heth and used the metallochromic dye, calmagite (1-(1-hydroxy-4-methyl-2-phenylazo)-2-napthol-4-sulfonic acid). The laboratory coefficient of variation, based on blinded split samples sent 1 week apart to the laboratory, was 3% (1), and repeated testing of 40 individuals over several weeks yielded a reliability coefficient of 0.69 (11). Plasma fibrinogen and the von Willebrand factor antigen were measured by the thrombin time titration method and enzyme-linked immunosorbent assay, respectively. Serum glucose was measured by a hexokinase/glucose-6-phosphate dehydrogenase method. Body mass index was calculated as weight (kg)/height (m)². We defined prevalent coronary heart disease and stroke at baseline, for exclusion, as a self-reported history of a physician-diagnosed heart attack, prior myocardial infarction by electrocardiogram, prior cardiovascular surgery, prior coronary angioplasty, or prior stroke or transient ischemic attack identified by a standardized interview (12).