Dietary magnesium intake in relation to plasma insulin levels and risk of type 2 diabetes in women.

OBJECTIVE Higher intake of magnesium appears to improve glucose and insulin homeostasis; however, there are sparse prospective data on the association between magnesium intake and incidence of type 2 diabetes. RESEARCH DESIGN AND METHODS In the Women's Health Study, a cohort of 39,345 U.S. women aged >/=45 years with no previous history of cardiovascular disease, cancer, or type 2 diabetes completed validated semiquantitative food frequency questionnaires in 1993 and were followed for an average of 6 years. We used Cox proportional hazard models to estimate multivariate relative risks (RRs) of type 2 diabetes across quintiles of magnesium intake compared with the lowest quintile. In a sample of 349 apparently healthy women from this study, we measured plasma fasting insulin levels to examine their relation to magnesium intake. RESULTS During 222,523 person-years of follow-up, we documented 918 confirmed incident cases of type 2 diabetes. There was a significant inverse association between magnesium intake and risk of type 2 diabetes, independent of age and BMI (P = 0.007 for trend). After further adjustment for physical activity, alcohol intake, smoking, family history of diabetes, and total calorie intake, the multivariate-adjusted RRs of diabetes from the lowest to highest quintiles of magnesium intake were attenuated at 1.0, 1.06, 0.81, 0.86, and 0.89 (P = 0.05 for trend). Among women with BMI >/=25 kg/m2, the inverse trend was significant; multivariate-adjusted RRs were 1.0, 0.96, 0.76, 0.84, and 0.78 (P = 0.02 for trend). Multivariate-adjusted geometric mean insulin levels for overweight women in the lowest quartile of magnesium intake was 53.5 compared with 41.5 pmol/l among those at the highest quartile (P = 0.03 for trend). CONCLUSIONS These findings support a protective role of higher intake of magnesium in reducing the risk of developing type 2 diabetes, especially in overweight women.