Dietary Calcium and Magnesium, Major Food Sources, and Risk of Type 2 Diabetes in U.S. Black Women

OBJECTIVE—Inverse associations between magnesium and calcium intakes and risk of type 2 diabetes have been reported for studies in predominantly white populations. We examined magnesium, calcium, and major food sources in relation to type 2 diabetes in African-American women. RESEARCH DESIGN AND METHODS—This is a prospective cohort study including 41,186 participants of the Black Women’s Health Study without a history of diabetes who completed validated food frequency questionnaires at baseline. During 8 years of follow-up (1995–2003), we documented 1,964 newly diagnosed cases of type 2 diabetes. RESULTS—The multivariate-adjusted hazard ratio of type 2 diabetes for the highest compared with the lowest quintile of intake was 0.69 (95% CI 0.59–0.81; P trend <0.0001) for dietary magnesium and 0.86 (0.74–1.00; P trend = 0.01) for dietary calcium. After mutual adjustment, the association for calcium disappeared (hazard ratio 1.04 [95% CI 0.88–1.24]; P trend = 0.88), whereas the association for magnesium remained. Daily consumption of low-fat dairy (0.87 [0.76–1.00]; P trend = 0.04) and whole grains (0.69 [0.60–0.79]; P trend <0.0001) were associated with a lower risk of type 2 diabetes compared with a consumption less than once a week. After mutual adjustment, the hazard ratio was 0.81 (0.68–0.97; P trend = 0.02) for magnesium and 0.73 (0.63–0.85; P trend <0.0001) for whole grains. CONCLUSIONS—These findings indicate that a diet high in magnesium-rich foods, particularly whole grains, is associated with a substantially lower risk of type 2 diabetes in U.S. black women.