Coffee, caffeine, and risk of type 2 diabetes - A prospective cohort study in younger and middle-aged US women

OBJECTIVE - High habitual coffee consumption has been associated with a lower risk of type 2 diabetes, but data on lower levels of consumption and on different types of coffee are sparse. RESEARCH DESIGN AND METHODS - This is a prospective cohort study including 88,259 U.S. women of the Nurses' Health Study II aged 26-46 years without history of diabetes at baseline. Consumption of coffee and other caffeine-containing foods and drinks was assessed in 1991, 1995, and 1999. We documented 1,263 incident cases of confirmed type 2 diabetes between 1991 and 2001. RESULTS - After adjustment for potential confounders, the relative risk of type 2 diabetes was 0.87 (95% CI 0.73 - 1.03) for one cup per day, 0.58 (0.49 - 0.68) for two to three cups per day, and 0.53 (0.41 - 0.68) for four or more cups per day compared with nondrinkers (P for trend < 0.0001). Associations were similar for caffeinated (0.87 [0.83 - 0.91] for a one-cup increment per day) and decaffeinated (0.81 [0.73 - 0.90]) coffee and for filtered (0.86 [0.82 - 0.90]) and instant (0.83 [0.74 - 0.93]) coffee. Tea consumption was not substantially associated with risk of type 2 diabetes (0.88 [10.64 - 1.23] for four or more versus no cups per day; P for trend = 0.81). CONCLUSIONS - These results suggest that moderate consumption of both caffeinated and decaffeinated coffee may lower risk of type 2 diabetes in younger and middle-aged women. Coffee constituents other than caffeine may affect the development of type 2 diabetes.