During 866,118 person-years of follow-up, we documented 1,263 cases of type 2 diabetes. Characteristics of the study population according to consumption of caffeinated and decaffeinated coffee and caffeine intake are presented in Table 1. Higher caffeinated coffee consumption, but not decaffeinated coffee consumption, was strongly associated with cigarette smoking and higher alcohol consumption. Both higher caffeinated and higher decaffeinated coffee consumption were associated with older age and lower consumption of sugar-sweetened soft drinks and tea. Women who did not consume caffeinated or decaffeinated coffee tended to have a higher BMI compared with women who did consume either type of coffee. Pearson correlations with caffeine intake were 0.83 for total coffee, 0.94 for caffeinated coffee, −0.05 for decaffeinated coffee, and 0.09 for tea consumption.
Higher coffee consumption was associated with a lower risk of type 2 diabetes (Table 2). Adjustment for potential confounders weakened this association, mainly due to adjustment for BMI and alcohol consumption. After multivariate adjustment, the relative risk (RR) 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 2–3 cups per day, and 0.53 (0.41–0.68) for four or more cups per day. Additional adjustment for magnesium, high- and low-fat dairy consumption, tea consumption, or sucrose intake; adjustment for BMI as a continuous variable; use of baseline coffee consumption instead of cumulative updated coffee consumption; use of baseline coffee consumption with exclusion of the first 4 years of follow-up; and exclusion of women who developed gestational diabetes during follow-up did not substantially change the association between coffee consumption and risk of type 2 diabetes (RR for four or more cups per day versus no cups per day ranged from 0.51 to 0.60; all P values