Coffee consumption and risk of type 2 diabetes - A systematic review

Context Emerging epidemiological evidence suggests that higher coffee consumption may reduce the risk of type 2 diabetes. Objective To examine the association between habitual coffee consumption and risk of type 2 diabetes and related outcomes. Data Sources and Study Selection We searched MEDLINE through January 2005 and examined the reference lists of the retrieved articles. Because this review focuses on studies of habitual coffee consumption and risk of type 2 diabetes, we excluded studies of type 1 diabetes, animal studies, and studies of short-term exposure to coffee or caffeine, leaving 15 epidemiological studies (cohort or cross-sectional). Data Extraction Information on study design, participant characteristics, measurement of coffee consumption and outcomes, adjustment for potential confounders, and estimates of associations was abstracted independently by 2 investigators. Data Synthesis We identified 9 cohort studies of coffee consumption and risk of type 2 diabetes, including 193 473 participants and 8394 incident cases of type 2 diabetes, and calculated summary relative risks (RRs) using a random-effects model. The RR of type 2 diabetes was 0.65 (95% confidence interval [CI], 0. 54-0.78) for the highest (>= 6 or >= 7 cups per day) and 0.72 (95% Cl, 0.62-0.83) for the second highest (4-6 cups per day) category of coffee-consumption compared with the lowest consumption category (0 or <= 2 cups per day). These associations did not differ substantially by sex, obesity, or region (United States and Europe). In the cross-sectional studies conducted in northern Europe, southern Europe, and Japan, higher coffee consumption was consistently associated with a lower prevalence of newly detected hyperglycemia, particularly postprandial hyperglycemia. Conclusions This systematic review supports the hypothesis that habitual coffee consumption is associated with a substantially lower risk of type 2 diabetes. Longer-term intervention studies of coffee consumption and glucose metabolism are warranted to examine the mechanisms underlying the relationship between coffee consumption and type 2 diabetes.