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% CI, 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
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.