Coffee consumption and health: umbrella review of meta-analyses of multiple health outcomes

For the meta-analysis of controlled clinical trials with binary outcome a test statistic for testing an overall treatment effect is proposed, which is based on a refined estimator for the variance of the treatment effect estimator usually used in the random-effects model of meta-analysis. In simulation studies it is shown that the proposed test keeps the prescribed significance level much better than the commonly used tests in the fixed-effects and random-effects model, respectively. Moreover, when using the test it is not necessary to choose between fixed effects and random effects approaches in advance. The proposed method applies in the same way to the analysis of a controlled multi-centre study with binary outcome, including a possible interaction between drugs and centres. Copyright 2001 John Wiley & Sons, Ltd.

We conducted a systematic review to summarize the epidemiological evidence on the association between cigarette smoking, coffee drinking, and the risk of Parkinson's disease. Case-control and cohort studies that reported the relative risk of physician-confirmed Parkinson's disease by cigarette smoking or coffee drinking status were included. Study-specific log relative risks were weighted by the inverse of their variances to obtain a pooled relative risk and its 95% confidence interval (CI). Results for smoking were based on 44 case-control and 4 cohort studies, and for coffee 8 case-control and 5 cohort studies. Compared with never smokers, the relative risk of Parkinson's disease was 0.59 (95% CI, 0.54-0.63) for ever smokers, 0.80 (95% CI, 0.69-0.93) for past smokers, and 0.39 (95% CI, 0.32-0.47) for current smokers. The relative risk per 10 additional pack-years was 0.84 (95% CI, 0.81-0.88) in case-control studies and 0.78 (95% CI, 0.73-0.84) in cohort studies. Compared with non-coffee drinkers, relative risk of Parkinson's disease was 0.69 (95% CI, 0.59-0.80) for coffee drinkers. The relative risk per three additional cups of coffee per day was 0.75 (95% CI, 0.64-0.86) in case-control studies and 0.68 (95% CI, 0.46-1.00) in cohort studies. This meta-analysis shows that there is strong epidemiological evidence that smokers and coffee drinkers have a lower risk of Parkinson's disease. Further research is required on the biological mechanisms underlying this potentially protective effect.

OBJECTIVE Previous meta-analyses identified an inverse association of coffee consumption with the risk of type 2 diabetes. However, an updated meta-analysis is needed because new studies comparing the trends of association for caffeinated and decaffeinated coffee have since been published. RESEARCH DESIGN AND METHODS PubMed and Embase were searched for cohort or nested case-control studies that assessed the relationship of coffee consumption and risk of type 2 diabetes from 1966 to February 2013. A restricted cubic spline random-effects model was used. RESULTS Twenty-eight prospective studies were included in the analysis, with 1,109,272 study participants and 45,335 cases of type 2 diabetes. The follow-up duration ranged from 10 months to 20 years. Compared with no or rare coffee consumption, the relative risk (RR; 95% CI) for diabetes was 0.92 (0.90–0.94), 0.85 (0.82–0.88), 0.79 (0.75–0.83), 0.75 (0.71–0.80), 0.71 (0.65–0.76), and 0.67 (0.61–0.74) for 1–6 cups/day, respectively. The RR of diabetes for a 1 cup/day increase was 0.91 (0.89–0.94) for caffeinated coffee consumption and 0.94 (0.91–0.98) for decaffeinated coffee consumption (P for difference = 0.17). CONCLUSIONS Coffee consumption was inversely associated with the risk of type 2 diabetes in a dose-response manner. Both caffeinated and decaffeinated coffee was associated with reduced diabetes risk.