Abstract
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.