I can't believe I didn't see this when it was first published... this study supports a (potentially causal) association between smoking and PD. It suggests again that smoking is protective and that nicotine itself may be the protective factor. Given that it is an observational study, it may well be affected by reverse causation, but it ties in nicely with what has been observed for cigarette smoking and supports the fact that there are clinical trials taking place using nicotine replacement therapy... this is a question that needs a definitive answer!
Int J Epidemiol. 2016 Dec 10. pii: dyw294. [Epub ahead of print]
Yang F, Pedersen NL, Ye W, Liu Z, Norberg M, Forsgren L, Trolle Lagerros Y, Bellocco R, Alfredsson L, Knutsson A, Jansson JH, Wennberg P, Galanti MR, Lager AC, Araghi M, Lundberg M, Magnusson C, Wirdefeldt K.
Cigarette smoking is associated with a lower risk of Parkinson's disease. It is unclear what constituent of tobacco smoke may lower the risk. Use of Swedish moist smokeless tobacco (snus) can serve as a model to disentangle what constituent of tobacco smoke may lower the risk. The aim of this study was to determine whether snus use was associated with a lower risk of Parkinson's disease.
Individual participant data were collected from seven prospective cohort studies, including 348 601 men. We used survival analysis with multivariable Cox regression to estimate study-specific relative risk of Parkinson's disease due to snus use, and random-effects models to pool estimates in a meta-analysis. The primary analyses were restricted to never-smokers to eliminate the potential confounding effect of tobacco smoking.
During a mean follow-up time of 16.1 years, 1199 incident Parkinson's disease cases were identified. Among men who never smoked, ever-snus users had about 60% lower Parkinson's disease risk compared with never-snus users [pooled hazard ratio (HR) 0.41, 95% confidence interval (CI) 0.28-0.61]. The inverse association between snus use and Parkinson's disease risk was more pronounced in current (pooled HR 0.38, 95% CI 0.23-0.63), moderate-heavy amount (pooled HR 0.41, 95% CI 0.19-0.90) and long-term snus users (pooled HR 0.44, 95% CI 0.24-0.83).
Non-smoking men who used snus had a substantially lower risk of Parkinson's disease. Results also indicated an inverse dose-response relationship between snus use and Parkinson's disease risk. Our findings suggest that nicotine or other components of tobacco leaves may influence the development of Parkinson's disease.