A study found a causally protective effect of tobacco smoking on the risk of Parkinson disease (PD), yet the mechanism behind this association was not made clear. Findings from this study were reported in Neurology.
The study was an analysis of smoking habits and risks of death associated with PD in 30,000 male doctors who participated in the British Doctors cohort study in 1951. Only participants from this study who were resurveyed for 5 subsequent decades were included. During 743,920 person-years, a total of 25,379 deaths were reported. Approximately 1.1% (n=283) of these deaths were considered attributable to PD. The researchers calculated the relative risks of PD associated with smoking habits, including smoking status, amount smoked, and years since smoking cessation, at baseline and resurveys.
From 1951 to 1998, the prevalence of current smoking declined from 67% to 8%, respectively. In a crude analysis, the rates of PD-associated death over follow-up were lower in the participants who reported current smoking at baseline vs the never smokers (30 vs 46/100,000 persons-years, respectively), with an unadjusted relative risk (RR) reduction of 34.7%. The mean age at death was 82 years in the doctors who died from PD over a mean of 42 years.
In the analysis adjusted for age at risk, the doctors who reported current smoking at baseline had a 30% lower risk of PD (RR, 0.71; 95% CI, 0.60–0.84). Current smokers at resurvey had a 40% lower risk of PD compared with never smokers (RR, 0.60; 95% CI, 0.46–0.77). There was an inverse dose-response association between PD and the amount of tobacco smoked per day at baseline (P =.0006) and resurvey (P =.002). Doctors who reported smoking cessation ≥10 years ago and 0 to 9 years ago had a 14% lower PD risk (RR, 0.86; 95% CI, 0.70–1.06) and 29% lower PD risk (RR, 0.71; CI, 0.54–0.93), respectively, compared with never smokers.
Limitations of this study were the inclusion of a small number of PD cases that were restricted to male British doctors only. Additionally, the researchers didn’t examine the mechanisms underlying the inverse association between tobacco smoking and PD risk, but they suggested it may partly be due to nicotine’s ability to stimulate dopamine release in the brain.
In spite of these findings, the researchers emphasized that “the adverse effects of smoking on risks of vascular and respiratory diseases, neoplasms, and other noncommunicable diseases for which tobacco is the chief risk factor greatly exceed any protective effect of current tobacco smoking on the risk of PD.”
Mappin-Kasirer B, Pan H, Lewington S, et al. Tobacco smoking and the risk of Parkinson disease: A 65-year follow-up of 30,000 male British doctors. Neurology. 2020;94(20):e2132‐e2138