Smoking and the risk of neurodegenerative diseases in a Chinese case–control study

Background: While smoking is inversely associated with Parkinson’s disease (PD) risk, its relationship with amyotrophic lateral sclerosis (ALS) and multiple system atrophy (MSA) remains unclear, particularly in Asian populations. We investigated these associations in a Chinese case–control study. Methods: We recruited newly diagnosed ALS (n=430), MSA (n=271), PD (n=523) cases and hospital-based controls (n=1033) in Sichuan, China. Logistic regression models were used to evaluate associations between smoking and disease risks, adjusting for demographic, lifestyle and occupational factors. Results: Compared with never-smokers, the adjusted ORs and 95% CIs of ALS for current and former smokers were 1.00 (0.61 to 1.65) and 1.79 (1.01 to 3.17), respectively. For MSA, ORs were 1.27 (0.73 to 2.23) for current smokers and 2.54 (1.41 to 4.60) for former smokers. Individuals who quit within 4 years before diagnosis showed the highest risk of ALS (OR=1.93, 95% CI 0.96 to 3.88) and MSA (OR=2.09, 95% CI 1.11 to 3.93). For both ALS and MSA, no consistent trend was found with increasing smoking duration or pack-years. In contrast, ever-smokers had a significantly lower PD risk (OR=0.49, 95% CI 0.33 to 0.71), particularly current smokers (OR=0.30, 95% CI 0.19 to 0.48). Longer smoking duration and higher cumulative smoking were also linked to PD risk with clear negative exposure-response patterns (P _trend=0.039 and 0.029, respectively). Conclusions: Consistent with findings in non-Asian populations, smoking was inversely associated with PD risks in the Chinese population. For ALS and MSA, we found evidence suggestive of positive relationships with cigarette smoking, but no clear exposure-response relationships were observed.