Smoking Cessation and Risk of Incident Dementia in Older Japanese Adults: The Ohsaki Cohort 2006 Study

概要

Background: Increasing evidence has shown that smoking is a risk factor for incident dementia, and that smoking cessation may be associated with a reduction of the dementia risk. However, it remains unknown how long a period of cessation would be necessary for the risk of dementia in ex-smokers to reach the same level as that in never smokers. Thus, the present study aimed to investigate the association of smoking status and years since smoking cessation with the risk of incident dementia among Japanese older adults, using the data from a large-scale cohort study.

Methods: A longitudinal analysis of smoking status and smoking cessation with dementia were conducted using data from a prospective cohort study of 12,489 Japanese individuals living in the Ohsaki City, Japan aged ≥65 years in 2006. Information on smoking status and other lifestyle factors was collected via a questionnaire at baseline. Participants were categorized into never smokers, current smokers, and ex-smokers. Ex-smokers were further divided into ≤2 years, 3 to 5 years, 6 to 10 years, 11 to 15 years, and >15 years of smoking cessation. Data on incident dementia were retrieved from the public Long-term Care Insurance database. Participants were followed up between April 2006 and November 2012. The Cox proportional hazards model was used to estimate the multivariable-adjusted hazard ratios (HRs) and 95% confidence intervals (95% CIs) for incident dementia with never smokers as the reference. Following covariates were adjusted in the models, which were sex, age, education level, obesity, time spent walking, alcohol drinking status, and history of diseases.

Results: During 61,613 person-years of follow-up, 1110 cases (8.9%) of incident dementia were ascertained. Compared with individuals who had never smoked, current smokers showed a higher risk of dementia (HR: 1.46, 95% CI: 1.17, 1.80) whereas the dementia risk did not increase among ex-smokers (HR:1.02, 95%CI: 0.85, 1.24). Among ex-smokers, the risk for those who had stopped smoking for ≤ 2 years was still high (HR 1.39, 95% CI: 0.96, 2.01), however, quitting smoking for 3 years and longer mitigated the increased risk incurred by smoking; the multivariable HRs (95% CIs) were 1.03 (0.70, 1.53) for those who had stopped smoking for 3 to 5 years, 1.04 (0.74, 1.45) for 6 to 10 years, 1.19 (0.84,1.69) for 11 to 15 years, and 0.92 (0.73, 1.15) for >15 years.

Conclusions: The present study confirmed the previous findings that current smokers showed an elevated risk of incident dementia. On the other hand, the present study also suggests that the risk of incident dementia among ex-smokers becomes the same level as that of never smokers if they maintain abstinence from smoking for at least 3 years. Thus, smoking cessation should be encouraged among older adults from the prospective of dementia prevention.