In a meta-analysis 1 of 19 prospective studies with at least 12 months of follow-up the association of smoking with dementia and cognitive decline was assessed. Studies included a total of 26,374 participants followed for dementia for 2-30 years and 17,023 participants followed up for 2-7 years to assess cognitive decline. Mean study age was 74 years. Current smokers at baseline, relative to never smokers, had risks of 1.79 (95% confidence interval (CI): 1.43 to 2.23) for incident Alzheimer's disease, 1.78 (95% CI: 1.28, 2.47) for incident vascular dementia, and 1.27 (95% CI: 1.02, 1.60) for any dementia. Compared with those who never smoked, current smokers at baseline also showed greater yearly declines in Mini-Mental State Examination scores over the follow-up period (effect size (beta) = -0.13, 95% CI: -0.18, -0.08). Compared with former smokers, current smokers at baseline showed an increased risk of Alzheimer's disease (relative risk = 1.70, 95% CI: 1.25, 2.31) and an increased decline in cognitive abilities (effect size (beta) = -0.07, 95% CI: -0.11, -0.03), but the groups were not different regarding risk of vascular dementia or any dementia.
Another meta-analysis 3 included 37 prospective cohort studies. Compared with never smokers, current smokers showed an increased risk of all-cause dementia (RR 1.30, 95% CI 1.18 to 1.45), Alzheimer's disease (AD) (RR 1.40, 95% CI 1.13 to 1.73) and vascular dementia (VaD) (RR 1.38, 95% CI 1.15 to 1.66). For all-cause dementia, the risk increased by 34% for every 20 cigarettes per day (RR 1.34, 95% CI 1.25 to 1.43). Former smokers did not show an increased risk of all-cause dementia (RR 1.01, 95% CI 0.96 to 1.06), AD (RR 1.04, 95% CI 0.96 to 1.13) and VaD (RR 0.97, 95% CI 0.83 to 1.13). Subgroup analyses indicated that the significantly increased risk of AD from current smoking was seen only in apolipoprotein E ε4 noncarriers; current smokers aged 65 to 75 years at baseline showed increased risk of all-cause dementia and AD compared to those aged over 75 or under 65 years; and sex, race, study location and diagnostic criteria difference in risk of dementia was not found.
A longitudinal analysis 2 of smoking status and smoking cessation with dementia in prospective cohort study included 12 489 Japanese individuals aged ≥ 65 years who were followed up for 6 years. Compared with individuals who had never smoked, current smokers showed a higher risk of dementia (HR 1.46, 95% CI 1.17 to 1.80). Among ex-smokers, the risk for those who had stopped smoking for ≤ 2 years was still high, however, quitting smoking for 3 years or longer mitigated the increased risk incurred by smokers; the multivariable HRs (95% CIs) were 1.03 (0.70 to 1.53) for those who had stopped smoking for 3-5 years and 0.92 (0.73 to 1.15) for > 15 years.
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