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PLoS One. 2015 Mar 12;10(3):e0118333. doi: 10.1371/journal.pone.0118333. eCollection 2015.

Smoking is associated with an increased risk of dementia: a meta-analysis of prospective cohort studies with investigation of potential effect modifiers.

Author information

1
The Second College of Clinical Medicine, Chongqing Medical University, Chongqing, China.
2
School of Public Health and Management, Chongqing Medical University, Chongqing, China.
3
Division of Pharmacy Practice and Administrative Sciences, College of Pharmacy, University of Cincinnati, Cincinnati, OH, 45221, United States of America.

Abstract

BACKGROUND:

Previous studies showed inconsistent results on the association of smoking with all-cause dementia and vascular dementia (VaD), and are limited by inclusion of a small number of studies and unexplained heterogeneity. Our review aimed to assess the risk of all-cause dementia, Alzheimer's disease (AD) and VaD associated with smoking, and to identify potential effect modifiers.

METHODS AND FINDINGS:

The PubMed, Embase, Cochrane Library and Psychinfo databases were searched to identify studies that provided risk estimates on smoking and incidence of dementia. A random-effects model was used to yield pooled results. Thirty-seven studies were included. Compared with never smokers, current smokers showed an increased risk of all-cause dementia (risk ratio (RR) 1.30, 95% confidence interval (CI) 1.18-1.45), AD (RR 1.40, 95% CI 1.13-1.73) and VaD (RR 1.38, 95% CI 1.15-1.66). For all-cause dementia, the risk increased by 34% for every 20 cigarettes per day (RR 1.34, 95% CI 1.25-1.43). Former smokers did not show an increased risk of all-cause dementia (RR 1.01, 95% CI 0.96-1.06), AD (RR 1.04, 95% CI 0.96-1.13) and VaD (RR 0.97, 95% CI 0.83-1.13). Subgroup analyses indicated that (1) the significantly increased risk of AD from current smoking was seen only in apolipoprotein E ε4 noncarriers; (2) 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 (3) sex, race, study location and diagnostic criteria difference in risk of dementia was not found.

CONCLUSIONS:

Smokers show an increased risk of dementia, and smoking cessation decreases the risk to that of never smokers. The increased risk of AD from smoking is more pronounced in apolipoprotein E ε4 noncarriers. Survival bias and competing risk reduce the risk of dementia from smoking at extreme age.

PMID:
25763939
PMCID:
PMC4357455
DOI:
10.1371/journal.pone.0118333
[Indexed for MEDLINE]
Free PMC Article

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