Smoking Is Associated with an Increased Risk of Dementia: A Meta-Analysis of Prospective Cohort Studies with Investigation of Potential Effect Modifiers

Abstract

BackgroundPrevious 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 FindingsThe 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.ConclusionsSmokers 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.

Clinical and Environmental Correlates of Serum BDNF: A Descriptive Study with Plausible Implications for AD Research

“…BDNF was also higher in smokers compared to non-smokers in other studies [43, 46], however unlike in the current study, the association did not always remain significant after adjustment for potential confounders [43]. Although for many years the relationship between smoking and dementia risk has been inconclusive, recent evidence suggests that smoking confers an increased risk [49–51]. An important candidate mechanism promoting AD pathophysiology and increased risk for AD in smokers is smoking-related cerebral oxidative stress [52].…”

Social class, dementia and the fourth age

“…Evidence for a link between smoking and risk of dementia in later life is strong (Zhong et al . ) and there may be cohort effects that are related to variations in declining smoking patterns by social class. One pathway focuses on higher levels of IQ in childhood, providing access to higher levels of social, cultural and economic capital in adulthood that protect against illnesses, including dementia.…”

“…Links have been drawn between low SES and harmful lifestyles across the life course with stress and a range of chronic health problems in later life that are also related to dementia (McEwen and Gianaros 2010). Evidence for a link between smoking and risk of dementia in later life is strong (Zhong et al 2015) and there may be cohort effects that are related to variations in declining smoking patterns by social class. One pathway focuses on higher levels of IQ in childhood, providing access to higher levels of social, cultural and economic capital in adulthood that protect against illnesses, including dementia.…”

Summary of the evidence on modifiable risk factors for cognitive decline and dementia: A population‐based perspective

“…According to several systematic reviews and metaanalyses, prospective and longitudinal studies have found strong evidence that current smoking increases the risk of cognitive decline and possibly dementia [7,26,35,[59][60][61][62][63][64]. Quitting smoking may reduce the associated risk to levels comparable to those who have not smoked [7,[63][64][65].…”

“…Quitting smoking may reduce the associated risk to levels comparable to those who have not smoked [7,[63][64][65]. One study of a large multi-ethnic cohort found heavy smoking in middle-age as much as doubled the risk of later-life dementia [66].…”