Alcohol consumption, smoking, and risk of dementia in community-dwelling Japanese people aged 40–74 years: The Murakami cohort study

•Light to moderate alcohol consumption is associated with a reduced risk of dementia.•Risk of dementia increases when those with poor general health are excluded.•Smoking is dose-dependently associated with risk of dementia.•The association between smoking and risk of dementia is stronger in middle-aged people.•There is an interaction between smoking and heavy drinking in their associations with dementia risk. Alcohol drinking and tobacco smoking have impacts on lifestyle-related diseases, but their association with dementia remains a debated topic. This study aimed to examine longitudinal associations between alcohol consumption, smoking, and dementia risk in middle-aged and older Japanese people. This study used a cohort design with an 8-year follow-up. Participants were community-dwelling Japanese people (N = 13,802) aged 40–74 years. The baseline survey, including a self-administered questionnaire, was conducted in 2011–2013. Predictors were alcohol consumption and tobacco smoking. The outcome was incident dementia obtained from a long-term care insurance database. Covariates were demographics, lifestyle factors, body mass index, general health status, and history of stroke, diabetes, and depression. Participant mean age was 59.0 years. The 1–149, 150–299, and 300–449 g ethanol/week groups had significantly lower adjusted hazard ratios (HRs) (0.62, 0.59, and 0.47, respectively) compared with the reference group, with no significant linear association. HRs increased toward 1 when past-drinkers and those with poor health status and a disease history were excluded (0.80, 0.66, and 0.82, respectively). Higher smoking levels were dose-dependently associated with a higher HR (adjusted P for trend = 0.0105), with the ≥20 cigarettes/day group having a significantly higher adjusted HR (1.80). Heavy drinkers (≥449 g ethanol/week) with smoking habits, but not those without smoking habits, had higher dementia risk (P for interaction = 0.0046). Light-to-moderate alcohol consumption is associated with decreased dementia risk, and smoking is dose-dependently associated with increased dementia risk, with an interaction between high alcohol consumption and smoking on dementia risk.