Middle-aged people who smoke are more likely to develop dementia later in life than those who do not, according to study findings published this week in the Archives of Internal Medicine.

U.S. and European researchers collected 23-year follow-up data from 21,123 people aged 50 to 60 years who participated in a California health survey from 1978 through 1985.  From 1994 to 2008, 5,367 of the original participants developed dementia (25.4%).

Using multivariate Cox proportional hazards, the researchers determined that smoking more than two packs of cigarettes a day was associated with a more than two-fold increase in risk for dementia (adjusted HR=2.14; 95% CI: 1.65-2.78), Alzheimer disease (adjusted HR=2.57; 95% CI:1.63-4.03) and vascular dementia (adjusted HR=2.72; (5% CI: 1.20-6.18).

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Furthermore, analyses revealed that the risk for dementia was dose dependent – increasing with the number of cigarettes smoked – and consistent regardless of race or gender.

“To our knowledge, this is the first study evaluating the amount of midlife smoking on long-term risk of dementia and dementia subtypes in a large multiethnic cohort,” the researchers wrote.  “The large detrimental impact that smoking already has on public heath has the potential to become even greater as the population worldwide ages and dementia prevalence increases.”

They called for further research to determine the mechanisms by which smoking affects the development of dementia.