People who smoke heavily in middle age could more than double their risk for Alzheimer's disease and other forms of dementia 20 years later, according to research by Finnish experts published in a US journal.
It has been documented that smoking increases the risk of most diseases and mortality, but some studies have shown that smoking can reduce the chances of developing Parkinson's disease and other neurodegenerative diseases.
"The link between smoking and risk of Alzheimer's disease, the most common subtype of dementia, has been somewhat controversial, with some studies suggesting that smoking reduces the risk of cognitive impairment," the authors wrote in the report, posted online on Monday in the Archives of Internal Medicine, published by the Journal of the American Medical Association (JAMA).
Minna Rusanen from the University of Eastern Finland and Kuopio University Hospital in Kuopio, Finland, analyzed data with colleagues from 21,123 members of a health care system who participated in a survey between 1978 and 1985, when they were between the ages of 50 and 60.
The diagnosis of Alzheimer's and other forms of dementia were made between January 1994 and end in July 2008. The patients were on average 71.6 years old at the time.
Of those patients studied, 5,367 participants, or 25.4 percent, were diagnosed with dementia during an average follow-up period of 23 years, including 1,136 and 416 in Alzheimer's and vascular dementia.
Those of the patients who smoked more than two packs of cigarettes per day in their fifties had a very high risk of developing dementia and Alzheimer's in particular compared to non-smokers.
Race and sex was not a factor in the study, the authors said.
Smoking is a known risk factor for stroke, and may help increase the risk of vascular dementia in a similar manner, the authors wrote.
Smoking also contributes to oxidative stress and inflammation, believed to be important in the development of Alzheimer's disease.
Smoking may affect dementia development via vascular and neurodegenerative pathways, the authors write.
"Our study suggests that heavy smoking in middle age increases the risk of both Alzheimer's disease and vascular dementia for men and women across different race groups. The large detrimental impact that smoking already has on public health has the potential to become even greater as the population worldwide ages and dementia prevalence increases."
The authors said it is the first study that studies "the amount of midlife smoking on long-term risk of dementia and dementia subtypes in a large multiethnic cohort."
Smoking is blamed for several million deaths per year from causes such as heart disease and cancer, according to background information in the article.