Cigarette smoking may be associated with an increased risk of Alzheimer's disease and other types of dementia, such as vascular dementia. Smoking is a risk factor for heart disease and some cancers, but its long-term link with dementia has been controversial. Some previous research has suggested that smoking does not cause dementia. Now, a study published in the medical journal Archives of Internal Medicine in October 2010 found that heavy smoking in mid-life may increase the risk of Alzheimer's and vascular dementia in late life by as much as 100 percent. Vascular dementia is an umbrella term that describes cognitive impairments caused by problems in blood vessels that feed the brain.
Researchers in Finland, Sweden, and the United States looked at data from a group of 21, 123 people from different ethnic backgrounds. All were members of the Kaiser Permanente Medical Care Program of Northern California and participated in a survey between 1978 and 1985. Of those, 5,367 (25.4 percent) were diagnosed with dementia over the 23-year follow-up period.
“We found a two-fold increase in risk among those who smoked two packs per day, a 44 percent increase in those who smoked one to two packs, and 37 percent increase in those smoking one-half a pack per day in mid-life,” says lead author Rachel A. Whitmer, Ph.D., an investigator at the Kaiser Permanente Northern California Division of Research.
The risk might be even higher, says Dr. Whitmer, because the initial data were self-reported—and smokers often claim to be nonsmokers or underreport daily use.
“We've probably underestimated the real risk, and we don't know how many of these people quit smoking between the time of the initial survey and being diagnosed with dementia. People need to understand that these are long-term consequences from mid-life smoking,” Dr. Whitmer says.
Middle-aged people who described themselves as former smokers did not appear to have an increased risk of later dementia.
Smoking might increase the risk of dementia by narrowing blood vessels in the brain, which leads to increased stroke risk, says Dr. Whitmer. But even those people who did not have a stroke were at higher risk for dementia, she says.
Although the study found an association between smoking and dementia, association does not necessarily imply causation, cautions John C. Morris, M.D., Friedman Distinguished Professor of Neurology at Washington University School of Medicine in Saint Louis, MO.
“We do not know whether it was the heavy smoking in mid-life that caused the later development of dementia. People who smoke heavily may have other attributes, such as socioeconomic background, nutritional preferences, or other factors that end up being the real culprit,” Dr. Morris says.
The findings were adjusted for age, sex, education, race, marital status, hypertension, hyperlipidemia (too much bad cholesterol or triglycerides), body mass index, diabetes, heart disease, alcohol consumption, and stroke. In other words, the investigators looked for associations between these factors and dementia in order to rule them out as the real cause.
“This is not new,” says Richard Mayeux, M.D., the Gertrude H. Sergievsky Professor of Neurology, Psychiatry and Epidemiology, and co-director of the Taub Institute on Alzheimer's Disease and the Aging Brain at Columbia University Medical Center, in New York, NY. “There are now three large studies demonstrating these risks. These studies have all said the same thing: Smoking increases Alzheimer's dementia risk,” Dr. Mayeux says.