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"We found that people who reported heavy smoking in midlife had more than a 100% increase in risk of Alzheimer's disease and vascular dementia," said lead researcher Rachel A. Whitmer, a research scientist in Kaiser Permanente's Division of Research in Oakland, Calif.
The report is published in the Oct. 25 online edition of the Archives of Internal Medicine.
For the study, Whitmer's group collected data on 21,123 ethnically diverse people in the Kaiser Permanente health care system who were surveyed between 1978 and 1985, when they were 50 to 60 years old.
During an average follow-up of 23 years, the researchers found that 25.4% were diagnosed with dementia, including Alzheimer's (1,136 people) or vascular dementia (416 people), which is the second most common form of dementia after Alzheimer's disease. Vascular dementia is caused by damage to the arteries in the brain.
Compared with non-smokers, those who smoked more than two packs of cigarettes a day in midlife had a "dramatic increase" in the incidence of dementia -- more than a 157% increased risk of developing Alzheimer's disease and a 172% increased risk of developing vascular dementia, Whitmer's team found.
Former smokers and people who smoked less than half a pack a day did not appear to be at increased risk of Alzheimer's or vascular dementia, the researchers note.
A link between Alzheimer's and smoking has been shown before, but this new study pinpoints the specific risk for middle-age smokers for developing both Alzheimer's and vascular dementia, the researchers say.
Smoking, an established risk factor for stroke, may contribute to the likelihood of vascular dementia by causing small clots in the brain. Smoking also contributes to oxidative stress and inflammation, which may be linked to the risk of developing Alzheimer's disease, the researchers say.
"The brain is not immune to long-term damage from smoking," Whitmer said.
Two smaller studies of predominantly white participants also suggested that mid-life smoking raised the risk of developing Alzheimer's, researchers noted.
Commenting on the new study, William Thies, chief medical and scientific officer at the Alzheimer's Association, said "this is a sound confirmation of something that's been known for a while."
Another expert, Dr. Samuel E. Gandy, the Mount Sinai Professor of Alzheimer's Disease Research at Mount Sinai School of Medicine in New York City, said the findings are promising.
"Environmental factors in Alzheimer's disease have been long sought, and, until now, only head injury has emerged," Gandy said. "Unlike head injury, a tobacco smoking association is especially important because that is a risk that can be modified."
Copyright © 2010 HealthDay. All rights reserved.
SOURCES: Rachel A. Whitmer, Ph.D., research scientist, Kaiser Permanente Division of Research, Oakland, Calif; William Thies, Ph.D., Chief Medical and Scientific Officer, Alzheimer's Association; Samuel E. Gandy, M.D., Ph.D., Mount Sinai Professor of Alzheimer's Disease Research, Mount Sinai School of Medicine, New York City; Oct. 25, 2010, Archives of Internal Medicine, online