For the study the team looked at records of 21 123 men and women in midlife and continued following them for 23 years. The records were included from 1994 to 2004. Compared with non-smokers, those who had smoked two packs of cigarettes a day increased their risk of developing Alzheimer's by more than 157% and had a 172% higher risk of developing vascular dementia - the second most common form of dementia after Alzheimer's. About 25% of the group, 5367 volunteers, were diagnosed with some form of dementia in the more than 20 years of follow up, including 1136 people who were diagnosed with Alzheimer's disease.
According to Whitmer, "People who smoke have increased inflammation, and we know inflammation also plays a role in Alzheimer's.
Dementia experts say the Kaiser research is strong. William Thies, the Alzheimer's Association's chief medical and scientific officer said that although some early studies show that smoking has a protective effect on dementia, "This study is particularly good because it separates out vascular dementia and Alzheimer's."
Brenda Plassman, director of the program in epidemiology, at Duke University's Dementia Department of Psychiatry also said, "The other novel aspect of it is that they've got a large enough sample to look at different ethnic groups, and it shows smoking's effect on dementia does not differ based on race."
According to the team of researchers more studies are needed to confirm the theories. However, the bottom line according to Thies is, "If there's somebody out there who hasn't heard smoking's bad for you, they must live in a cave somewhere... This is another good reason not to smoke."
The study was funded by the National Graduate School of Clinical Investigation, EVO grants from Kuopio University Hospital, and grants from the Juho Vainio Foundation and Maire Taponen Foundation. The study was also supported by a Kaiser Permanente Community Benefits Grant and National Institute of Health and Academy of Finland Grant.
The World Health Organisation says 5 million people die every year from tobacco-related heart attacks, strokes and cancers. Another 430 000 adults die annually from breathing second-hand smoke.
