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Sir Richard Doll and colleagues from the Clinical Trial Service Unit and Epidemiological Studies Unit, Radcliffe Infirmary at Oxford reported on observations of over 34,000 male British doctors whose smoking habits have been reviewed every six to 12 years since 1951 to determine the impact on their health. The research team also reviewed the published data on the associations between smoking and Alzheimer's disease.
Over 24,000 of the doctors had died by the end of 1998. Dementia was mentioned on the death certificates of 483. Among 473 whose smoking habits were recorded at least 10 years before their death, when they would not have been influenced by the start of the disease, the prevalence of both Alzheimer's disease and of other types of dementia was similar in both smokers and non-smokers.
If anything, persistent smoking may increase rather than decrease the age specific onset rate of dementia, conclude the authors.
The prior suggestions that smoking might be protective, say the authors, came from studies that were flawed because they were too small, or had relied on information about smoking habits from people other than the sufferers themselves.
In an accompanying editorial Carol Brayne from the Institute of Public Health at Cambridge writes that a protective effect for nicotine is biologically plausible. This is because of the boosting effect of the drug on neurotransmitter systems in the brain, which are damaged in Alzheimer's's disease. But she adds that these effects would be likely to be short-lived.
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In the long term, smoking increases the risk of vascular dementia, because it increases the risk of vascular disease in general. "The public health message is clear: at the population level there is no protective effect of smoking in dementia."
(1) The original research report: Richard Doll, Richard Peto, Jillian Boreham, and Isabelle Sutherland. Smoking and dementia in male British doctors: Prospective study. BMJ 2000;320:1097-1102 (22 April).
(2) The accompanying editorial: Carol Brayne. Smoking and the brain. No good evidence exists that smoking protects against dementia. BMJ 2000;320:1087-1088 (22 April).