LONDON - Contrary to the belief that smoking may help ward off dementia and Alzheimer's disease, the most rigorous study ever to investigate the link has concluded that cigarettes provide no protection against mental decline.

In research published this week in the British Medical Journal, scientists from Oxford University found male doctors who smoked were just as likely to develop dementia and Alzheimer's as their counterparts who had never smoked or had quit years before. Earlier studies on the relationship between smoking and dementia, a major component of Alzheimer's, produced conflicting results, with some showing an apparent benefit. Alzheimer's is one of the few diseases said to occur more often in nonsmokers than smokers. "We've reached a point now where the suggestion that smoking is protective is no longer an issue. This wipes out the question of it somehow being a good thing," said Dr. Carol Brayne, an epidemiologist at Cambridge University who was not connected with the study. "It may not put the whole discussion to rest because that almost never happens, but this is very strong and it certainly does squash the notion of a benefit," added Bill Thies, vice president of medical and scientific affairs at the U.S. Alzheimer's Association. The findings are the latest from the longest-running study of smoking and disease. It has tracked British doctors since 1951 and has previously confirmed the link between smoking and lung cancer, showed that about half of smokers die from the habit and that quitting can make a difference. Many have believed smoking could protect against mental decline, partly because it is well-established cigarettes can help ward off the brain disorder Parkinson's disease and partly because it seems plausible that nicotine might boost brain chemical networks that are damaged in Alzheimer's patients. But the scientific studies that have indicated smokers were less likely to develop dementia were based on too few people to be reliable, looked back at smoking habits when the people already had become demented or were otherwise fundamentally flawed, according to the authors of the Oxford study. "I think there is a popular belief, but there weren't any serious studies showing it," said Richard Peto, an Oxford professor of epidemiology who was involved in the study. "This study supercedes the other studies." In the study, 34,439 male doctors were followed for nearly 50 years, with their smoking habits recorded every six to 12 years up until 1998. By then, 24,133 had died, with dementia mentioned on the death certificate in 473 of those cases. Smokers accounted for 40 percent of doctors who developed dementia and 40 percent of those who didn't. Peto's group reviewed the previous studies and concluded, based on all the reliable evidence, that if anything, smoking might marginally increase the risk of dementia. Part of the reason is that some dementia cases occur when the brain is deprived of oxygen because of blockages in the blood vessels. Smoking is bad for blood vessels and can encourage blockages. Alzheimer's disease, characterized by the degeneration of brain cells, is the most common form of dementia, accounting for about two-thirds of cases. It is estimated that the disease afflicts about 12 million people worldwide including 4 million in the United States or 3 percent of people 65 or older, according to the nonprofit organization Alzheimer's Disease International.

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