LOS ANGELES -- Heavy smoking at middle age more than doubles the risk of Alzheimer's disease and other types of dementia later in life, according to one of the first long-term studies to examine the issue.
Smoking has a clear effect on the heart and lungs, but whether it also damages the brain has been controversial. The study, published Monday in the Archives of Internal Medicine, overcomes some of the obstacles that have made it difficult to assess such a link. For example, some previous research suggesting that smoking doesn't cause dementia mostly examined elderly people only for a short period of time.
To get a more complete look, researchers in Finland, Sweden and the health plan Kaiser Permanente's Division of Research followed 21,123 middle-aged Kaiser members who participated in a survey between 1978 and 1985 and then were followed for an average of 23 years.
After controlling for other factors that can contribute to dementia -- such as education level, race, age, diabetes, heart disease and substance abuse -- the study found a significant link with heavy smoking at middle age.
Compared to non-smokers, people who smoked two packs a day or more had a 114 percent increased risk of dementia (more than double), while people who smoked one to two packs a day had a 44 percent increased risk. Those who smoked a half-pack to one pack a day had a 37 percent increased risk.
Middle-aged people who described themselves as former smokers did not appear to have an increased risk of later dementia.
One way that smoking might increase the risk of dementia would be via the narrowing of blood vessels in the brain, a process that leads to the well-established increased risk of stroke from the habit, said Rachel A. Whitmer, a research scientist at Kaiser Permanente's Division of Research and the principal investigator of the study. However, even people who smoked heavily at mid-life and did not have any subsequent strokes were at higher risk for dementia, Whitmer said.
Of the 5,367 participants who were eventually diagnosed with dementia, just 416 were diagnosed with vascular dementia, a condition in which reduced blood flow to the brain triggers strokes that steadily erode memory.
The majority of the cases were diagnosed simply as dementia, while 1,136 cases were diagnosed as Alzheimer's disease.
"Stroke is certainly one of the pathways that smoking causes dementia, but it's not the only pathway," Whitmer said. Oxidative stress and inflammation caused by smoking may also damage the brain and lead to dementia, she said.
The link between smoking and later dementia did not differ according race, ethnicity or sex. The researchers do not know how many of the smokers continued smoking into old age, quit or cut back. Therefore, Whitmer said, the actual rates of smoking-related dementia might be even higher if people who quit or cut back were left out of the analysis.
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Whether the threat of developing yet another smoking-related disease will influence the roughly 20 percent of U.S. adults who smoke to quit, or persuade teenagers not to start, is unknown. But dementia, Whitmer noted, is one of the most feared diseases because there is no cure and no effective treatments to slow its course.
"People know that smoking is bad for them," said Gloria Soliz, a smoking-cessation trainer for the American Lung Association in California. However, she added, "that is not necessarily what motivates them to quit or work on staying quit. One of the reasons why smoking is very insidious is that you don't see the effects until years in the future."
A recent health care strategy shift toward focusing on the prevention and early detection of Alzheimer's disease may prompt smokers to confront the risk of dementia sooner, Whitmer said.
"We know dementia has a 10-to-15-year process before people become really demented," she said. "People have to understand that it's not a disease of old age. It's a disease of a lifetime. We need to think about risk factors early on."
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