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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Minding Your Mind Minding Your Mind

Smoking and Dementia

December 21, 2012

By Michael Craig Miller M.D.

Harvard Medical School

Here is one more reason to kick the smoking habit. Recent research suggests that smoking increases the risk of developing Alzheimer's disease and other types of dementia. These are degenerative brain disorders that take years to develop. We can't change some factors that predispose people to develop dementia — such as advanced age and genetic profile.

Scientists are therefore searching for modifiable risk factors, like smoking, that people could change to help them reduce their risk of developing dementia.

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What the Research Shows

Most studies looking at this problem have involved elderly smokers and have lasted only a few years. These studies can only provide a snapshot view of the relationship between dementia and elderly smokers rather than a longer-range view of how smoking affects risk over a lifetime.

Two longer-term studies that enrolled middle-aged participants found that midlife smoking raised the risk of Alzheimer's disease, the most common type of dementia. But one study enrolled only Japanese Americans, while the other involved white people living in Finland. Therefore, results may not apply to other groups of people. That's an important factor, as rates of both smoking and dementia vary by race.

Researchers in California and Finland wanted to do a study that was more representative of the United States as a whole. They analyzed the medical records of thousands of members of a health maintenance organization in San Francisco and Oakland. The members came from various racial groups. Nine percent of participants identified themselves as Asian Americans, 23% as African Americans, and 63% as white Americans (a category that included Caucasians and Hispanics).

Participants completed a health survey during routine medical visits between 1978 and 1985, when they were 50 to 60 years old. At this time, clinicians measured their blood pressure, cholesterol levels, weight and other to heart disease risk factors. Participants answered questions about their health habits, including how many packs of cigarettes they smoked per day. The researchers then examined medical records for 21,123 participants still alive between 1994 and 2008 (an average of 23 years after the first assessments) to determine who developed Alzheimer's or another dementia.

They found that 5,367 people (roughly 25% of the sample) were diagnosed with some type of dementia, including 1,136 with Alzheimer's. After adjusting for age, gender, race, education and cardiac risk factors, the researchers found that:

  • People who smoked more than two packs of cigarettes a day at midlife had more than double the risk of developing dementia in old age when compared with nonsmokers.
  • People who had stopped smoking by midlife and those who smoked less than half a pack a day had a risk of dementia similar to that of people who had never smoked.

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Tobacco's Toll

This analysis strengthens the evidence that smoking is a risk factor for dementia. But, it was not designed to explain how tobacco use hurts the brain.

One theory is that smoking indirectly causes dementia by harming the heart and blood vessels (heart disease increases risk of dementia). Because smoking can cause blood clots, it's also possible that midlife smoking may cause multi-infarct dementia, the type that results from a series of small strokes. However, even after the researchers controlled for stroke, they found that cigarette smoking significantly increased the risk of vascular dementia. This suggests that something else in cigarettes may be harmful.

The study was not designed to show whether people who quit smoking reduced their risk of dementia. That's because the researchers collected data on smoking status only once, at midlife. Still, there are many other reasons to quit smoking, including the most obvious: Tobacco is the leading cause of preventable death in the United States, accounting for about one of every five American deaths.

Some people are able to give up smoking through willpower alone. They are the minority. Maybe this new research will be the extra reason someone needs to quit.

On the subject of quitting: Most people need help, in the form of counseling, nicotine replacement therapy and medications. Most smokers also have to try to quit multiple times before they succeed, but the health benefits — better breathing, less chance of heart disease, and possibly protection of the brain — suggest the effort is worth it. (Check out our Tobacco Cessation section.

Michael Craig Miller, M.D Michael Craig Miller, M.D., is Senior Editor of Mental Health Publishing at Harvard Health Publications. He is an assistant professor of psychiatry at Harvard Medical School. Dr. Miller is in clinical practice at Beth Israel Deaconess Medical Center, where he has been on staff for more than 25 years.

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