While age, family history, and genetics are well-accepted as the major risk factors for Alzheimer's disease, cigarette smoking has not been considered a major contributor to Alzheimer's risk. In fact, some older studies and news stories even perpetuated the notion that smoking may protect against Alzheimer's risk. But more convincing recent evidence seems to set the record straight: Cigarette smoking is clearly associated with a higher risk of developing Alzheimer's disease, and heavy smoking in mid-life may double the risk of dementia in later years.
In a meta-analysis of 43 individual studies that examined the relationship between smoking and Alzheimer's disease, the authors concluded that smoking is in fact a significant risk factor for Alzheimer's disease. Interestingly, one of the variables that was taken into account in making this conclusion was whether or not the authors of the original studies had an affiliation with the tobacco industry. Eleven of the 43 studies had authors with tobacco industry affiliations, and these studies (not surprisingly) showed that smoking actually lowers Alzheimer's risk.
In another more recent study, heavy smoking in mid-life almost doubled the risk of both Alzheimer's disease and vascular dementia in later life. The study, published in the Archives of Internal Medicine in October 2010, was the first to look at the long-term risk of mid-life smoking and late-life dementia. It involved teams from the United States, Sweden, and Finland, and evaluated more than 5,300 people who developed dementia over 23 years. There were 1,136 cases of Alzheimer's disease and 416 cases of vascular dementia.
Compared with non-smokers, people who smoked two or more packs of cigarettes per day were twice as likely to develop dementia, while people who smoked one to two packs per day had a 44% increase in dementia risk. Even people who only smoked half a pack per day had a 37% increase in dementia risk. The authors cautioned that the study couldn't prove that it was the heavy smoking in mid-life that caused the dementia, as opposed to some other factor that heavy smokers may have, like a propensity to head trauma.
Since other mid-life vascular risk factors, like hypertension and diabetes, have been linked to higher rates of dementia, it was not surprising to the authors to see the link with smoking as well.
For people who are still smoking, there is some hopeful news from this study. First of all, people who had quit smoking by age 50 had no higher risk of Alzheimer's disease, so don't give up in your efforts to quit. Education, persistence, and support are the keys to quitting, and using a quitting smoking toolbox may help you towards ending your nicotine addiction.
In addition, people who smoked less than half a pack a day also had no higher risk of developing dementia, so don't think that cutting back on the number of cigarettes you smoke per day has no health benefits. Fortunately, there are now many aids to quitting smoking that are readily available.
Sources:
Cataldo, JK, Prochaska JJ, Glantz SA, "Cigarette Smoking is a Risk Factor for Alzheimer's Disease: An Analysis Controlling for Tobacco Industry Affiliation", Journal of Alzheimer's Disease 19;2:465-80, January 2010.
Rusanen M, Kivipelto M, et al.,"Heavy Smoking in Midlife and Long-term Risk of Alzheimer Disease and Vascular Dementia", Arch Intern Med. Published online October 25, 2010.
