Men who are currently smoking or those with a history of smoking may be at risk for cognitive decline as they transition from midlife to older age, new research suggests.
In a cohort study of more than 7,000 people (mean age at first assessment, 56 years), investigators found that men who continued to smoke over the next decade showed a significantly more rapid decline in cognition than men who never smoked.
Men who had recently quit smoking also showed some cognitive decline, especially in executive functioning tasks. In long-term ex-smokers, the degree of cognitive decline was similar to the decline seen in those who had never smoked.
"There is increasing evidence that smoking is a risk factor for dementia, but its impact on cognitive decline, and particularly on cognitive decline in early old age, has been unclear," lead author Séverine Sabia, PhD, research associate in the Department of Epidemiology and Public Health at the University College London, told Medscape Medical News.
However, results from this study showed that male smokers who were in their 50s "had a cognitive decline as fast as nonsmokers 10 years older than they were," said Dr. Sabia.
In addition, she noted that the fact that no residual detrimental effects of smoking on cognitive decline was observed 10 years after smoking cessation "should encourage people to stop smoking as early as possible."
The study was published online February 6 in the Archives of General Psychiatry.
Doubling of Dementia Cases
According to the investigators, there were approximately 36 million cases of dementia worldwide in 2010 — a number that is projected to double by 2020.
"Smoking is a possible risk factor for dementia, although its impact may have been underestimated in elderly populations because of the shorter life span of smokers," write the researchers.
For this analysis, investigators examined a cohort of 5099 men and 2137 women who participated in the Whitehall II study, which includes employees of the British Civil Service.
All participants underwent their first cognitive assessments between 1997 and 1999, followed by examinations between 2002 and 2004 and between 2007 and 2009.
The assessments comprised 5 different cognitive tests, including examinations of memory, vocabulary, and executive function. The results were then combined to form a global cognitive score for each participant.
In addition, smoking status was recorded over the entire study period.
Results at the 10-year follow-up showed that men who were currently smokers had a faster decline in global cognition score (mean difference, -0.09; 95% confidence interval [CI], -0.15 to -0.03) and in executive function (mean difference, -0.11; 95% CI, -0.17 to -0.05) than men who had never smoked.
Men who had recently quit smoking also showed a greater decline in executive function compared with the men who had never smoked (mean difference, -0.08; 95% CI, -0.14 to -0.02).
No Association in Women
There were no significant differences in cognitive decline between the male long-term ex-smokers and those who had never smoked, nor was an association found between smoking status and cognitive decline in the women who were assessed.
Several possible reasons cited by Dr. Sabia for this last finding included the fact that the sample had significantly fewer women than men (only 262 of the women were current smokers at the time of the first cognitive assessment) and that "it is possible that smoking behavior is differently associated with other behaviors in men and women."
"For example, we found that current male smokers drank much more alcohol than female smokers (23 vs 10 drinks/week). More research is needed to understand this difference before drawing conclusions," she reported.
Nevertheless, at least for men, "it is increasingly recognized that age-related cognitive pathologies such as dementia result from long-term processes, perhaps beginning as long as 20 to 30 years before the clinical diagnosis of dementia," write the investigators.
"Our study illustrates the importance of examining risk factors for cognitive decline much earlier in the life course.... Public health messages on smoking should continue to target smokers at all ages," they add.
"What makes this study stand out is that they were looking at cognitive function over 10 years," Rachel Whitmer, PhD, epidemiologist at the Kaiser Permanente Division of Research in Oakland, California, told Medscape Medical News.
"This was looked at in people who we would consider to be 'young-old' and not very elderly. And that's a very important question to tackle when you're thinking about the influence of smoking on cognitive function or dementia or other diseases that manifest late in life," said Dr. Whitmer.
She added that although she was "a little surprised" to see the results' gender differences, she was not surprised to find that smoking was associated with a greater 10-year cognitive decline in the men.
"I also think there are some really nice implications for a public health message about not only not smoking but that if you do smoke, quitting has positive effects on disease and cognitive abilities."
Dr. Whitmer, who was not involved in this research, was principal investigator for a recent study published in the Archives of Internal Medicine that looked at the association between smoking in midlife and the long-term risks for dementia, Alzheimer's disease, and vascular dementia.
"In our study, we were able to retrospectively go back and look at data over 30 years and then follow our people going forward. We found that the smokers had about a 2-fold greater risk for dementia; and the risk was even greater in the heavy smokers," she explained.
"Even though it was looking at different outcomes, this new study supports the same body of evidence that smoking is bad for the brain."
Dr. Whitmer added that it will be exciting for the investigators to continue following the cohort to measure the association between smoking patterns over the long term and actual dementia.
"I think the take-home message from these types of studies is that clinicians need to realize that smoking elevates not only risk for dementia but that people also don't perform as well," she said.
"And patients need to hear that smoking can really affect the brain — but quitting can have good effects even up to 10 years later. It's a really positive message."
The study was funded by the National Institute on Aging, the Academy of Finland, the Bupa Foundation, and the British Heart Foundation; by a European Young Investigator Award from the European Science Foundation; and by grants from the National Institutes of Health and the British Medical Research Council. The study authors and Dr. Whitmer have disclosed no relevant financial relationships.
Arch Gen Psychiatry. Published online February 6, 2012. Abstract