A recent meta-analysis concluded that smoking is a risk factor for dementia, according to background information in the article.
However, research regarding the link between smoking and cognitive (thinking, learning and memory) function is difficult in older adults because many study participants do not return for follow-up visits or die of smoking-related diseases.
Severine Sabia, M.Sc., of the Institut National de la Sante et de la Recherche Medicale, Villejuif, France, and colleagues analyzed data from 10,308 London-based civil servants age 35 to 55 who enrolled in the Whitehall II study between 1985 and 1988.
Smoking habits were assessed at that time and again between 1997 and 1999. A total of 5,388 participants completed tests of memory, reasoning, vocabulary and verbal fluency between 1997 and 1999 and 4,659 were re-tested five years later.
Individuals who smoked at the beginning of the study were more likely to die during the average 17.1 years of follow-up, and also were less likely to participate in the cognitive testing. At the first round of cognitive testing, those who smoked were more likely to be in the lowest-performing group (lowest 20 percent) compared with those who had never smoked.
Those who reported being ex-smokers at the beginning of the study were 30 percent less likely than smokers to have poor vocabulary and low verbal frequency scores. Individuals who stopped smoking during the study also experienced more improvement in other health habits, such as drinking less alcohol, being more physically active and eating more fruits and vegetables.
"This study presents four key findings. First, smoking in middle age is associated with memory deficit and decline in reasoning abilities. Second, long-term ex-smokers are less likely to have cognitive deficits in memory, vocabulary and verbal fluency," the authors wrote.
"Third, giving up smoking in midlife is accompanied by improvement in other health behaviors. Fourth, our results based on a large prospective cohort study of middle-aged British civil servants suggests that the association between smoking and cognition, even in late midlife, could be underestimated because of higher risk of death and non-participation in cognitive tests among smokers," they added.
The results are important because individuals with cognitive impairment in midlife may progress to dementia at a faster rate, the authors noted.
"During the past 20 years, public health messages about smoking have led to changes in smoking behaviour. Public health messages on smoking should continue to target smokers of all ages," the concluded.
The study appears in the June 9 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.