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"Dementia is rare among middle-aged people, but cognitive function at this age in closely related to dementia," said lead researcher Severine Sabia, of the Institut National de la Sante et de la Recherche Medicale in Villejuif, France. "Our results show that smoking is associated with poorer cognition and decline over five years.
"Another interesting finding," Sabia added, "was that ex-smokers improved their other health behaviors, and among them there was little residual adverse effect of smoking on cognition."
The findings are published in the June 9 issue of the Archives of Internal Medicine.
For the study, the researchers collected data on 10,308 British civil servants who ranged in age from 35 to 55 and were enrolled in the Whitehall II study. Between 1985 and 1988 and again in 1997 and 1999, they were asked about their smoking habits. Nearly 5,400 people completed tests of memory, reasoning, vocabulary and verbal fluency in 1999. Five years later, 4,659 of the study participants were retested.
During the first round of cognitive testing, people who smoked ranked in the lowest 20 percent of all those examined, compared with people who had never smoked. But, people who were ex-smokers were 30 percent less likely to have poor vocabulary and low verbal frequency scores than current smokers.
"Smoking is associated with poorer cognitive function in midlife," Sabia said. "However, 10 years after smoking cessation, there is little adverse effect of smoking on cognition."
The study also found that ex-smokers had better overall health habits than smokers. They drank less alcohol, exercised more and ate more fruits and vegetables, the researchers found.
"With the ageing population and the projected increases in older adults with dementia, it is important to identify modifiable risk factors," Sabia said. "Our results suggest that smoking had an adverse effect on cognitive function. Thus, public health messages should target smokers at all ages."
But one expert said he wasn't convinced that the study had proven a connection between smoking and memory loss.
"There are two things that are a little concerning about the [study] results that would give us pause before definitely concluding that smoking leads to a decrease in memory," said Dr. Michael Siegel, a professor of social and behavioral sciences at Boston University.
First, the results for smoking as a cause of memory loss were only significant when other health factors weren't taken into account, Siegel noted. "That suggests that smoking is associated with other types of health behaviors that are affecting the outcome," he said. "When other health behaviors were included, it completely wiped out the effect of smoking."
And second, Spiegel said he wasn't sure why ex-smokers performed better on the memory tests. "The reason their memory is better is not because they smoked -- that's not plausible," he said.
Smoking may not be the reason for memory impairment, Siegel said. Rather it may be that people who never smoked or quit smoking have better overall health habits, may be better educated, and may have a higher level of cognitive functioning to begin with, he said.
Dr. Norman H. Edelman, scientific consultant to the American Lung Association, said: "This study is generally a confirmation of previous work. But there is a fundamental question: Are they stupid because they smoke or do they smoke because they are stupid?"
SOURCES: Severine Sabia, M.Sc., Institut National de la Sante et de la Recherche Medicale, Villejuif, France; Michael Siegel, M.D., M.P.H., professor, social and behavioral sciences, Boston University; Norman H. Edelman, M.D., scientific consultant, American Lung Association, New York City, and professor, preventive medicine, internal medicine, physiology & biophysics, Stony Brook University, New York; June 9, 2008, Archives of Internal Medicine
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