Damage to brain from smoking can be reversed
But study of thinning grey matter warns that repair could long process after quitting
Damage to the brain's outer layer caused by smoking may be reversible after quitting, but it could take years, a study said.
Brain scans of 500 Scottish septuagenarians confirmed a link between smoking and an acceleration of age-related thinning of the cortex - the outer layer of grey matter.
But the study also pointed, for the first time, to a potential for recovery after quitting.
The cortex of ex-smokers in the group "seems to have partially recovered for each year without smoking," the multinational research team wrote in the Nature journal Molecular Psychiatry. But they warned that it could be a long process.
Many studies have linked cigarette smoking with cognitive decline and dementia, and some also with brain degeneration.
"Evidence suggests that smokers have, on average, slightly poorer global cognitive functioning in later life, as well as lower mean scores on several cognitive domains such as cognitive flexibility and memory," said the study authors.
The team used people who had participated in the Scottish Mental Survey as school children in 1947, when their cognitive function was tested.
The survivors underwent MRI (magnetic resonance imaging) scans again in 2007, and the results of 504 of them were analysed.
There were 36 current smokers, 223 ex-smokers and 245 who had never smoked in the group, which had an average age of 73, the study paper said.
There was no significant difference between their ages or childhood IQ, and the group was split almost equally between men and women.
Analysis of the scans showed that current smokers had a generally thinner cortex than those who had never smoked.
As for the quitters, who had on average smoked a pack a day for 30 years, "it took roughly 25 years without smoking for differences in cortical thickness to no longer be observed between ex-smokers and those that never smoked", the authors wrote.
"Heavy ex-smokers remained with a thinner cortex at age 73 years even after more than 25 years without smoking."
"Smokers need to be informed that cigarettes are associated with accelerated cortical thinning, a biomarker of cognitive ageing," said the study.
"Importantly, cortical thinning can persist for many years after smoking cessation.
"The potential to at least partially recover from smoking-related thinning might serve as a strong motivational argument to encourage smoking cessation."