NEW YORK (Reuters Health) - UK researchers have found that middle-aged smokers show a faster decline in scores on tests of word memory, relative to non-smokers.

Furthermore, people who smoked in their 40s did worse on tests that measure how fast they could pick out certain letters from a page full of different letters, the authors write in the June issue of the American Journal of Public Health.

The relationship between smoking and cognitive decline appeared strongest in people who smoked more than 20 cigarettes each day, and persisted even when controlled for socioeconomic status, gender and a range of medical conditions.

Just why smoking may speed up age-related cognitive decline is not yet clear, lead author Dr. Marcus Richards, from University College London, told Reuters Health.

"Our results for memory still held up after taking blood pressure into account, but smoking could have been causing changes in the brain's blood supply that we were not able to measure," he said.

Alternatively, chemicals in cigarette smoke could also damage the brain directly, Dr. Richards added.

In the study, the researchers analyzed longitudinal data collected from 5362 people born in 1946. Study participants were contacted 21 times by the time they reached 53 years of age.

Although smokers in their 40s performed just as well as non-smokers on verbal memory tests, smokers' performance deteriorated much faster from their 40s to their 50s.

And people who said they smoked while in their 40s scored worse during speed tests conducted in their 40s than non-smokers.

But the findings also suggest that quitting may help, for the researchers discovered that people who stopped smoking before age 53, and especially those who stopped before age 43, tended to exhibit a slower decline in cognitive function.

Am J Public Health. 2003 Jun;93(6):994-8

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