Long-term smoking linked to diminished mental proficiency

Oct 11 2005

The association between long-term smoking and diminished mental proficiency in 172 alcoholic and non-alcoholic men was a surprising finding from a study that set out to examine alcoholism's long-term effect on the brain and thinking skills.

While the researchers confirmed previous findings that alcoholism is associated with thinking problems and lower IQ, their analysis also revealed that long-term smoking is too. The effect on memory, problem-solving and IQ was most pronounced among those who had smoked for years. Among the alcoholic men, smoking was associated with diminished thinking ability even after alcohol and drug use were accounted for.

The findings are the first to suggest a direct relationship between smoking and neurocognitive function among men with alcoholism. And, the results suggest that smoking is associated with diminished thinking ability even among men without alcohol problems.

The new findings, released online before publication by the journal Drug and Alcohol Dependence, were made by a team from the U-M Medical School's Addiction Research Center, or UMARC, and their colleagues at the VA Ann Arbor Healthcare System and Michigan State University.

Lead author Jennifer Glass, Ph.D., a research assistant professor in the U-M Department of Psychiatry, cautions that the findings need to be duplicated by other studies before any conclusions are made about smoking's effect on the brain, or before the findings can be considered relevant to women.

But, she says, the findings should prompt alcoholism researchers to re-examine their data for any impact from smoking -- a factor that is not usually taken into account in studies of alcoholism's effects on the brain, despite the fact that 50 percent to 80 percent of alcoholics smoke. Meanwhile, the U-M-led team is launching a study that will examine the issue in adolescents, and plans to test the 172 men again soon.

"We can't say that we've found a cause-and-effect relationship between smoking and decreased thinking ability, or neurocognitive proficiency," says Glass. "But we hope our findings of an association will lead to further examination of this important issue. Perhaps it will help give smokers one more reason to quit, and encourage quitting smoking among those who are also trying to control their drinking."

Many alcoholism-recovery programs don't emphasize quitting smoking, even though smoking can be a social and possibly chemical "cue" associated with alcohol consumption.

Glass notes that her team's paper is being published, coincidentally, at the same time as a paper from a team at the University of California, San Francisco, in which brain scans showed that alcoholics who smoke have lower brain volume than alcoholics who don't smoke, and that cognitive function decreases with brain volume among non-smoking alcoholics, but not smoking alcoholics.

Taken together with previous epidemiological studies, the two new papers feed a growing body of evidence for a link between long-term smoking and thinking ability, says Robert Zucker, Ph.D., professor of Psychology in the U-M Departments of Psychiatry and Psychology, and director of the UMARC. Zucker is senior author on the new paper led by Glass.

"The exact mechanism for smoking's impact on the brain's higher functions is still unclear, but may involve both neurochemical effects and damage to the blood vessels that supply the brain," Zucker says. "This is consistent with other findings that people with cardiovascular disease and lung disease tend to have reduced neurocognitive function."

The data for the new paper by Glass, Zucker and their colleagues at U-M and Michigan State University, come from an ongoing longitudinal, or long-term, project that uses interviews and standardized research questionnaires to look at mental and physical health issues in families, measured every three years.

The study, which has run for more than fifteen years and recently was funded for another five, is supported by the National Institute of Alcoholism and Alcohol Abuse, part of the National Institutes of Health. The new work that will explore these relationships further in youth is being funded by the National Institute on Drug Abuse, also a part of the NIH.

In their ninth year in the study, participants completed the MicroCog Assessment of Cognitive Function, a well-established standard battery of tests that assess short-term memory, immediate and delayed story recall, verbal analogies, mathematical reasoning and visual-spatial processing.

Scores for each test, and a global proficiency score, are based on the speed and accuracy of a person's responses, adjusted for age and education level. The participants also took a short form of the standard IQ test, and their scores were adjusted for age.

Forty of the men had clinically diagnosable alcoholism at the time of the test, though none had been drinking within an hour of the tests. Twenty-four of these men also were smokers. The study also included 63 men who had had alcoholism earlier in life, 29 of whom smoked; and 69 men who had never been alcoholic, 13 of whom smoked. All smokers were allowed to smoke at will during the testing session, so none were in a nicotine-deprived state when they took the neurocognitive tests.

Glass and her colleagues analyzed the participants' scores using two standard measures of long-term drinking and smoking behavior: lifetime alcohol problem severity, or LAPS, and pack-years, a measure that takes into account the number of packs of cigarettes a person smoked each day and the number of years they smoked that much.

Across the board, both smoking and drinking showed an effect: Higher pack-years and LAPS scores were both significantly associated with lower global cognitive proficiency scores and IQ.

When the researchers limited the analysis to those participants who had ever had a diagnosis of alcoholism during their lifetime, they still found a significant association between LAPS scores and IQ, and between pack-years and both IQ and overall cognitive proficiency. In fact, the impact of heavy lifetime smoking history was greater than the effect of lifetime drinking history.

This finding, Glass says, means that alcoholism researchers who have consistently found evidence of cognitive deficits among alcoholics -- but who have not taken smoking into account in their analysis -- may actually be seeing a combined effect of smoking and alcohol consumption among alcoholic study participants who smoke. Further analyses of these data, with smoking separated out as a variable just as hard drug use is often separated, is needed, she says.