Smoking cigarettes irreversibly shrinks the brain, study warns

ST. LOUIS, Mo. — There are already a myriad of reasons to avoid cigarettes, but new research is adding yet another major motivator to steer clear of tobacco. Scientists at Washington University School of Medicine in St. Louis have found that smoking actively shrinks the human brain. On a more positive note, study authors add that quitting cigarettes appears to prevent further loss of brain tissue. However, quitting won’t restore the brain to its original size.

The human brain naturally shrinks as one ages and grows older. So, the research team believes smoking effectively causes the brain to age prematurely. They also note this work helps explain why smokers are at a higher risk of age-related cognitive decline and Alzheimer’s disease.

“Up until recently, scientists have overlooked the effects of smoking on the brain, in part because we were focused on all the terrible effects of smoking on the lungs and the heart,” says senior author Laura Bierut, MD, the Alumni Endowed Professor of Psychiatry, in a university release. “But as we’ve started looking at the brain more closely, it’s become apparent that smoking is also really bad for your brain.”

It’s been known for quite some time that there’s a connection between smoking and smaller brain volume, but scientists have always remained uncertain which is the instigator. There is also a third factor in all of this: genetics. Both the size of one’s brain and smoking behavior are heritable. Roughly half of a person’s risk of smoking traces back to his or her genes.

In an effort to unravel the relationship between genes, brains, and behavior, Prof. Bierut and first author Yoonhoo Chang, a graduate student, analyzed data drawn from the U.K. Biobank, a publicly available biomedical database that contains genetic, health, and behavioral information on half a million people. A cohort of over 40,000 U.K. Biobank participants underwent brain imaging, which can gauge brain volume. All in all, researchers analyzed de-identified data on brain volume, smoking history, and genetic risk for smoking among 32,094 individuals.

Each pair of factors turned out to have a connection, including history of smoking and brain volume, genetic risk for smoking and history of smoking, and genetic risk for smoking and brain volume. Moreover, the connection linking smoking and brain volume depended on the dose. In other words, the more packs a person smokes per day, the smaller his or her brain volume.

When the study authors decided to consider all three factors together, the link between genetic risk for smoking and brain volume disappeared, but the link between each and smoking behaviors remained. Then, using a statistical approach known as mediation analysis, study authors were able to determine the specific sequence of events: Genetic predisposition leads to smoking, which in turn leads to decreased brain volume.

“It sounds bad, and it is bad,” Prof. Bierut adds. “A reduction in brain volume is consistent with increased aging. This is important as our population gets older, because aging and smoking are both risk factors for dementia.”

Unfortunately, the brain shrinkage tied to smoking appears to be permanent. By analyzing data on people who had quit smoking years before, researchers uncovered that their minds remained permanently smaller than those of people who had never smoked.

“You can’t undo the damage that has already been done, but you can avoid causing further damage,” Chang concludes. “Smoking is a modifiable risk factor. There’s one thing you can change to stop aging your brain and putting yourself at increased risk of dementia, and that’s to quit smoking.”

The study is published in the journal Biological Psychiatry.

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