The positive news is that ceasing smoking prevents additional brain tissue loss. However, even though quitting smoking doesn’t restore the brain to its original size, it does halt further decline. Since natural ageing results in a decrease in brain volume, smoking effectively accelerates the ageing process of the brain, as highlighted by the researchers.
Published in Biological Psychiatry: Global Open Science, these findings provide insights into why smokers face an elevated 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,” explained senior author Laura J. Bierut, MD, the Alumni Endowed Professor of Psychiatry. “But as we’ve started looking at the brain more closely, it’s become apparent that smoking is also really bad for your brain.”
The association between smoking and smaller brain volume has been known, but identifying the instigator has been a challenge. Genetics, as a third factor, plays a role since both brain size and smoking behaviour are heritable. Approximately half of an individual’s smoking risk is attributed to their genes.
To untangle this complex relationship, Bierut and first author Yoonhoo Chang analyzed data from the UK Biobank, a biomedical database containing genetic, health, and behavioural information from over 40,000 participants. Brain imaging data were available for a subset of these participants, allowing the researchers to assess brain volume.
The analysis revealed interconnected links between smoking history and brain volume, genetic risk for smoking and smoking history, and genetic risk for smoking and brain volume. Moreover, the association between smoking and brain volume was dose-dependent, with higher daily cigarette consumption correlating with smaller brain volume.
When considering all three factors simultaneously, the link between genetic risk for smoking and brain volume disappeared, while the associations between each of these factors and smoking behaviours persisted.
Through mediation analysis, the researchers delineated the sequence: genetic predisposition leads to smoking, resulting in decreased brain volume. Bierut emphasized the significance, stating, “A reduction in brain volume is consistent with increased ageing. This is important as our population gets older because ageing and smoking are both risk factors for dementia.”
Unfortunately, the observed brain shrinkage appears to be irreversible. Even among individuals who had quit smoking years prior, their brains remained permanently smaller than those who never smoked.
“You can’t undo the damage that has already been done, but you can avoid causing further damage,” Chang emphasized. “Smoking is a modifiable risk factor. There’s one thing you can change to stop ageing your brain and putting yourself at increased risk of dementia, and that’s to quit smoking.”
