New research has found that smoking causes the brain to shrink and that genetics could be important as about half of one's risk of smoking can come from their genes.

Researchers at Washington University School of Medicine, US, said that since a natural reduction in brain volume is usually seen to occur with age, smoking therefore effectively ages the brain prematurely.

The findings help explain why smokers are at a high risk of age-related cognitive decline and Alzheimer's disease, which is a neurodegenerative disease affecting some people progressively with age, the researchers said.

"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," said Laura J. Bierut, professor of psychiatry and senior author of the study published in the journal Biological Psychiatry: Global Open Science.

"But as we've started looking at the brain more closely, it's become apparent that smoking is also really bad for your brain," said Bierut.

The researchers said that quitting smoking can prevent further damage, even as it cannot restore the brain to its original size.

Previous studies have found both brain size and smoking behaviour to be heritable.

Thus, in this study, the scientists wanted to better understand the relationship between genes, brains and smoking behaviour.

The team overall analysed data on brain volume (determined through brain imaging), smoking history and genetic smoking risk for 32,094 people from the UK Biobank. The public biomedical database contains genetic, health and behavioural information on half a million people, mostly of European descent.

The researchers found that each of the pair of factors were linked - smoking history and genetic smoking risk, genetic smoking risk and brain volume, and brain volume and smoking history. They also found that the more packs a person smoked per day, the smaller their brain volume.

Further, when the team considered all the three factors together, they found that the link between genetic smoking risk and brain volume disappeared.

The other two links - those between smoking history and genetic smoking risk, and brain volume and smoking history - still remained, however, they found.

Using statistical analysis, the researchers determined the chain of events as thus - genetic risk leads to smoking, which leads to a reduced brain volume.

"It sounds bad, and it is bad. 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," said Bierut.

The researchers also found that the shrinkage effects were irreversible by analysing data of people who had quit smoking years before. They found that these people's brains 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," said first author Yoonhoo Chang, a graduate student at the university.