Long-term smoking 'may cause' brain shrinkage

“Smokers have thinner brain cortex and could have impaired thinking,” The Independent reports. MRI scans of long-term smokers show signs that the cerebral cortex – the grey matter of the brain – which plays a key role in thinking and memory, was thinner than expected.

The study looked at brain scans of more than 500 people aged 73 to see if there were any noticeable differences between smokers, ex-smokers and people who never smoked.

Smokers had the thinnest cortex on the MRI scans. However, despite some media reports, none of the participants had dementia or memory loss, and the researchers did not reveal any differences between the groups in terms of cognitive ability. The smoking group was limited in size to 36 participants (possibly because smokers are less likely to live until they are 73).

Thinning was also seen in ex-smokers compared to never smokers (these groups both had more than 200 participants). However as the study only took one measurement at one point in time, it cannot tell us either if this thinning in ex-smokers is due to smoking or if it partially recovers once a person quits smoking.

The authors acknowledge that this study doesn’t prove that smoking caused the cortex to thin as the measurement was only taken once. However, we already know that smoking is unhealthy and it is always a good idea to quit however long you have been smoking.

Where did the story come from?

The study was carried out by researchers from Edinburgh University, McGill University in Montreal, and Harvard Medical Schools in Massachusetts. It was funded by a Research Ageing Program grant, the Age UK-funded Disconnected Mind project, the UK Medical Research Council, the Scottish Funding Council, the UK Biotechnology and the Biological Sciences Research Council.

The study was published in the peer-reviewed medical journal Molecular Psychiatry on an open-access basis so it is free to read online.

The media has implied that there are direct links between having a thinner cortex and experiencing memory and cognitive problems, but this was not a result presented by this research.

What kind of research was this?

This was a cross sectional study comparing the thickness of the cortex of the brain between people who currently smoked, ex-smokers and non-smokers. The people are part of an ongoing, longstanding cohort study of people born in 1936.

This type of study can show associations, but it is not able to prove that one factor (smoking in this case) causes the other (thinning cortex). Ideally the study would assess people’s brain and smoking habits over time to see whether the changes come after a person takes up smoking and not before.

However, such a study is likely to be expensive to do and take a long time, so often researchers start with a cross sectional study. (And for this cohort, such a study would have been impossible, as MRI scanners weren’t invented until the 1970s).

A randomised controlled trial would not be ethical for this type of research, so an observational study like this is appropriate.

What did the research involve?

The researchers compared the thickness of the cortex of people aged 73 who were current smokers, ex-smokers and non-smokers. The cortex thins naturally as we age, but researchers wanted to see if this process was accelerated in smokers.

A group of 504 people from a longstanding study called the Lothian Birth Cohort 1936 (LBC 1936) was recruited into the study. This original study had started collecting data on people born in the Lothian region of Scotland in 1936 including mental ability and intelligence, which was tested when they were 11 years old.

The 504 participants (260 women and 244 men) were invited to have a brain MRI scan to measure the thickness of their cerebral cortex – the grey matter of the brain. None of them had any evidence of dementia, according to self-report and scoring more than 24 out of 30 on Mini Mental State Examination (MMSE) – a test commonly used to look for cognitive problems.

The participants were assessed on a range of factors, including:

smoking status, including age starting, age stopping (if they stopped) and average number smoked per day
recent alcohol consumption
socioeconomic status
cognitive testing and other psychological assessments
history of any medical conditions
physical examination, including blood pressure and lung function
blood tests

The researchers then analysed the results to look for any association between the thickness of the cortex and smoking history. They adjusted their results for gender and exact age.

What were the basic results?

There were 36 current smokers, 223 ex-smokers and 245 non-smokers. There was no significant difference between the groups in terms of gender, intelligence or socioeconomic status aged 11. However, those who never smoked were less likely to have a history of cardiovascular disease, had better lung function and drank fewer units of alcohol per week.

Current smokers had a significantly:

thinner cortex over most of the brain than people who had never smoked
thinner cortex than ex-smokers, but the difference was less than compared to never smokers

Ex-smokers had a significantly thinner cortex than non-smokers, but the difference was not as large as for current smokers compared to non-smokers. Those ex-smokers who had stopped smoking a longer time ago tended to have less difference in cortex thickness than those who stopped more recently.

How did the researchers interpret the results?

The researchers concluded that “smokers need to be informed that cigarettes are associated with accelerated cortical thinning, a biomarker of cognitive aging”. They also say that “the potential to at least partially recover from smoking-related thinning might serve as a strong motivational argument to encourage smoking cessation”.

Conclusion

This study has shown an association between smoking and a thinner cortex, though it cannot prove that smoking caused the cortex to thin. The study was cross sectional, so cannot say which came first – the smoking or the cortex differences. Also, confounding factors other than smoking may be contributing.

Strengths of the study include:

Having access to measurements of cognitive ability when the participants were 11 years old, before most of them would have started smoking, as a potential indicator of cortex thickness.
The radiologists were blinded to which MRIs came from each group, reducing the risk of them introducing bias based on interpreting scans differently for people they knew to be smokers.
All participants were the same age when they had the MRI scan, so age did not need to be adjusted for in the results. This is important because the cortical thickness naturally decreases with age.
The fact that those who gave up smoking seemed to have less difference to the non-smokers than those who continued to smoke fits with the idea that the two factors might be related to each other rather than arising just by chance.

Limitations acknowledged by the authors include:

The number of current smokers was small, only 36.
It is possible there were differences in cortex thickness before a person started smoking. They say that structural changes in the frontal part of the brain related to impulse control may have made it more likely for people to start smoking in the first place.

All this considered, it has long been known that smoking is bad for you.

Aside from the risk of cancer, heart disease and stroke, smoking may also increase the risk of dementia type conditions, such as vascular dementia and Alzheimer’s disease.

So, if smoking by itself does not directly cause cortical thinning, it is a good idea to quit if you smoke, whatever your age. Quitting, however long you have smoked, should lead to an increase in life expectancy and quality of life. Help and advice on quitting can be found here.