How smoking harms the brain

We know that smoking is bad for our lungs and heart, but now we know that it harms the brain as well.  As part of our Staying Sharp series, Dr Sherif Karama of McGill University, Canada, reveals the evidence.

Lifestyle choices affect brain ageing

People with good cognitive abilities (i.e. thinking skills) in old age tend to be those who have had good cognitive abilities throughout their lives. Nonetheless, in old age, most, if not all, of us exhibit a certain degree of decline in some thinking skills, like learning new information and being able to quickly shift from one mental task to another.

However, some of us show a steeper rate of decline than others. The factors that affect this rate of decline are still being researched but some are already clear and one of them is smoking.

Smoking speeds up brain ageing

While the effect of smoking on cognitive abilities is relatively small and requires large studies to be able to be detected, it is nonetheless present. Recently, we also looked at the effect on the structure of the brain by examining data from the Lothian Birth Cohort 1936 (LBC1936) at age 73.

We found that smokers had a thinner cerebral cortex than non-smokers – in other words, smoking was destroying the grey matter in smokers. This is important because the cerebral cortex is a part of the brain that is crucial for thinking skills including memory and learning, so thicker is better.

The cortex does tend to thin with age naturally but we found that, all else being equal, the more people had smoked, the more they tended to have a thin cortex. These results suggest that smoking accelerates the normal thinning of the cortex that occurs with age.

Why quitting matters

Importantly, people in the LBC1936 who stopped smoking early enough in their life seemed to partially recover with time. However, this recovery can be a long process.

In this group of people, we worked out the average number of cigarettes that they had smoked in their lifetime. This was around 196,000 cigarettes, that is, 20 a day for 26.9 years.

For those who had smoked the average number, it took roughly 25 years for complete recovery of the affected areas of the cortex. Those who had smoked less than the average recovered faster, while those who had smoked the most still had a long way to go to recover at age 73.

Overall, this shows that cognitive and brain ageing is a dynamic lifelong process. It is not something that simply happens in old age and does not appear to be determined simply by one thing that you do or avoid as you reach old age.

Rather, it appears to be the consequence of multiple factors that include genetic and environmental factors and the lifestyle choices we make, that have been playing a role throughout our lives.

Key references

Karama, S., Ducharme, S., Corley, J., Chouinard-Decorte, F., Starr, J.M., Wardlaw, J.M., Bastin, M.E. and Deary, I.J., 2015. Cigarette smoking and thinning of the brain’s cortex. Molecular psychiatry, 20(6), pp.778-785.