The harmfulness of tobacco is well known, not only to the smoker, but also to those exposed to second-hand smoke. Smoking is the main cause of lung cancer, one of the most deadly cancers, but it also increases the risk of cancers of the throat, bladder, pancreas, kidney, stomach, liver and cervix. People who smoke daily are also more prone to cardiovascular disease, such as high blood pressure or stroke, deep-vein thrombosis and arteritis of the lower limbs, for example.
Smoking also weakens the respiratory system, leading to diseases such as chronic bronchitis and emphysema. But smoking also affects brain size. While it is already accepted that the brain volume of smokers is smaller than that of non-smokers, the unanswered question is whether it is smoking that causes brain shrinkage, or whether it is people with smaller brains who have a greater propensity to smoke. A study carried out on 28,000 patients has just confirmed the first hypothesis.
To determine whether there is a causal link between smaller brain size and smoking, researchers at Washington University in St. Louis used brain imaging data from the UK Biobank. The UK Biobank is a long-term cohort study, launched in 2006, which investigates risk factors for mainly chronic diseases such as heart disease, diabetes, cancer and mental disorders. The study involves 500,000 participants from all over the UK, who are being followed over the long term and have provided blood, urine and saliva samples, as well as detailed information about their lifestyle, environment and health, to enable future analyses.
As part of the smoking study, researchers analysed participants’ self-reported smoking habits in surveys conducted between 2006 and 2010 and between 2012 and 2013 and then used magnetic resonance imaging to map participants’ brains. The results of the analyses showed that people who smoked daily before entering the study had on average 7.1 cubic centimetres lower brain volumes, including 5.5 cubic centimetres of grey matter, the part of the brain where neurons are located and responsible for information processing and decision-making, and 1.6 cubic centimetres of white matter, the part of the brain where nerve fibres are located that enable neurons to communicate with each other.
The researchers noted even greater differences in grey matter volume in daily smokers who smoked intensively. According to the so-called pack-year measurement, which corresponds to the consumption of one pack a day for one year, each additional year of consumption is associated with a reduction of around 0.15 cubic centimetres in grey matter volume, bearing in mind that the frequency with which people smoke has no impact on white matter volume.
Analyses also showed that people who had stopped smoking a long time ago had more grey matter than those who had stopped recently, suggesting that quitting smoking slightly reverses the decline in brain volume. Quitting smoking for a year, for example, enabled former daily smokers to regain 0.09 cubic centimetres of grey matter volume. The discovery of the link between smoking and brain shrinkage could provide clues as to whether there is also a link between smoking and certain neurological diseases, such as Alzheimer’s, characterized by brain shrinkage.
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