It’s no secret that smoking cigarettes can ruin your body.
The list of illnesses that smoking causes — heart disease, stroke, diabetes, emphysema, bronchitis and several types of cancer — is long and troubling.
However, most people don’t realize that smoking is also associated with mental illness.
In recent years, studies have suggested that there’s a strong link between the two.
But researchers haven’t been able to decide whether smoking causes mental disorders, or if individuals who already have a mental disorder smoke to ease their symptoms.
Now, researchers from Aarhus University in Denmark believe they’ve proven that smoking is a cause of mental illnesses like depression, bipolar disorder and schizophrenia.
“The numbers speak for themselves — smoking does cause mental illness,” Dr. Doug Speed, statistical geneticist at Aarhus University’s Center for Quantitative Genetics and Genomics, said in a statement.
“Although it’s not the only cause, smoking increases the risk of being hospitalized with a mental illness by 250%,” Speed added.
To conduct this study, Speed and two colleagues from Canada gained access to the UK Biobank, one of the world’s largest databases of human health information, containing genetic data on more than half a million people.
The genetic data was combined with other health-related factors, such as participant-provided information on lifestyle.
And to make their data analysis more robust, the researchers considered the timing of when people started smoking.
“Smoking typically comes before the mental illness. In fact, a long time before,” Speed said. “On average, people from the dataset began smoking at the age of 17, while they were typically not admitted to hospital with a mental disorder until after the age of 30,” he added.
Speed and his colleagues uncovered a genetic link between smoking and mental illness — genes, they found, play a part in whether someone becomes a smoker.
“There are a number of genetic variants that we can refer to as ‘smoking-related genes.’ The people in the dataset who carried the smoking-related genes but did not smoke were less likely to develop mental disorders compared to those who carried the genes and smoked,” Speed explained.
He added, “In this study, we demonstrate that it’s probable that the risk of starting to smoke causes the risk of developing mental disorders to increase due to the ‘smoking-related genes.’”
But now that the study, published in Acta Psychiatrica Scandinavica, has found that smoking can cause mental illness, the question of how smoking can do that remains unanswered.
“We still need to find the biological mechanism that causes smoking to induce mental disorders. One theory is that nicotine inhibits absorption of the neurotransmitter serotonin in the brain, and we know that people with depression don’t produce enough serotonin,” Speed said.
Nicotine in a cigarette activates the production of serotonin in the brain, which in part makes a smoker feel relaxed after smoking.
But after being consumed over a period of time, nicotine doesn’t help produce serotonin — it inhibits it, causing anxiety and emotional disturbance.
“Another explanation could be that smoking causes inflammation in the brain, which in the long term can damage parts of the brain and lead to various mental disorders. But as I said: We don’t know for sure as yet,” Speed said.