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AARHUS, Denmark — By now, it’s pretty obvious that smoking cigarettes is unhealthy, simply based on its link to lung cancer alone. However, a new study finds a consequence of smoking that many people may not know about is an increase in the risk of developing mental illness.

Recently, evidence has been mounting which demonstrates a strong relationship between smoking and mental illness. Still, researchers are debating on whether smoking specifically causes depression or other types of disorders, or if people simply smoke for relief from disorders they already have. Now, researchers from Aarhus University are showing that smoking can indeed raise depression risk, as well as bipolar disorder, by over 100 percent.

“The numbers speak for themselves. Smoking does cause mental illness. Although it’s not the only cause, smoking increases the risk of being hospitalized with a mental illness by 250 percent,” says Doug Speed, professor at the Center for Quantitative Genetics and Genomics at Aarhus University, in a media release.

“Smoking typically comes before the mental illness. In fact, a long time before. On average, people from the data set 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 adds.

To conduct this work, Speed and his colleagues needed to collect lots of data. Mental health is complex, and there can be more than one reason for developing a disorder. For this reason, it was important for the team to have as much data as possible to rule out other things that could impact their findings. They used the UK Biobank, one of the largest databases in the world for health information, which contains genetic data on over half a million people. This data was paired with other lifestyle information provided by the participants to get the full picture.

As many as 90 percent of the people in the data set who were still smokers or former smokers began doing so before turning 20. The likelihood of starting smoking after this age is significantly lower. Speed adds that your genes can help determine if you will become a smoker or not.

“When we looked at the many smokers in the database, we found a number of recurring genetic variants. By looking at twin studies, in which the twins had the same genes but grew up in separate homes, we could see that their genes could explain 43 percent of the risk of becoming a smoker.”

cigarette smoke
(Credit: Lukas from Pexels)

The team also found that in homes where adoptive parents were smokers, the likelihood that the child would smoke increased. Unsurprisingly, if the parents didn’t smoke, the risk was lower. However, if the child’s biological parents were smokers, the risk was still higher because of the genes passed on.

“There are a number of genetic variants that we can refer to as ‘smoking-related genes’. The people in the data set 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 says.

“Because the genetic variants also seem to be linked with the risk of mental illness, this used to be a bit blurry. But 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.’”

Statistically, Speed and the team found a correlation here, but they couldn’t explain why they noticed what they did. One possible reason is that nicotine induces brain damage.

“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,” the researcher adds.

Nicotine activates the production of serotonin in the brain, which is partly why smoking can relax you. However, chronic smoking can cause the opposite effect by inhibiting serotonin and making you more anxious and mentally unstable.

“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 says.

Since this research shows that people hardly ever start smoking cigarettes after the age of 20, the team poses the idea that it could be beneficial to raise the age limit for buying them.

“This could be a good way to prevent people from starting smoking. Again, we don’t know why people don’t start smoking after the age of 20, but perhaps it’s because we become less and less willing to take risks with age,” says Speed.

This study offers much more expansive knowledge on smoking and mental health, but a key limitation of the research is that it includes U.K. participants and not Danish people. Despite this, Speed thinks the differences would be minimal.

“Denmark and the UK are very similar, and I would say that they are quite comparable. Having said that, our next step is actually to conduct the same study with figures from Denmark and Finland. However, getting access to this data is more expensive, which is why we did a pilot study with the British data to see if there was a correlation,” the study author concludes.

The findings are published in the journal Acta Psychiatrica Scandinavica.

About Shyla Cadogan, RD

Shyla Cadogan is a DMV-Based acute care Registered Dietitian. She holds specialized interests in integrative nutrition and communicating nutrition concepts in a nuanced, approachable way.

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1 Comment

  1. PJ London says:

    And so the piling on of BS continues.
    They proved that smoking causes cancer in exactly the same way that they proved that coffee, sugar, and potatoes cause cancer. Just keep on increasing the uptake until the subject dies.
    What a curious person may find curious is that even though smoking has decreased by enormous numbers, no one hails the victory by publishing the equally enormous decrease in deaths from lung cancer! Do you wonder why? Me too!
    What is proven is that smoking increases cognitive ability, this has never been disputed. Unsurprisingly, the better your cognitive ability the more likely you are to be depressed. Sometimes, even in the midst of your depression, you find a way to be happy, this is called “A bipolar disorder”.
    “The real hopeless victims of mental illness are to be found among those who appear to be most normal. Many of them are normal because they are so well adjusted to our mode of existence, because their human voice has been silenced so early in their lives, that they do not even struggle or suffer or develop symptoms as the neurotic does.” They are normal not in what may be called the absolute sense of the word; they are normal only in relation to a profoundly abnormal society. Their perfect adjustment to that abnormal society is a measure of their mental sickness. These millions of abnormally normal people, living without fuss in a society to which, if they were fully human beings, they ought not to be adjusted.” –
    Aldous Huxley – Brave New World Revisited

    “The supreme trick of mass insanity is that it persuades you that the only abnormal person is the one who refuses to join in the madness of others, the one who tries vainly to resist. We will never understand totalitarianism if we do not understand that people rarely have the strength to be uncommon.” — Eugene Ionesco

    As long as they want you to be stupid, the war on smoking will continue.