Smoking cigarettes doubles the risk of developing depression, bipolar disorder

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.

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