New Research Links Smoking to Greatly Increased Risk of Mental Illness

A new study indicates a link between smoking and an increased risk of mental illness. Utilizing data from the UK Biobank, the research suggests smoking may contribute to depression and bipolar disorder, with genetic factors playing a significant role. The findings imply a potential impact of nicotine on mental health and raise considerations for policy changes in cigarette sales.

Most of us know that smoking is unhealthy. Cigarette packets bear graphic warnings of physical side effects like diseased lungs and rotting teeth, and the media often reports on the association between smoking and various cancers.

However, you might not know that smoking may also increase your risk of developing mental illnesses.

Together with two colleagues from Canada, Doug Speed from the Center for Quantitative Genetics and Genomics at Aarhus University has shown that smoking can probably lead to depression and bipolar disorder.

“The figures in our study show that smoking might increase the risk of being hospitalized with a mental illness. It does not mean, however, that smoking is the only cause, but it does look like smoking increases that risk by 250 percent”, he says.

For Doug Speed and his colleagues to be able to answer whether smoking can cause mental disorders, they needed very large volumes of data. There can be many different reasons why we develop a mental disorder. It was therefore important that they had enough data to clean their figures from other possible effects.

They gained access to the UK Biobank, one of the largest databases in the world of human health information. The database contains genetic data on more than half a million people. The genetic data was paired with a lot of other health information and answers provided by the participants regarding their lifestyle.

They fed the data into a computer and began looking for patterns. Doug Speed and his colleagues are far from the first researchers to investigate this correlation, but they found a new way to do it, as he explains.

“Previous research hasn’t really considered that there may be a temporal dimension at play. People typically start to smoke before the age of 20, but aren’t admitted to hospital with a mental disorder until they’re between 30 and 60 on average.”

“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.”

As many as 90 percent of the people in the data set who were still smokers or former smokers started before the age of 20. The likelihood that you will start smoking later in life is therefore quite small. In fact, your genes help determine whether or not you will become a smoker, explains Doug Speed.

“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.”

In the homes where the adoptive parents also smoked, the risk of the child starting to smoke increased. However, if the parents didn’t smoke, the risk was lower, but still greater when the children’s ‘real’ parents had been smokers and passed on certain genes.

“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,” he says and continues:

“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’”.

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