Smoking May Double Risk Of Depression And Bipolar Disorder

“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. “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.

According to the researchers to investigate these correlations, health data from 350,000 people who were enrolled in the UK Biobank, paired with health information and answers provided regarding their lifestyle was fed into a computer to look for patterns. As much as 90% of the participants who were still smokers or former smokers began before the age of 20. Since people rarely start smoking after reaching 20 years old, the researchers suggest that it may be a good idea to look into increasing the age limit for buying cigarettes.

“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,” said 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."

"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,” says Speed: "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.”

"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,” said Speed. “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.”

"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," concludes Speed.