Researchers have given people another reason to quit smoking, by finding that regular smokers may have a higher risk of developing depression in comparison to those who never smoke.
The study also found that while smokers who have quit have an elevated risk of depressive symptoms in short run, this risk declines to the level of never smokers in the long run.
In other words, both completely smoke-free life style and successful smoking cessation in long run seem to protect from depressive symptoms.
Though it is known that depression is associated with cigarette smoking, the nature of this association is discussed under various hypotheses. The first is that those who suffer from depressive symptoms smoke cigarettes in order to alleviate their symptoms.
The second is that chronic persistent smoking may have a role in the etiology of depression. The third hypothesis suggests that there is a reciprocal mechanism between smoking and depression, and the fouth says that there are shared underlying genetic factors explaining this co-morbidity.
This study conducted in the Department of Public Health at the University of Helsinki explored, which of those assumptions would be supported by the data, when smoking behaviour and changes in it is considered as a predictor of depressive symptoms.
The researchers had access to the data collected within the Finnish Adult Twin Cohort Project. There were about 4,000 male and 5,000 female twins, whose health and health behaviour were followed-up through 15 years.
Data on smoking behaviour and changes in it between 1975 and 1981 were analyzed as a predictor of depressive symptoms measured in 1990. The analyses were adjusted for other factors known to predict depression.
Because the data consisted of twins it was possible to test the causality between smoking and depression by using twin pairs discordant for depression, where the twin without depression served as a matched control for his/her co-twin with depression. Additionally, it was possible to explore potential shared genetic influences underlying the association.
The results suggest that first, persistent chronic cigarette smoking predicts depressive symptoms. However, when adjusted for other factors associated with depression, the elevated risk of persistent smoking remained significant among men only.
"The result that chronic smoking may actually have a role in the etiology of depression, may be surprising, as nicotine as such - in short term - is assumed to have some positive effects on mood", says Dr. Tellervo Korhonen from the Department of Public Health at the University of Helsinki.
"We should look for explanation to our result from long term effects of cigarette smoking, from addiction mechanisms and from other substances than nicotine within tobacco smoke," Dr Korhonen added.
The research is published in the scientific journal Psychological Medicine.