Current heavy smokers have a more than threefold increased risk for major depression, compared to former heavy smokers.
And while the link between smoking and depression is well documented, the finding adds another twist to the debate between so-called "shared-vulnerability" and causal hypotheses about smoking and depression.
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"Our findings are consistent with the view that the heavy-smoking-to-major depression pathway is causal in nature, rather than mainly due to confounding by shared vulnerability factors," wrote Dr. Salma Khaled and her associates.
"Under the shared-vulnerability hypothesis, ever-heavy smokers may be expected to have similar elevated risk for major depressive episode irrespective of their smoking status during follow-up," wrote Salma Khaled, Ph.D., in the April issue of the Journal of Psychiatric Research.
"Our results point to the contrary."
Dr. Khaled, who was at the Mental Health Center for Research and Teaching, Canada in Toronto at the time of this research, and colleagues looked at a total of 3,824 adults from the Canadian National Population Health Survey. Participants in the survey completed a baseline interview between 1994 and 1995 and were prospectively followed since then, with new interviews conducted every second year through 2006-2007.
To be included in the study, subjects had to have maintained their smoking status as current, former, and never smokers throughout the survey follow-up duration. "Heavy" smokers were defined as those subjects who smoked 20 or more cigarettes per day (J. Psychiatr. Res. 2012;46:436-43).
"Ever-heavy smokers (current and former) may share similar genetic, behavioral, and environmental vulnerabilities, at least for heavy smoking initiation," according to Dr. Khaled, who is now at the University of Calgary (Alta.), and her associates.
If these factors were wholly to blame for depression – as dictated by the shared vulnerability hypothesis – then we would expect former-heavy smokers and current smokers would have an equal likelihood of having a major depressive episode (MDE), she reasoned.
"However, if the persistence of the exposure (current as opposed to former) had the dominant effect on the risk for MDE, then current-heavy smokers would be expected to have higher risks of MDE relative to former-heavy smokers."
Overall, the 12-year risk of MDE for the entire sample was 13.2% (95% confidence interval, 11.8-14.6), the authors found.
Stratified by smoking status, the risk of MDE among current-heavy smokers was 26.7%; among former-heavy smokers it was 7.1%, and among those who never smoked it was 12.2%.
That amounted to a significant hazard ratio of 3.1 for current heavy smokers, compared with former smokers, even after adjustment for age, sex, and stress (P less than .001).
Moreover, the hazard ratios for MDE among former-heavy smokers, compared with current smokers, decreased incrementally according to time passed since smoking cessation, from 0.5 for those who quit for between 1 and 5 years ago (P less than .05) to a hazard ratio of 0.2 among those who quit smoking greater than 21 years ago (P less than .001).
"Our findings are consistent with the view that the heavy-smoking-to-major depression pathway is causal in nature, rather than mainly due to confounding by shared vulnerability factors," noted Dr. Khaled and her associates.
Nevertheless, she acknowledged that "shared vulnerability factors including genetic vulnerability in the context of smoking and depression may not be limited to smoking initiation and heavy smoking onset, but may also influence the ability to quit smoking and maintain smoking cessation."
The authors declared no conflicts of interest in relation to this study, which they said was supported by the Canadian Institutes of Health Research.