Current every day smokers have a more than triple increased risk for big depression, in comparison with ex-smokers. And while the link between smoking and depression is fully supported, the new finding adds another weave to the discussion between “shared-brittleness” and causal hypotheses about smoking cigarettes and depression.
“Our recent findings are compatible with the view that the heavy smoking to big depression pathway is causative in nature, rather than principally due to confounding by shared vulnerability factors,” reported Dr. Salma Khaled and her colleagues.
“Under the shared-vulnerability hypothesis, heavy smokers may be supposed to have the same raised risk for big depressive episode free of their smoking habit status during follow-up,” added Salma Khaled, Ph.D., in the April issue of the Journal of Psychiatric Research.
Dr. Khaled, who was at the Mental Health Center for Investigation and Teaching, Canada in Toronto at the time of this study, and colleagues looked at 3,824 smoking adults from the Canadian National Population Health Research. Participants in the study completed an interview between 1994 and 1995 and were potentially followed since then, with new interviews conducted every second year through 2006-2007.
To be included in the study, investigated people had to have maintained their smoking status as current, former, and never smokers throughout the study follow-up duration. “Heavy” smokers were defined as those subjects who smoked 20 or even more cigs per day.
“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 all factors were completely to censure 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 smokers.”
In general, the 12-year risk of MDE for the whole sample was 13.2%, the authors discovered.