Led by researchers from the University of Bristol’s Tobacco and Alcohol Research Group (TARG), the study was supported by Bristol’s MRC Integrative Epidemiology Unit (IEU) and the NIHR Bristol Biomedical Research Centre (BRC). The study authors used UK Biobank data from 462,690 individuals of European ancestry, which comprised of 8% current smokers and 22% former smokers.
The research team applied the Mendelian randomisation, an analytic approach which uses genetic variants associated with an exposure, such as exposure to cigarette smoke, in order to derive conclusions about cause-and-effect relationships. The analysis indicated that tobacco smoking increased the risk for depression and schizophrenia, and also that vice versa, depression and schizophrenia increase the likelihood of smoking.
“There was strong evidence to suggest smoking is a risk factor for both schizophrenia (odds ratio (OR) 2.27, 95% confidence interval (CI) 1.67–3.08, p<0.001) and depression (OR 1.99, 95% CI 1.71–2.32, p<0.001). Results were consistent across both lifetime smoking and smoking initiation. We found some evidence that genetic liability to depression increases smoking (β = 0.091, 95% CI 0.027–0.155, p = 0.005) but evidence was mixed for schizophrenia (β = 0.022, 95% CI 0.005–0.038, p = 0.009) with very weak evidence for an effect on smoking initiation,” read the study Abstract.
Smoking linked to bipolar disorder
Earlier this year, the same research team published a study in the British Journal, indicating that tobacco smoking increases the risk for bipolar disorder. In line with these findings, in 2016, the UK government’s mental health task force had recommended that psychiatric hospitals should be smoke free by 2018.
“Individuals with mental illness are often overlooked in our efforts to reduce smoking prevalence, leading to health inequalities. Our work shows that we should be making every effort to prevent smoking initiation and encourage smoking cessation because of the consequences to mental health as well as physical health,” said Dr. Robyn Wootton, Senior Research Associate in the School of Experimental Psychology and the study’s lead author.
Senior study author and Professor of Biological Psychology in Bristol’s School of Psychological Science, Marcus Munafò, added: “The increasing availability of genetic data in large studies, together with the identification of genetic variants associated with a range of behaviours and health outcomes, is transforming our ability to use techniques such as Mendelian randomisation to understand causal pathways. What this shows is that genetic studies can tell us as much about environmental influences—in this case the effects of smoking on mental health—as about underlying biology.”
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