Objective: Previous studies have shown an association between cigarette smoking and coeliac disease, but it has yet to be established whether this relationship is causal. The aim of this study was to assess causality using the Bradford Hill criteria.
Methods: A matched case-control study using a questionnaire to establish a detailed smoking history for 138 incident cases of adult coeliac disease and 276 age-matched and sex-matched controls. Subjects were categorized according to their active cigarette exposure prior to diagnosis of the matched case, and odds ratios and tests for linear trends were calculated.
Results: At the time of diagnosis, 10% of cases and 30% of controls were current smokers (odds ratio, 0.21 and 95% confidence interval, 0.11-0.40 for coeliac disease in current smokers versus never smokers). A biological gradient was demonstrated for total, recent and current cigarette exposure. The greatest risk reduction related to current exposure (odds ratio, 0.15, and 95% confidence interval, 0.06-0.37 for coeliac disease in current heavy smokers versus never smokers).
Conclusions: This study strengthens the case for a causal relationship between smoking and coeliac disease by demonstrating a strong, temporally appropriate and dose-dependent effect, thus meeting the Bradford Hill criteria. This suggests that cigarette smoking truly protects against the development of adult coeliac disease.