Risk of inflammatory bowel disease attributable to smoking, oral contraception and breastfeeding in Italy: a nationwide case-control study. Cooperative Investigators of the Italian Group for the Study of the Colon and the Rectum (GISC)

BACKGROUND: Using data from a case-control study carried out in Italy 1989-1992, we estimated the odds ratios (OR) and the population attributable risks (AR) for inflammatory bowel diseases (IBD) in relation to smoking, oral contraception and breastfeeding in infancy. METHODS: The study focused on 819 cases of IBD (594 ulcerative colitis: UC; 225 Crohn's disease: CD) originating from populations resident in 10 Italian areas, and age-sex matched paired controls. RESULTS: Compared with non-smokers, former smokers were at increased risk of UC (OR = 3.0; 95% confidence interval CI: 2.1-4.3), whereas current smokers were at increased risk of CD (OR = 1.7; 95% CI: 1.1-2.6). Females who reported use of oral contraceptives for at least one month before onset of symptoms had a higher risk of CD (OR = 3.4; 95% CI: 1.0-11.9), whereas no significant risk was observed for UC. Lack of breastfeeding was associated with an increased risk of UC (OR = 1.5; 95% CI: 1.1-2.1) and CD (OR = 1.9; 95% CI: 1.1-3.3). Being a 'former smoker' was the factor with the highest attributable risk of UC both in males (AR = 28%; 95% CI: 20-35 %) and in females (AR = 12%; 95% CI: 5-18%). Smoking was the factor with the highest attributable risk for CD in males (AR = 31%; 95% CI: 11-50%). Lack of breastfeeding accounted for the highest proportion of CD in females (AR = 11%; 95% CI: 1-22%). Oral contraceptive use accounted for 7% of cases of UC and for 11% of cases of CD. CONCLUSIONS: Taken together, the considered factors were responsible for a proportion of IBD ranging from 26% (CD females) to 36% (CD males). It is concluded that other environmental and genetic factors may be involved in the aetiology of IBD.