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)

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.

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.

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.

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.