The role of environmental factors in the risk for Crohn's disease (CD), an inflammatory bowel disease (IBD), was investigated in a North Island-based New Zealand case-control cohort. A total of 315 CD patients and 536 controls were recruited through various sources to the Auckland CD Risk Factor Study. As well as demographic characteristics, the self-reported questionnaire included (1) smoking and drinking alcohol, (2) breastfeeding in infancy, (3) early life exposures to allergens and microbes, (4) health conditions lasting 6 months or longer and (5) taking antibiotics and any medications. There was strong evidence for familial associations of the disease, and minor effects of birth order and number of siblings. Being a smoker, especially over a long time period, and exposure to smoking during childhood and adolescence periods increased risk, whereas drinking alcohol at least once per week showed a slight protective effect. Long term use of the oral contraceptive pill increased the risk of developing CD, but breastfeeding and immunisation during infancy showed no significant association. Long term and debilitating illness (lasting 6 months or more), taking antibiotics prior to developing CD, or taking four or more antibiotics or any regular medication in a year during adolescence substantially increased the CD risk. Having a pet during childhood was a protective factor, but regularly feeding an animal was not sufficient to protect. Many of these significant factors are likely to impact on the colonic microflora and/or immune system. We conclude that, in addition to strong evidence for genetic associations, factors likely to impact on immune response or reduce early exposure to microbes provide a main risk factor for CD in this New Zealand population.
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