Introduction There are plausible biological mechanisms for how obesity may be involved in the development of inflammatory bowel disease (IBD) due to the pro-inflammatory cytokines synthesised by adipose tissue including TNF-α. The aim of this work was to conduct the first prospective cohort study of obesity in the aetiology of IBD.

Methods The cohort consisted of 366 351 healthy men and women aged 30–74 years from centres in Europe (EPIC—European Prospective Investigation into Cancer) recruited between the years 1991 and 1998. At recruitment participants' weight, height and energy intake were measured and physical activity from questionnaires and their body mass index (BMI) calculated. The cohort was monitored to 2004 to identify those who developed Crohn's disease (CD) or ulcerative colitis (UC). Each diagnosis was medically confirmed via review of the medical notes. Each case was matched with four controls, for gender, age at recruitment and centre. BMI was divided into five categories. Conditional logistic regression was used to calculate OR and 95% CI adjusted for smoking.

Results In the cohort, 102 participants developed incident CD (73% female, mean age of 55.9 years, SD=11.0 years) and 213 participants developed incident UC (61% female, mean age of 57.5 years, SD=10.8 years). There were no associations between the four higher categories of BMI and the development of CD (Trend=1.00, 95% CI 0.79 to 1.27, p=0.91) or UC (Trend=1.03, 95% CI 0.83 to 1.29, p 0.76). Dividing BMI in two categories.

Conclusion No associations were found between BMI and either CD or UC. Further studies are required to confirm these results. If confirmed then BMI does not need to be measured in future epidemiological studies of IBD.

Competing interests None declared.