Abstract Background Previous epidemiological studies observed inconsistent effects of carbohydrate intake on incident inflammatory bowel disease (IBD). We aimed to evaluate the effects of glycemic index and glycemic load, the two main indicators to measure the quality and quantity of carbohydrates, on the risk of two main subtypes of IBD, Crohn’s disease (CD) and ulcerative colitis (UC). Methods This analysis included 121,148 participants who were free of IBD at baseline in the UK Biobank. We collected dietary information from a validated web-based 24-hour dietary recalls questionnaire and estimated dietary glycemic index and glycemic load based on carbohydrate foods. The primary outcome was the incident CD and UC. Cox proportional hazard models were used to calculate multivariable-adjusted hazard ratios (HRs) and 95% confidence intervals (CIs). Substitution analyses were conducted to test associations with risk for CD and UC after replacing medium or high glycemic index foods with low glycemic index foods. Results In the current analysis, participants with lowest quartile of glycemic index were more likely to be females, and with lower BMI, and those with highest quartile of glycemic load were more likely to be none to moderate alcohol consumers and non-smokers. During a median follow-up of 10.6 years, 133 incident CD and 335 incident UC cases were identified. In the fully multivariable-adjusted model, a 13% (95% CI 1%-27%) higher risk of UC was associated with each 1-SD increment of dietary glycemic index. Comparing the lowest glycemic index, we found that highest dietary quartile of glycemic index was associated with increased risk of UC (HR 1.46, 95%CI 1.07-1.99, P = 0.016; P for trend = 0.005), but not with CD. We did not find significant associations between dietary glycemic load with risk of CD and UC. Additionally, replacing medium or medium and high glycemic index foods with low glycemic index foods was associated with a lower risk of UC, respectively. Conclusion Conclusion: Our findings provide evidence that a higher dietary glycemic index, but not glycemic load, was associated with an increased risk of UC, further demonstrated by the substitution analyses, suggesting the importance of dietary glycemic index in dietary recommendations for UC prevention. Therefore, the large interventional trial should be considered to evaluate the role of low glycemic index diets in preventing UC and expected to be applied in dietary recommendations for UC prevention.
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