The Protective Effect of Hard Water on Ulcerative Colitis and Crohn's Disease: A Cross-Sectional and Cohort Study of 481,468 Participants in the UK Biobank

Background Environmental factors play a significant role in the pathogenesis of inflammatory bowel disease. This study aims to investigate the association between water hardness and both the prevalence and incidence of Crohn’s disease and ulcerative colitis. Methods Cross-sectional study and cohort study were respectively conducted based on 481,468 participants in the UK Biobank. Multivariate logistics regression and Cox proportional hazards regression were used to evaluate the association between water hardness and Crohn’s disease and ulcerative colitis. Results A total of 481,468 participants were included in the cross-sectional study. Increased water hardness was correlated with reduced prevalence of Crohn’s disease (OR = 0.88, 95% CI: 0.79-0.99, p = 0.027, and p = 0.039 for the trend) and ulcerative colitis (OR = 0.83, 95% CI: 0.77-0.89, p < 0.001, and p < 0.001 for the trend). A total of 475,895 participants with the mean follow-up of 14.6 years were included in the cohort study. During the follow-up period, 1033 developed Crohn’s disease and 2165 developed ulcerative colitis. Increased water hardness was significantly associated with a decreased risk of Crohn’s disease (HR = 0.84, 95% CI: 0.73-0.97, p = 0.016, and p = 0.007 for the trend) but not ulcerative colitis. We also found that the protective effect of very hard water is more significant for individuals with low genetic risk for Crohn’s disease (HR = 0.74, 95% CI: 0.57-0.95, p = 0.019). Conclusion Higher water hardness is associated with a decreased prevalence of both Crohn’s disease and ulcerative colitis, as well as a reduced incidence of Crohn’s disease. However, no significant association was observed with the incidence of ulcerative colitis.