A study of the protective association 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 significant roles in the pathogenesis of inflammatory bowel disease. This study investigates the associations between hard water exposure and the prevalence and incidence of inflammatory bowel diseases, including Crohn’s disease and ulcerative colitis.

METHODS

A cross-sectional and cohort study was conducted using 481,468 participants in the UK Biobank. Logistic regression was used to examine associations between hard water exposure and disease prevalence at baseline, while Cox regression assessed associations with incident cases during follow-up.

RESULTS

A total of 481,468 participants were included in the cross-sectional study. Higher levels of hard water exposure were associated with reduced prevalence of Crohn’s disease (OR = 0.88, 95% CI: 0.79–0.99,  = 0.027, and  = 0.039 for the trend) and ulcerative colitis (OR = 0.83, 95% CI: 0.77–0.89,  < 0.001, and  < 0.001 for the trend). 475,895 participants with a mean follow-up of 14.6 years were included in the cohort study. Hard water was significantly associated with a decreased risk of Crohn’s disease (HR = 0.84, 95% CI: 0.73–0.97,  = 0.016, and  = 0.007 for the trend) but not ulcerative colitis. We also observed a stronger inverse association between very hard water and Crohn’s disease among individuals with low genetic risk. (HR = 0.74, 95% CI: 0.57–0.95,  = 0.019).

CONCLUSION

Hard water is linked to a lower prevalence of both CD and UC, as well as a lower incidence of CD. However, no significant association was observed with the incidence of UC.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s12876-025-04230-1.