Comment in Aliment Pharmacol Ther. 2019 Nov;50(10):1137-1138. Aliment Pharmacol Ther. 2020 Feb;51(3):405. BACKGROUND: Obesity is associated with increased risk for various gastrointestinal and liver diseases. However, the relationship between obesity and abnormal bowel habits is poorly understood. AIM: To investigate the relationship between body mass index (BMI) and bowel habit, controlling for clinical, demographic and dietary factors, in a representative sample of the United States adult population METHODS: Data were extracted from the 2009-2010 National Health and Nutrition Examination Survey. Survey responses were included in this study if respondents completed the bowel health questionnaire (BHQ), were ≥20 years of age, and did not report history of IBD, celiac disease or colon cancer. BMI was divided into the following categories: underweight, normal weight, overweight, obese and severely obese. Stepwise logistic regression provided risk ratios of constipation and diarrhoea controlling for confounding factors (dietary, life-style, psychological and medical). RESULTS: A total of 5126 respondents completed the BHQ, had BMI data available, and met eligibility criteria. Of these, 70 (1.40%) were underweight, 1350 (26.34%) were normal weight, 1731 (33.77%) were overweight, 1097 (21.40%) were obese and 878 (17.13%) were severely obese. Up to 8.5% of obese and 11.5% of severely obese individuals had chronic diarrhoea, compared to 4.5% of normal weight individuals. Stepwise regression revealed that severe obesity was independently associated with increased risk of diarrhoea. CONCLUSION: Obesity is positively associated with chronic diarrhoea in a nationally representative US adult population after adjusting for several known confounding factors. © 2019 John Wiley & Sons Ltd.