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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. AIMS. 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 (NHANES). 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 inflammatory bowel disease, 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 5,126 respondents completed the BHQ, had BMI data available, and met eligibility criteria. Of these, 70 (1.40%) were underweight, 1,350 (26.34%) were normal weight, 1,731 (33.77%) were overweight, 1,097 (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. CONCLUSIONS. In conclusion, this study provides strong evidence that obesity is positively associated with chronic diarrhoea in a nationally representative US adult population after adjusting for several known confounding factors