Lactose Intolerance in Patients with Diarrhea-Predominant Irritable Bowel Syndrome in a Tertiary Care Hospital in Bangladesh

Background: Diarrhea-predominant irritable bowel syndrome (IBS-D) and lactose intolerance frequently coexist, presenting overlapping gastrointestinal symptoms that complicate diagnosis and management. This study aimed to determine the prevalence of lactose intolerance among IBS-D patients in Bangladesh and to identify associated clinical features and risk factors. Methods: A cross-sectional observational study was conducted among 392 IBS-D patients at Bangabandhu Sheikh Mujib Medical University (BSMMU). Lactose intolerance was diagnosed using the Lactose Tolerance Test (LTT). Clinical symptoms, dietary patterns, comorbidities, and risk factors were assessed using structured interviews and validated scoring systems. Statistical analysis included Chi-square tests and logistic regression. Results: Lactose intolerance was identified in 57.7% of IBS-D patients using LTT. Lactose-intolerant patients experienced significantly higher rates of abdominal pain (95.1% vs. 89.8%, p = 0.045), bloating (85.0% vs. 71.1%, p = 0.001), flatulence (77.9% vs. 60.8%, p < 0.001), and diarrhea occurring three or more times a day (67.7% vs. 49.4%, p < 0.001) compared to lactose-tolerant patients. Severe IBS symptoms were more common in the lactose-intolerant group (43.4% vs. 15.7%, p < 0.001). Significant associations were observed with family history of gastrointestinal illness (p = 0.015) and inverse association with lactose-rich diet (p < 0.001). Trends were noted for higher prevalence of PPI use, diabetes mellitus, and thyroid disorders among lactose-intolerant individuals. Conclusion: Lactose intolerance is highly prevalent among IBS-D patients in Bangladesh and is associated with more severe symptom profiles. Routine screening for lactose intolerance and individualized dietary interventions is recommended to improve clinical outcomes in this population.