Lactose sensitivity and malabsorption in functional dyspepsia and functional dyspepsia-irritable bowel syndrome overlap.

INTRODUCTION: Lactose intolerance is frequently suspected in disorders of gut-brain interaction, yet symptoms often do not align with objective malabsorption. While lactose-related symptoms have been investigated in irritable bowel syndrome, data on functional dyspepsia, particularly patients with overlapping irritable bowel syndrome, remain limited. This study examined the relative contribution of lactose sensitivity and lactose malabsorption to symptom burden, and evaluated whether functional dyspepsia-irritable bowel syndrome overlap is associated with greater symptom severity than functional dyspepsia alone. METHODS: In this cross-sectional study, adults underwent standardized lactose hydrogen breath and symptom testing. Diagnoses followed Rome IV criteria. Lactose malabsorption was defined as ≥20 ppm hydrogen increase; lactose sensitivity as ≥20 mm rise on the adult Carbohydrate Perception Questionnaire. Group differences were tested non-parametrically; adjusted effects used a general linear model with age and gender. RESULTS: Among 587 patients, lactose sensitivity was more prevalent than lactose malabsorption (54.9% versus 41.2%; p < 0.001) and occurred more frequently in patients with functional dyspepsia-irritable bowel syndrome overlap than in those with functional dyspepsia alone (69.8% versus 52.7%; p < 0.001). Patients with overlap exhibited the highest symptom burden (p < 0.001). In multivariate analysis, lactose sensitivity and younger age were independently associated with functional dyspepsia-irritable bowel syndrome overlap, whereas lactose malabsorption and gender were not. CONCLUSION: Lactose sensitivity, not malabsorption, was closely associated with symptom burden in patients with functional dyspepsia. Functional dyspepsia-irritable bowel syndrome overlap was linked to amplified burden, underscoring the importance of validated symptom assessment alongside breath testing for diagnostic precision.