Interventions: Patients will be assigned to experimental (biofeedback for functional outlet obstruction) and active comparator (fiber supplementation) arms. Biofeedback for functional outlet obstruction: sessions of biofeedback guided by anorectal manometry (performed during the first 3 weeks of the intervention period) combined with instructions for daily exercising for 4 weeks. Fiber supplementation: 2.5 g plantago ovata per day for 4 weeks. Main outcome and measures. Clinical symptoms of functional dyspepsia measured by daily questionnaires for 7 consecutive days before and during the last week of intervention.