BACKGROUND AND AIM: Partially hydrolyzed guar gum (PHGG) is a water-soluble, non-gelling dietary fiber with a wide range of uses in clinical nutrition. The aim of this prospective study was to investigate the effect of guar gum on colonic transit time (CTT) and symptoms of chronic constipation.METHODS: We enrolled patients fulfilling Rome III criteria for chronic constipation. CTT was measured before and at the end of treatment. After a 2-week run-in period, patients received 5 mg PHGG daily for 4 weeks. During study period, patients kept daily symptoms, stool and laxative usage diaries. They also recorded their symptom-related satisfaction weekly and treatment adverse events.RESULTS: Forty-nine patients received treatment; 39 (80 %) completed the study. Treatment significantly reduced colon transit time, from 57.28 ± 39.25 to 45.63 ± 37.27 h (p = 0.026), a reduction more prominent in slow transit patients (from 85.50 ± 27.75 to 63.65 ± 38.11 h, p = 0.016). Overall, the weekly number of complete spontaneous and spontaneous bowel movements increased significantly (p < 0.001); the latter correlated significantly with the acceleration of CTT in the overall population and in slow transit patients (B = 0.382; p = 0.016 and B = 0.483; p = 0.023, respectively). In addition, the number of bowel movements with straining decreased (p < 0.001) and stool form improved (p < 0.001), while days with laxative intake and days with abdominal pain decreased (p = 0.001 and p = 0.027, respectively).CONCLUSION: Four-week PHGG use accelerates colon transit time in patients with chronic constipation, especially in those with slow transit, and improves many of their symptoms including frequency of bowel movements.