BACKGROUND: Knowledge about adverse symptoms over time from fiber supplementation is lacking. PURPOSE: The aim of this study was to compare the severity of adverse gastrointestinal (GI) symptoms during supplementation with dietary fiber or placebo over time in adults with fecal incontinence. A secondary aim was to determine the relationship between symptom severity and emotional upset and their association with study attrition and reducing fiber dose. METHODS: Participants (N = 189; 77% female; 92% White; age, M = 58 years, SD = 14 years) with fecal incontinence were randomly assigned to a placebo or a supplement of 16 g total dietary fiber per day from 1 of 3 sources: gum arabic, psyllium, or carboxymethylcellulose. They reported GI symptoms daily during baseline (14 days), incremental fiber dosing (6 days), and 2 segments of steady full fiber dose (32 days total). RESULTS: Severity of symptoms in all groups was minimal. Adjusting for study segment and day, a greater feeling of fullness in the psyllium group was the only symptom that differed from symptoms in the placebo group. The odds of having greater severity of flatus, belching, fullness, and bloating were 1.2-2.0 times greater in the steady dose segment compared with baseline. There was a positive association between symptom severity and emotional upset. Participants with a greater feeling of fullness or bloating or higher scores for total symptom severity or emotional upset were more likely to withdraw from the study sooner or reduce fiber dose. CONCLUSIONS: Persons with fecal incontinence experience a variety of GI symptoms over time. Symptom severity and emotional upset appear to influence fiber tolerance and study attrition. Supplements seemed well tolerated.