The participants had evidence of chronic constipation, based on diagnostic criteria for constipation, laxative use, or self-reports.

The forms of fiber used in the supplements included fructooligosaccharides, inulin, isomaltooligosaccharides, oat bran, partially hydrolyzed guar gum, and wheat bran. The dosages ranged from 5 to 22 grams of fiber per day. The studies ran for 4 weeks to 5 months.

Taking fiber did not affect stool frequency (based on 6 studies) but did reduce laxative/enema use, with a large effect size (based on 4 studies).

Adverse events (e.g., bloating and flatulence) were higher with fiber (based on 2 studies).

Fiber supplements and constipation in older people Original paper