Dietary fiber supplements are widely used as a first-line treatment for constipation although little Level 1 evidence exists to support its use in adults. The effectiveness of different fiber supplements is difficult to compare given wide differences in water-retention capabilities and effects on the colonic microbial ecology. Given the promising, yet largely unsubstantiated, benefits of fiber supplementation on symptoms of functional constipation, the objective of this clinical trial is to evaluate tolerance and effectiveness of four week supplementation of a fiber product, in a dose-ranging fashion, on whole gut transit time and gastrointestinal symptoms in adults with functional constipation.