Insoluble fibers, such as cellulose and lignin, are mostly unfermentable by colonic microflora and increase fecal bulk by their particle formation and water-holding capacity. Wheat bran is among the best bulking agents. Some fermentable hemicellulose fibers, including cabbage, increase fecal bulk by increasing fecal flora. In contrast, highly fermentable fibers, such as pectin, have little effect on fecal bul