Dietary . fiber. is actually a mixture of soluble and insoluble plant polysaccharides. Soluble fibers like oat and psyllium absorb a lot of water and slow the release of nutrients into circulation. Insoluble fibers like wheat bran, soy, and other grain fibers, scour the intestinal lining, increasing surface area for absorptions. Which type to use depends on what sort of digestive symptoms are present.