Dietary fiber is plant-derived material that is resistant
to digestion by human alimentary enzymes. Fiber may be
divided into two broad chemical classes: 7) non-a-glucan polysaccharides
(cellulose, hemicelluloses, and pectins) and 2) lignins.
Dietary fiber behaves within the gastrointestinal tract as
a polymer matrix with variable physicochemical properties including
susceptibility to bacterial fermentation, water-holding
capacity, cation-exchange, and adsorptive functions. These
properties determine physiological actions of fiber and are dependent
on the physical and chemical composition of the fiber.
Fiber undergoes compositional changes as a consequence of
bacterial enzymatic action in the colon. Dietary fiber is of clinical
significance in certain disorders of colonic function and in
glucose and lipid metabolism. Dietary fiber increases stool bulk
by acting as a vehicle for fecal water and by increasing fecal
bacterial volume. Use of fiber in the treatment of constipation
and uncomplicated diverticular disease is well established. By
increasing stool bulk, fiber also reduces the fecal concentration
of bile acids and other substances. Certain types of fiber decrease
the rate of glucose absorption and attenuate postprandial rises
in blood glucose and insulin. Plasma cholesterol levels are reduced
by mucilaginous forms of fiber. This effect appears to
be mediated in part by an increase in fecal acidic sterol excretion.-