Chronic functional constipation. Which supplementary fiber to choose? [editorial]

Dietary fiber (DF) increases fecal weight by mechanisms that depend on the structure of its components and on the intricate relationship between its physico-chemical properties (water-holding capacity solubility particle size degree of lignification pentose concentrations etc.) and the bacterial population of the colon. As the insoluble components of the fiber resist bacterial breakdown they are eliminated intact and keep the retained water generating high fecal weight. During the colonic transit this large bulk stimulates propulsive contractions and transit time being shortened there might be less water reabsorption feces becoming wetter. In contrast 90-100% of the soluble fibers (but just 30-80% of the insoluble) are fermented by the intestinal flora liberating the retained water and producing shortchain fatty acids. These acids are absorbed generating water absorption and electrolyte co-absorption which together with the absorption of the liberated water tends to reduce fecal water content. However fermentation is also a great stimulus for bacterial mass increase which making up around 50% of dry feces contributes to fecal weight. There are indications that fermentation increases with prolonged fiber consumption since the bacterial flora polysaccharidases are inducible. Summing up the main characteristics required for good laxation are attributed to the insoluble fraction of fiber the soluble fraction playing a contributory role although it does fulfill other important metabolic functions. In addition excess fermentation has undesirable clinical effects. (excerpt)