Translations:Dietary fiber/42/en

1. Caloric energy of foods through a bulking effect
2. Slowing of gastric emptying time
3. A glycemic index type of action on absorption
4. A slowing of bile acid absorption in the ileum so bile acids escape through to the cecum
5. Altered or increased bile acid metabolism in the cecum
6. Indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fiber fermentation affecting the cholesterol metabolism in the liver.
7. Binding of bile acids to fiber or bacteria in the cecum with increased fecal loss from the entero-hepatic circulation.

One action of some fibers is to reduce the reabsorption of bile acids in the ileum and hence the amount and type of bile acid and fats reaching the colon. A reduction in the reabsorption of bile acid from the ileum has several direct effects.

1. Bile acids may be trapped within the lumen of the ileum either because of a high luminal viscosity or because of binding to a dietary fiber.
2. Lignin in fiber adsorbs bile acids, but the unconjugated form of the bile acids are adsorbed more than the conjugated form. In the ileum where bile acids are primarily absorbed the bile acids are predominantly conjugated.
3. The enterohepatic circulation of bile acids may be altered and there is an increased flow of bile acids to the cecum, where they are deconjugated and 7alpha-dehydroxylated.
4. These water-soluble form, bile acids e.g., deoxycholic and lithocholic are adsorbed to dietary fiber and an increased fecal loss of sterols, dependent in part on the amount and type of fiber.
5. A further factor is an increase in the bacterial mass and activity of the ileum as some fibers e.g., pectin are digested by bacteria. The bacterial mass increases and cecal bacterial activity increases.
6. The enteric loss of bile acids results in increased synthesis of bile acids from cholesterol which in turn reduces body cholesterol.

The fibers that are most effective in influencing sterol metabolism (e.g. pectin) are fermented in the colon. It is therefore unlikely that the reduction in body cholesterol is due to adsorption to this fermented fiber in the colon.

1. There might be alterations in the end-products of bile acid bacterial metabolism or the release of short chain fatty acids which are absorbed from the colon, return to the liver in the portal vein and modulate either the synthesis of cholesterol or its catabolism to bile acids.
2. The prime mechanism whereby fiber influences cholesterol metabolism is through bacteria binding bile acids in the colon after the initial deconjugation and dehydroxylation. The sequestered bile acids are then excreted in feces.
3. Fermentable fibers e.g., pectin will increase the bacterial mass in the colon by virtue of their providing a medium for bacterial growth.
4. Other fibers, e.g., gum arabic, act as stabilizers and cause a significant decrease in serum cholesterol without increasing fecal bile acid excretion.