The principal metabolites of fermentable fibers (including any starch thats has passed into the cecum and been degraded by bacteria) are lactate and short-chain fatty acids (SCFAs), formerly called volatile fatty acids (VFAs) because of their volatility in acidic aqueous solutions. Many different fibers including pectin, gums, oat and wheat bran, and psyllium, which is mostly mucilages and nonpolysaccharides, are degraded to short chain fatty acids. The short-chain fatty acids include primarily acetic, butyric, and propionic acids. In addition to these acids, other products of fiber fermentation are hydogen, carbon dioxide, and methane gases that are exceted as flatus or are expired by the lungs.
Different fibers are fermented to different short-chain fatty acids in different amounts by different bacteria. For example, ingestion of pectin resulted in higher propionate concentrations in the colon of rats versus wheat bran, which resulted in higher butyrate concentrations. Bacteria that act on pectin include, for example :
- Bacteriodes that generate acetate, propionate, and succinate
- Eubacteria that yield acetate, butyrate, and lactate
- Bifidobacteria that produce acetate and lactate
- Increased water and sodium absorption in the colon
- Mucosal cell proliferation
- Provision of energy
- Acidification of luminal environment
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