Dietary Fiber - Soluble Fiber Fermentation

Soluble Fiber Fermentation

The American Association of Cereal Chemists has defined soluble fiber this way: "the edible parts of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine." In this definition:

edible parts of plants

indicates that some parts of a plant we eat—skin, pulp, seeds, stems, leaves, roots—contain fiber. Both insoluble and soluble sources are in those plant components.

carbohydrates

complex carbohydrates, such as long-chained sugars also called starch, oligosaccharides, or polysaccharides, are sources of soluble fermentable fiber.

resistant to digestion and absorption in the human small intestine

foods providing nutrients are digested by gastric acid and digestive enzymes in the stomach and small intestine where the nutrients are released then absorbed through the intestinal wall for transport via the blood throughout the body. A food resistant to this process is undigested, as insoluble and soluble fibers are. They pass to the large intestine only affected by their absorption of water (insoluble fiber) or dissolution in water (soluble fiber).

complete or partial fermentation in the large intestine

the large intestine comprises a segment called the colon within which additional nutrient absorption occurs through the process of fermentation. Fermentation occurs by the action of colonic bacteria on the food mass, producing gases and short-chain fatty acids. It is these short-chain fatty acids—butyric, acetic (ethanoic), propionic, and valeric acids—that scientific evidence is revealing to have significant health properties.

As an example of fermentation, shorter-chain carbohydrates (a type of fiber found in legumes) cannot be digested, but are changed via fermentation in the colon into short-chain fatty acids and gases (which are typically expelled as flatulence).

According to a 2002 journal article, fibers compounds with partial or low fermentability include:

• cellulose, a polysaccharide
• hemicellulose, a polysaccharide
• lignans, a group of phytoestrogens
• plant waxes
• resistant starches

fiber compounds with high fermentability include:

• beta-glucans, a group of polysaccharides
• pectins, a group of heteropolysaccharides
• natural gums, a group of polysaccharides
• inulins, a group of polysaccharides
• oligosaccharides, a group of short-chained or simple sugars
• resistant dextrins