Fiber has numerous effects in the large intestine.
Fiber is attacked and broken down by the huge population of bacteria that live in the colon.
The breakdown products are acids and gases. This process is called fermentation. Dietary fiber is only partly fermented, because some plant cell walls resist bacterial attack.
The simple organic acids produced by fermentation are mostly absorbed, and in doing so they nourish the lining of the colon. They also provide fuel for the rest of the body, especially the liver. This may have important consequences for metabolism; half the calories in fiber are made available to the body.
The gases arising from fermentation soften and enlarge the stool. They are also passed as wind (flatus) and can contribute to bloated feelings in some people.
Here is a summary of the actions of dietary fiber at different levels of the digestive system:
Mouth
Makes work, slows ingestion, cleans teeth
Stomach
Dilutes contents, distends, prolongs retention of food
Small intestine
Dilutes contents, distends, slows absorption
Right colon
Dilutes, distends, feeds bacteria and so acidifies contents
Left colon
Nourishes the colon, speeds up passage, lowers pressures
Rectum, anus
Softens and enlarges stool, prevents trauma to the body, reduces strain of defecation.
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