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Fiber is defined as material made by plants that is not digested by the human gastrointestinal tract. Fiber is one of the mainstays in the treatment of constipation though it also may have additional uses such as the treatment of diverticular disease of the colon. Many types of fiber bind to water and keep the water within the intestine. The fiber and water adds bulk (volume) to the stool, and the water softens the stool. Increased gas (flatulence) is a common side effect of high-fiber diets. The gas occurs because bacteria within the colon produce gas as a by-product of their digestion of fiber.
There are different sources of fiber, and the type of fiber varies from source
to source. Some types of fiber are digested to a greater extent by colonic
bacteria than other types of fiber. The better-digested fiber produces more gas.
All fibers, no matter their source, can cause flatulence; however, since bacteria vary in their ability to digest different types of fiber, different sources of fiber may produce different amounts of gas. To complicate the situation, the ability of bacteria to digest one type of fiber can vary from individual to individual. This makes the selection of the best type of fiber for each individual (i.e., a fiber that improves the quality of the stool without causing flatulence) more difficult. The choice becomes a matter of trial and error.
What are the different types of fiber?
Types of fiber can be categorized in several ways, for example, by their source. The most common natural sources of fiber include fruits and vegetables as well as wheat or oat bran. These fibers are most likely to cause flatulence. Fibers used medicinally to treat constipation include psyllium seed, synthetic cellulose (methyl cellulose), and calcium polycarbophil. (A lesser known source is an extract of malt; however, this extract may soften stools in ways other than by increasing fiber.) Psyllium and methyl cellulose (and probably calcium polycarbophil) do not increase the production of gas; however, they still may result in the sensation of bloating. This may be due to an effect of these fibers that slows the transit of gas through the intestine.
The different sources of fiber should be tried one by one. The fiber should be started at a low dose. The dose should be increased every 1-2 weeks until either the desired effect on the stool is obtained or troublesome bloating or flatulence (or other gastrointestinal symptoms) supercedes. (Fiber does not work overnight!) If bloating or flatulence occurs, the dose of fiber can be reduced for a few weeks, and then the higher dose can be re-instituted. (It generally is said that the amount of bloating or gas that is produced by fiber decreases when it is ingested for a prolonged period of time; however, this has never been carefully studied or proven.) If bloating or flatulence remains a problem and prevents the dose of fiber from being raised to a level that affects the stool satisfactorily, it is time to try a different source of fiber.
When using increasing amounts of fiber, it is recommended that increasing amounts of water be ingested, presumably to provide water for the fiber to bind and prevent "hardening" of the fiber and obstruction of the intestine. This is simple and reasonable advice; however, it has never been shown that the amount of ingested water has any beneficial effect. (There is a lot of water in the normal intestine, and extra water is just absorbed and excreted in the urine.) Despite the lack of demonstrated benefit, it probably is a good idea not to become dehydrated while taking fiber supplements.
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NIH.gov. Gas in the Digestive Tract.