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, unlike the intestine of humans, are capable of digesting fiber to a small extent. The bacteria 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.