What is Dietary Fiber?
Dietary fiber is a type of complex structured carbohydrate which does not get digested and absorbed in the small intestine. It reaches the large intestine mostly intact where it is partially or fully fermented by microbes. 
Fiber is naturally present in plants and its functional derivatives are Cellulose, Resistant Dextrins, Hemicellulose, Oligosaccharides, Resistant Starch, Chitins, Pectins, Beta-Glucans and Lignin. Generally, most of the carbohydrates are broken down into sugar molecules by various enzymatic hydrolysis in human system, however dietary fiber are not completely broken down into digestible sugar molecules in the small intestine (for the reason, humans lack the necessary enzymes to split the glycosidic bond which is present in fiber). Undigested fiber passes through the large intestine, where some are fermented by bacteria. The by-products of this fermentation are carbon dioxide, methane, hydrogen and short-chain fatty acids (SCFAs). It should be noted that certain types of fiber may cause an increase in gas production which leads to bloating.
Types of Dietary Fiber
Fiber is classified as Soluble Fiber and Insoluble Fiber.
Soluble dietary fiber:
It is fiber which readily dissolves in liquids. Some types of soluble fiber are viscous and hydrophilic and will form a gel-like substance in the gastrointestinal tract during the digestion process. The other non-viscous type of soluble fibers are fermented in the large intestine to provide several physiological benefits.
Soluble fiber may increase the fecal stool bulk as well as soften it so that it slides easily through the large intestine.
Soluble fiber is present in corn, apples, oranges, grapes, dry beans, lentils, peas, barley, oats and the likes thereof.
Insoluble dietary fiber:
It is the type of fiber which does not readily dissolve in liquids and remains relatively unchanged in the gastrointestinal tract during the digestion process. Insoluble fiber prevents constipation by adding bulk to fecal content in the large intestine. Generally, insoluble fiber is minimally fermented. Insoluble fiber is present in vegetables, whole grain products, whole wheat bread, bran, pasta, crackers, edible seeds, brown rice and the likes thereof. 
Why do you need it?
Adding fiber to your daily diet is imperative and has numerous benefits. It helps provide satiety without the need to consume large quantities of food. Thus, this prevents intake of excessive calories and fosters weight management.
Dietary fiber helps to regulate daily bowel movements. Soluble dietary fiber helps mainly by softening stools so that they slide easily through the large intestine. Hence, reducing the frequency of constipation. It also helps increase peristaltic movements and reduces the time that waste matter remains in the colon. Also, in episodes of loose watery stools, soluble fiber absorbs the excess water and adds bulk to the stools. This contributes to the maintenance of a healthy bowel movement routine.
A diet rich in soluble fiber may help to manage elevated cholesterol levels in the blood. Fiber may also have a role in maintaining blood glucose level by slowing the absorption of glucose. Along with intake of high fiber diet, it is important to also increase water intake.
The US Dietary Guidelines recommends a daily requirement of dietary fiber of 21g to 38g, depending on calorie requirements.
Sources of fiber
Dietary sources: Fiber can be naturally obtain from food sources such as fruits, vegetables, beans or wholegrain cereals.
Other sources: In addition to food sources, fiber intake can be increased from dietary supplements that contain ingredients such as resistant maltodextrin, psyllium, methylcellulose or inulin.
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