Dietary fiber or roughage is the indigestible portion of food derived from plants. It has two main components:

• Soluble fiber, which dissolves in water, is readily fermented in the colon into gases and physiologically active byproducts and can be prebiotic and viscous. This delays gastric emptying which, in humans, can result in an extended feeling of fullness.
• Insoluble fiber, which does not dissolve in water, is metabolically inert and provides bulking, or it can be fermented in the colon. Bulking fibers absorb water as they move through the digestive system, easing defecation.

Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, pectins, beta-glucans, and oligosaccharides.

There are two types of fiber: insoluble and soluble. Both come from food material, both are indigestible by the human body, and both play a critical role in supporting digestion.

Insoluble fiber is what you’ve maybe heard referred to as “roughage”. It’s the tough matter found in food stalks, skins, and seeds and it doesn’t dissolve in water or get absorbed into the bloodstream. It adds bulk to the waste produced in the gut, which helps keep you regular.

Meanwhile, soluble fiber absorbs water to form a gel-like substance inside the digestive system. Sources include beans, oats, fruits, and avocados. Soluble fiber helps soften stool so it can slide through the GI tract more easily.

Fiber is in every fruit, vegetable, whole grain, and legume that you eat (so we’re still wondering why prunes get all the glory). Fiber in the daily diet contributes to a feeling of fullness, lowers cholesterol, and helps control blood sugar levels.

Fiber confers major benefits on humans but for many decades scientists didn’t really know why. Recent studies of the gut microbiome have revealed that the trillions of microbes in the colon may be responsible for some of the beneficial effects of fiber, such as the production of butyrate, a short-chain fatty acid responsible for powering cellular energy.