Dietary fiber, also known as roughage, refers to the parts of plant-based foods that cannot be fully broken down by human digestive enzymes. Dietary fibers vary in chemical composition and are generally classified by their solubility, viscosity, and fermentability — all of which affect how fiber is processed in the body. Dietary fiber includes two main types: soluble and insoluble fiber, found in plant foods such as legumes, whole grains, cereals, vegetables, fruits, nuts, and seeds.
A high-fiber diet is generally associated with better health and a reduced risk of various diseases. Traditionally, fiber sources have been categorized by whether they provide soluble or insoluble fiber, although most plant-based foods contain both in varying amounts depending on the properties of the fiber, such as viscosity and fermentability.
The health benefits of fiber depend on the type consumed and its effects on the digestive system. Bulking fibers—like cellulose and hemicellulose (including psyllium)—absorb and retain water, promoting regular bowel movements. Viscous fibers—such as beta-glucan and psyllium—thicken stool mass. Fermentable fibers—such as resistant starch, xanthan gum, and inulin—feed bacteria and microbes in the large intestine and are metabolized to produce short-chain fatty acids (SCFAs), which support gut health.
Soluble fibers (fermentable or prebiotic fibers) dissolve in water and are typically fermented in the colon, producing gases and physiologically active byproducts such as SCFAs, which are produced by gut bacteria. Examples include beta-glucan (found in oats, barley, and mushrooms) and raw guar gum. Psyllium, a soluble, viscous, non-fermentable fiber, holds water as it moves through the digestive tract, helping ease bowel movements.
Soluble fibers are generally viscous and slow down gastric emptying, which may extend the feeling of fullness in humans. Inulin (from chicory root), wheat dextrin, oligosaccharides, and resistant starches (found in legumes and bananas) are non-viscous, soluble fibers. Regular intake of soluble fibers like beta-glucan from oats or barley has been linked to lowering LDL cholesterol, a risk factor for cardiovascular disease.
Insoluble fibers, which do not dissolve in water, are resistant to digestive enzymes in the upper digestive tract. Examples include wheat bran, cellulose, and lignin. Ground insoluble fibers stimulate mucus secretion in the colon, increasing bulk. However, finely ground insoluble fiber may not have the same bulking effect and could potentially contribute to constipation. Some types of insoluble fibers, such as resistant starch, can be fermented in the colon.
Dietary fiber includes non-starch polysaccharides and other plant-based components like cellulose, resistant starch, resistant dextrins, inulin, lignin, chitin (in fungi), pectin, beta-glucan, and oligosaccharides.
