Dietary fiber in the diet consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants.1 Added fiber refers to isolated, nondigestible carbohydrates added to foods or taken as a supplement. Total fiber in foods is the sum of dietary fiber and added fiber. This review specifically focuses on explaining the differences between soluble and insoluble dietary fiber.
Dietary fiber is classified by physical and chemical properties using several categories, including the following: soluble or insoluble, viscous or non-viscous, and fermentable or non-fermentable in the gastrointestinal tract.1-3 The physiologic effects of fiber vary with the type. The classification of fiber into soluble and insoluble is based on water-dissolvability. 2
Soluble and insoluble fiber can be viscous or non-viscous, fermentable or non-fermentable. Viscosity is defined by the gel-forming capacity of the fiber. Beneficial physiological effects of viscous fibers include improved LDL-cholesterol levels and glycemic control.3 Fermentability refers to how readily gut microbiota metabolize fiber into various metabolites including short-chain fatty acids. Fermentable fiber consumption is associated with improved health of the gut microbiota.4 Non-fermentable fibers, by staying intact throughout the colon, aid in relieving constipation and facilitating bowel movement regularity.3 Fiber-rich foods most often contain fibers with more than one physicochemical property. Thus, a single food may be a source of both insoluble and soluble fiber.1-3 Classification of foods as higher in insoluble or soluble fiber is variable and often inconsistent.5
The designation of fiber as soluble or insoluble is method-dependent and may fail to predict physiological effects accurately. In 2001, the National Academies of Science, Engineering, and Medicine recommended replacing the terms soluble fiber and insoluble fiber with the alternative properties of viscosity and fermentability to more clearly distinguish dietary fiber and its physiological effects.1 However, distinguishing the impact of soluble versus insoluble fiber on cardiovascular disease risk and other disease states remain areas of current research. Thus, understanding the differences in the two types of fiber and food sources of each remains relevant.
A review of the literature, clinical practice guidelines, and national association recommendations and publications, for relevant information on dietary insoluble and soluble fiber was conducted. The detailed methods are available from the authors upon request.
Soluble fiber is the edible part of the plant that is resistant to digestion and dissolves in water.6,7 Soluble fiber encompasses pectin, gums, carrageen, raffinose, xylose, mucilage extracted from psyllium husk, β-glucan, fructans, as well as some hemicellulose.6,7,8 Examples of foods most commonly listed as rich in soluble fibers include fruits like bananas, apples, pears, and berries as well as oats, barley, and legumes (i.e., beans, peas, lentils). 6
Most soluble fiber can be partially or totally fermented by colonic bacteria to short-chain fatty acids.4 Fermentable soluble fibers do not provide a laxative benefit but may be beneficial to the gut microbiota.3,4 However, non-fermentable soluble fibers, such as psyllium, can provide a benefit to bowel movement regularity.3
Soluble fiber is often viscous, as opposed to non-viscous. Soluble fibers that are viscous absorb water, leading to gel formation, which increases food transit time through the gastrointestinal tract, delays gastric emptying, decreases nutrient absorption, and slows digestion.2,3,6,8,9 Improved cardiovascular health and attenuation of blood glucose levels are associated with higher viscosity soluble fiber.3
Insoluble fiber passes through the digestive tract intact and is not soluble in water.6,7 Insoluble fiber includes cellulose, some hemicellulose, chitin, xanthan, and lignin. 6,7,8 Examples of foods more often noted to be rich in insoluble fibers include some fruits and vegetables, whole grains, wheat bran, nuts, and seeds.6
Insoluble fibers are often non-fermentable. Non-fermentable insoluble fiber decreases transit time in the gastrointestinal tract and increases fecal bulk to relieve constipation.2,6 Mechanical irritation of the gut mucosa stimulates water and mucous secretion resulting in bulky, soft, easier-to-pass stools.2,3,8,9 Insoluble fiber may reduce the risk for colon cancer, hemorrhoids, and diverticulitis and improve colon health.2,3,7,8,9,10 Insoluble fiber does not have a significant effect on LDL cholesterol or glucose levels.3 Insoluble fibers that are fermentable, such as cellulose, may benefit the gut microbiota.11-14
Summary of evidence
Dietary fiber can be classified by various physicochemical properties. Soluble fiber and insoluble fiber constitute two of the ways that dietary fiber can be identified. Soluble fiber and insoluble fiber can be viscous or non-viscous and fermentable or non-fermentable. Soluble fibers are more often viscous and fermentable, and insoluble fibers are more often non-fermentable in the human gastrointestinal tract leading to differences in physiologic effects on health. Foods can contain either or both soluble and insoluble fiber. Many clinical practice guidelines recommend diets or dietary patterns rich in both soluble and insoluble fiber for the prevention of cardiovascular disease and promotion of gastrointestinal health.15,16
- Institute of Medicine (US) Panel on the Definition of Dietary Fiber and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary reference intakes proposed definition of dietary fiber. Washington (DC): National Academies Press (US); 2001.
- Stephen AM, Champ MM., Cloran SJ, et al. Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutrition research reviews. 2017;30(2), 149–190.
- McRorie JW, Jr & McKeown NM. Understanding the physics of functional fibers in the gastrointestinal tract: an evidence-based approach to resolving enduring misconceptions about insoluble and soluble Fiber. JAND. 2017;117(2), 251–264.
- So D, Whelan K, Rossi M, et al. Dietary fiber intervention on gut microbiota composition in healthy adults: a systematic review and meta-analysis. Am J Clin Nutr. 2018;107(6):965-983.
- Stephen AM, Champ MM, Cloran SJ, et al. Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutr Res Rev. 2017;30(2):149-190.
- Soliman GA. Dietary fiber, atherosclerosis, and cardiovascular disease. Nutrients. 2019;11(5), 1155.
- Ciudad-Mulero M, Fernández-Ruiz V, Matallana-González MC, Morales P. Dietary fiber sources and human benefits: the case study of cereal and pseudocereals. Adv Food Nutr Res. 2019;90:83-134.
- Williams BA, Mikkelsen D, Flanagan BM, & Gidley MJ. "Dietary fibre": moving beyond the "soluble/insoluble" classification for monogastric nutrition, with an emphasis on humans and pigs. J Anim Sci Biotechnol. 2019;10, 45.
- Dhingra D, Michael M, Rajput H, & Patil RT. Dietary fibre in foods: a review. J Food Sci Technol, 2012;49(3), 255–266.
- Veronese N, Solmi M, Caruso MG, et al. Dietary fiber and health outcomes: an umbrella review of systematic reviews and meta-analyses. Am J Clinl Nutr. 2018;107(3), 436–444.
- Fischer F, Romero R, Hellhund A, et al. Dietary cellulose induces anti-inflammatory immunity and transcriptional programs via maturation of the intestinal microbiota. Gut microbes. 2020;12(1), 1–17.
- Tuncil YE, Thakkar RD, Marcia A, et al. Divergent short-chain fatty acid production and succession of colonic microbiota arise in fermentation of variously-sized wheat bran fractions. Scientific reports. 2018;8(1), 16655.
- Titgemeyer EC, Bourquin LD, Fahey GC, Jr, & Garleb KA. Fermentability of various fiber sources by human fecal bacteria in vitro. Am J of Clin Nutr. 1991;53(6), 1418–1424.
- Yang L, Zhao Y, Huang J, et al. Insoluble dietary fiber from soy hulls regulates the gut microbiota in vitro and increases the abundance of bifidobacteriales and lactobacillales. J Food Sci Technol. 2020;57(1):152-162.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans 2020-2025. 9th Edition. 2020.
- Dietary Guidelines Advisory Committee and Nutrition Evidence Systematic Review Team. Scientific Report of the 2020 Dietary Guidelines Advisory Committee: Advisory Report to the Secretary of Agriculture and the Secretary of Health and Human Services. Washington, DC: US Department of Agriculture; 2020.
Table 1. Soluble and total fiber content in commonly consumed foods.
Table 2. Insoluble and total fiber content in commonly consumed foods.
aNutrition information can vary based on product brand, serving size, and nutrient analysis method. The fiber amounts listed for these products may not match nutrition labels. The information in these tables are from the following reference: Marlett JA, Cheung TF. Database and quick methods of assessing typical dietary fiber intakes using data for 228 commonly consumed foods. J Am Diet Assoc. 1997;97:1139-1148, 1151.