Dietary Fiber

Dietary fibre, also called roughage, refers to the components of plant-derived foods that are resistant to digestion by human gastrointestinal enzymes. Fibre is chemically diverse and includes non-starch polysaccharides and other plant constituents such as cellulose, hemicelluloses, resistant starch, resistant dextrins, inulins, lignins, chitins, pectins, β-glucans and various oligosaccharides. Its physiological roles in supporting digestive health and systemic well-being have led to widespread recommendations for regular fibre intake as part of a balanced diet.
Fibre is commonly categorised by its solubility, viscosity and fermentability. These characteristics help determine how fibre behaves in the gastrointestinal tract, influencing bowel function, gut microbiota composition and metabolic effects. Many plant foods contain both soluble and insoluble fibres in varying proportions.

Types of Dietary Fibre

Soluble FibreSoluble fibre dissolves in water to form gels of varying viscosity. It is generally fermentable by colonic bacteria, producing gases and physiologically active short-chain fatty acids (SCFAs). Typical sources include β-glucans in oats, barley and certain mushrooms; guar gum; pectins; inulin from chicory root; resistant dextrins; and oligosaccharides found in legumes and bananas. Psyllium is a notable soluble fibre that is viscous but minimally fermented; it retains water in the stool and facilitates defecation.
Soluble viscous fibres can slow gastric emptying, contributing to prolonged satiety. Regular consumption of certain soluble fibres, particularly β-glucans, has been shown to reduce low-density lipoprotein (LDL) cholesterol levels. Soluble non-viscous fibres such as inulin and fructooligosaccharides function primarily as prebiotics, supporting beneficial gut bacteria.
Insoluble FibreInsoluble fibre does not dissolve in water and resists digestion in the upper gastrointestinal tract. Examples include wheat bran, cellulose and lignin. Coarsely ground insoluble fibre stimulates mucus secretion and promotes stool bulk, whereas finely ground forms may not provide the same benefit and may even contribute to constipation. Some insoluble fibres, particularly resistant starches, undergo partial fermentation in the colon, producing SCFAs.

Physiological Roles

Fibre’s physiological effects vary according to its chemical and physical properties:

• Bulking fibres (e.g., cellulose, hemicellulose) absorb water, increasing stool mass and promoting regular bowel movements.
• Viscous fibres (e.g., β-glucans, psyllium) form gels that slow nutrient absorption and can aid in cholesterol reduction.
• Fermentable fibres (e.g., resistant starch, inulin, xanthan gum) serve as substrates for gut microbiota, supporting SCFA production which benefits colonic health and systemic metabolism.

SCFAs such as acetate, propionate and butyrate contribute to maintaining colonic epithelial integrity, regulating inflammation and supporting mineral absorption, particularly calcium, magnesium and iron.

Definitions

Dietary fibre is defined as the fraction of edible plant material not digested by endogenous enzymes in the human digestive tract. Historically this included primarily lignin and certain non-starch polysaccharides. Contemporary definitions encompass resistant starches and specific oligosaccharides, reflecting improved understanding of their physiological significance. Nutrition scientists often apply either a physiological definition (resistance to mammalian enzymatic digestion) or a chemical definition (non-starch polysaccharides plus lignin).

Food Sources

Dietary fibre is abundant in fruits, vegetables, legumes and whole grains. High-fibre foods include bran products, with crude corn bran containing approximately 79 g of fibre per 100 g and crude wheat bran around 43 g per 100 g. Whole fruits and vegetables provide a mixture of fibre types: for example, prunes offer insoluble fibre from the skin and soluble fibre from the pulp. Grapes, berries, citrus fruits, avocados and bananas contribute varying proportions of both types. Vegetables such as broccoli, carrots, sweet potatoes, onions, green beans and courgettes are also significant sources. Nuts, seeds and legumes are important contributors of both soluble and insoluble fibres.

Fibre Supplements

A range of fibre supplements is available to support digestive health or provide additional nutrients:
Soluble Fibre SupplementsThese include psyllium, inulin, oligosaccharides and dextrins. Such supplements may alleviate symptoms of irritable bowel syndrome, including constipation, diarrhoea and abdominal discomfort. Prebiotic fibres such as inulin may benefit individuals with inflammatory bowel diseases by enhancing SCFA production and moderating inflammatory responses. Higher doses, however, can cause bloating or flatulence.
Insoluble Fibre SupplementsResistant starch derived from high-amylose maize is used to improve bowel regularity and has been studied for its positive effects on insulin sensitivity and glycaemic control. Some preliminary findings suggest it may aid symptom reduction in ulcerative colitis.
Inulin and FructansInulin is a naturally occurring fructan extracted from chicory root or Jerusalem artichokes. It can replace sugar or fat in food products and improve powder flow in nutritional formulations. As a fermentable prebiotic fibre, inulin promotes SCFA production and enhances mineral absorption. Excess intake may lead to gastrointestinal discomfort, particularly in sensitive individuals.
Vegetable Gum FibresGum arabic and guar gum are examples of soluble gums used as supplements. They dissolve readily and have shown promise in managing irritable bowel syndrome.

Activity in the Gastrointestinal Tract

Fibre reaches the large intestine largely intact due to the absence of human enzymes capable of breaking its glycosidic bonds. Once in the colon, fermentable fibres support the growth and activity of gut microbiota. Different types of fibre exhibit varying rates and extents of fermentation, influencing gas production, SCFA output and microbial composition.
Fibre’s broader physiological contributions include maintenance of healthy bowel function, modulation of postprandial glycaemia, enhancement of lipid metabolism and support for immune function via interactions with the gut microbiome. Regular consumption is widely recommended as part of dietary patterns aimed at reducing the risk of cardiovascular disease, type 2 diabetes, obesity and certain gastrointestinal disorders.