Fibre and gastrointestinal health

Chapter 2.1
Fibre and gastrointestinal health

Vikas Kumar,¹ Amit Kumar Sinha,² Han-ping Wang¹ and Gudrun De Boeck²

¹Ohio State University, Ohio, USA

²University of Antwerp, Antwerp, Belgium

The term ‘fibre’ was first used by Eben Hipsley in 1953. During his observations he found that populations with fibre-rich diets tended to have lower rates of pregnancy toxaemia [1]. In 1976, Trowell defined fibre as the component of plant foods that resisted digestion by enzymes produced by humans [2].

Dietary fibre is a group of non-digestible plant polysaccharides which escapes digestion and absorption in the upper gastrointestinal (GI) tract and is termed non-starch polysaccharides (NSPs). Plant ingredients generally contain a mixture of both water-soluble and insoluble fibre. Soluble fibre disperses when mixed with water and has the ability to increase the viscosity of digesta which slows down the diffusion of digestive enzymes and the absorption of nutrients. This reduction in absorption lowers postprandial glucose, cholesterol and insulin responses, which has significant implications for the prevention and management of insulin-resistant and type 2 diabetes.

Soluble fibre is mainly found inside the cells of fruits, vegetables, beans and oat bran, and consists of pectins, gums and mucilages. Insoluble fibre possesses high water-binding capacity, renders faecal content softer and bulkier, and allows easy luminal passage [3], thereby playing a crucial role in the correct functioning of the GI tract. Cellulose, lignin and hemicellulose contain insoluble fibre and are widely found in wheat, whole grains, fruits and vegetables.

A major proportion of NSPs escapes the small intestine nearly intact, and is fermented by commensal microbiota residing in the caecum and colon into short-chain fatty acids (SCFAs), promoting normal laxation. Short-chain fatty acids have a number of health-promoting effects such as maintaining normal GI structure and function, preventing or alleviating colon-based diarrhoea, lowering colonic pH, inhibiting growth of pathogenic organisms [4], improving mineral utilisation, stimulating proliferation of colonic epithelial cells, thereby increasing the absorptive capacity of the epithelium, and stimulating colonic blood flow and fluid and electrolyte uptake. Non-starch polysaccharides may benefit human health by reducing and/or limiting the risk of both acute and chronic diseases, such as infectious diarrhoea, obesity, diabetes, colorectal cancer, neonatal necrotising enterocolitis and inflammatory bowel disease through various mechanisms including a physiological effect on the GI tract, colonic fermentation, immunomodulation and a prebiotic effect [4,5]. Subsequently, NSP intake can be viewed as a marker of a healthy diet and higher dependency on fibre-supplemented food reflects a healthier lifestyle [5].

2.1.1 Classification of non-starch polysaccharides

According to Phillips [6], the 2009 meeting of the Codex Committee on Nutrition and Foods for Special Dietary Uses finally agreed upon the following definition – that NSPs are carbohydrate polymers with ≥10 monomeric units which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories.

Edible carbohydrate polymers naturally occurring in food as consumed.
Carbohydrate polymers, which have been obtained from raw food material by physical, enzymatic or chemical means and which have been shown to have a physiological effect or benefit to health as established by mostly accepted scientific proof from competent authorities.
Synthetic carbohydrate polymers which have been shown to have a physiological benefit to health as demonstrated by generally accepted scientific evidence from competent authorities [7].

Fibre from different sources may vary in a number of physical and chemical characteristics. Even though fibre was conventionally classified according to solubility, additional properties, such as viscosity and fermentability, are now being recognized as more important in terms of specific physiological benefits [8] (Table 2.1.1). Generally, soluble fibres are fermented and have a higher viscosity than insoluble fibres. Nevertheless, most soluble fibres are not viscous (e.g. acacia gum, partially hydrolysed guar gum), whereas a few of the insoluble fibres may be well fermented (e.g. finely ground soy polysaccharides) [8].

Table 2.1.1 Classification of non-starch polysaccharides based on three physicochemical characteristics [5,8,55]

Types	Properties	Solubility	Fermentability^a	Viscosity^a
Acacia gum	A non-viscous, soluble fibre obtained as an exudate from the branches and stems of Acacia senegal and Acacia seyal. A highly branched, high molecular weight molecule consisting of galactose, arabinose, rhamnose and glucuronic acid units.	Soluble	Fermentable
Guar gum	Guar gum is produced by milling the endosperm of the guar seed. The major polysaccharide is galactomannan. Galactomannans are highly viscous and are therefore used as food ingredients for their thickening, gelling and stabilising properties.	Soluble		Non-viscous
Partially hydrolysed guar gum (PHGG)	The structure consists of a mannose backbone with galactose side units. PHGG is a soluble fibre with only marginal effects on viscosity, yet it seems to retain the ability of native guar gum to lower glucose and insulin levels.	Soluble	Fermentable	Viscous
Inulin, oligofructose and fructo-oligosaccharides	Inulin, oligofructose (OF) and fructo-oligosaccharides (FOS) belong to a larger class called inulin-type fructans, which refers to all linear fructans that contain beta-2,1 fructosyl-fructose glycosidic bonds. These molecules differ in chain length and method of extraction or synthesis, yet nomenclature is inconsistent in the literature. In general, inulin refers to molecules with an average degree of polymerisation ≥10, whereas FOS and OF refer to shorter chain molecules. FOS, OF and inulin are non-viscous, soluble fibres obtained from a number of foods (primarily chicory root) or produced synthetically by adding fructose units to a sucrose molecule via beta-1,2 linkages (FOS only).	Soluble	Fermentable	Non-viscous
Pectin	Pectins are mainly present in the cell wall and intracellular tissues of many fruits and berries, and consist of galacturonic acid units with rhamnose interspersed in a linear chain. Pectins frequently have side chains of neutral sugars, and the galactose units may be esterified with a methyl group, a feature that allows for its viscosity.	Soluble	Fermentable	Non-viscous
Hemicellulose A	Its structure is that of an acidic xylan, a linear chain of beta-D-xylopyranosyl units, bonded together by (1– > 4) glycosidic links, containing single alpha-D-xylopyranosyl and 4-O-methyl-alpha-D-glucopyranuronosyl residues.	Soluble
Oat fibre	Oat beta-glucans are non-starch polysaccharides. Like starch, oat beta-glucans are composed of glucose molecules in long chains but the binding between glucose mononers differs from starch. It has both (1 → 4)- and (1 → 3) linkages. About 70% of the links are beta(1 → 4)- and the rest are beta(1 → 3). The distribution is not random. The mixed linkages that form oat beta-glucans are important for their physical properties, such as viscosity and solubility. The GI tract does not contain enzymes that can digest oat beta-glucans.	Soluble		Viscous
Mixed-linked beta-glucans	Mixed-linked beta-glucans occur exclusively in members of the monocotyledon family Poaceae, to which the cereals and grasses belong, and in related families of the order Poales. Mixed-linked beta-glucans are also referred as (1 → 3,1 → 4)- beta-D-glucans or cereal beta-glucans. They are linear, unbranched polysaccharides in which beta-D-glucopyranosyl monomers are polymerised through both beta(1 → 4)- and beta(1 → 3) linkages.	Soluble
Polydextrose polyol	Polydextrose polyol is a water-soluble specialty carbohydrate which is manufactured from glucose. It is a complex carbohydrate and traditionally used as a bulking agent in foods. It is like other polyols and can be used as a sugar and fat replacer. It is particularly valuable in the production of low-calorie foods.	Soluble
Psyllium	Psyllium refers to the husk of psyllium seeds and is a very viscous mucilage in aqueous solution. The psyllium seed, also known as plantago or flea seed, is small, dark, reddish-brown, odourless and nearly tasteless. P. ovata, known as blond or Indian plantago seed, is the species from which husk is usually derived.	Soluble
Wheat dextrin	Wheat dextrins are a group of low molecular weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by alpha(1 → 4) or alpha(1 → 6) glycosidic bonds. Dextrins can be produced from starch from malting and mashing and during digestion by the enzyme amylase.	Soluble
Cellulose	Cellulose is the main structural component of all cell walls in cereal grains and is a linear homopolymer of beta(1 → 4) linked glucose units. Cellulose chains are long flat linear ribbons of glucose units with molecular weights of over 1,000,000. The beta(1 → 4) linkage between the glucose units holds the chain in a flat conformation so cellulose chains can align next to each other and form numerous hydrogen bonds between the sugar hydroxyl groups. Cellulose quantity in whole grains can vary from species to species and is largely a consequence of the thickness of the husk and seed coat.	Insoluble	Non-fermentable	Viscous
Soy polysaccharide	Soy polysaccharides are obtained from soy cotyledon and consist of a number of fibre components, including cellulose, hemicelluloses, lignin and pectin-like molecules. Although soy polysaccharides are typically 75–85% insoluble, they have been shown to be highly fermentable in humans, probably because of their small particle size.	Insoluble	Fermentable	Viscous
Resistant starch	Resistant starch refers to starch and products of starch digestion that are not absorbed in the small intestine and pass to the colon. It can be classified according to the characteristics that make it resistant to digestion (physically inaccessible, granular form, retrograded or chemically modified).	Insoluble	Fermentable	Viscous
Hemicellulose B	This hemicellulose seems to be a homogeneous polysaccharide with an apparent molecular weight of 35,000. Its structure is that of an acidic arabinoxylan, a linear chain of beta-D-xylopyranosyl units, bonded together by (1 → 4) glycosidic links, containing a single L-arabinofuranosyl, alpha-D-xylopyranosyl and 4-O-methyl-alpha-D-glucopyranuronosyl residues joined by glycosidic links.	Insoluble
Lignin	Lignin is a highly branched polymer composed of phenylpropanoid units and is found within ‘woody’ plant cell walls, covalently bound to fibrous polysaccharides.	Insoluble
Outer pea fibre	Outer pea fibre is an insoluble fibre obtained from the hulls of the field pea and is composed of hemicelluloses, cellulose and pectic substances. It is primarily used to enhance the fibre content of products, without modifying functional or technical properties, and increases stool weight in healthy individuals.	Insoluble	Non-fermentable	Viscous
Resistant dextrins	Indigestible components of starch hydrolysates, as a result of heat and enzymatic treatment, yield indigestible dextrins that are also called resistant maltodextrins. The average molecular weight of resistant maltodextrins is 2000 daltons and they consist of polymers of glucose containing alpha(1 → 4) and alpha(1 → 6) glucosidic bonds, as well as 1 → 2 and 1 → 3 linkages.	Other
Chitin and chitosan	Chitin is an amino-polysaccharide containing beta(1 → 4) linkages as are present in cellulose. Chitosan is the deacetylated product of chitin. Both chitin and chitosan are found in the exoskeletons of shrimp, crabs and lobsters and in the cell walls of most fungi.	Other
Polydextrose	Polydextrose is a polysaccharide that is synthesised by random polymerisation of glucose and sorbitol. Polydextrose serves as a bulking agent in foods and sometimes as a sugar substitute. It is not digested or absorbed in the small intestine and is partially fermented in the large intestine, with the remnants excreted in the faeces.	Other

^aFermentability and viscosity will vary depending on degree of water solubility and additionally for viscosity, physical form, molecular weight and concentration.

2.1.2 Clinical effect of non-starch polysaccharides

Dietary supplementation of fibre has a wide array of beneficial health effects as already described. It is well known that the human GI tract is less suited to the modern high-fat, energy-rich and low-volume diets of urbanised countries but better suited to cope with a diet rich in NSP with a large volume. Consequently, low NSP intake is associated with many Western diseases such as obesity, type 2 diabetes and gastrointestinal disorders. Based on various scientific reports, it is apparent that the majority of the clinical effect of NSP intake is promoted by the fibre-mediated alterations in the functional processes of the GI tract. As such, this chapter will highlight the health-promoting effects of NSPs on the GI tract and the key mechanisms associated with modulating the clinical benefits of fibre intake.

Physiological effects of fibre on the GI tract

Dietary intake of fibre influences the entire GI tract from mouth to anus. Foods rich in fibre have lower energy density and take longer to empty from the stomach. Water-soluble fibre usually delays gastric emptying and slows the transit of food materials through the small intestine, consequently increasing nutrient absorption time [9]. In the GI tract (particularly the small intestine), fibre can elicit responses of a wide variety of gut hormones that serve as incretins to stimulate insulin release and affect appetite [10]. Fibre can also bind with bile acids and impede micelle formation, thus increasing faecal excretion of bile acids and cholesterol [11]. Consumption of a diet low in NSP could lead to watery stools and the addition of pectin significantly reduces the occurrence of watery stools and promotes normalisation of colonic fluid composition [12]. Reports suggest that consumption of pectin and soy polysaccharides increases colonic water absorption, probably mediated via SCFA production, suggesting that reduction in luminal SCFA levels in antibiotic-associated colitis may be responsible for diarrhoea [13]. Ramakrishna and Mathan (1993) further confirmed that acute watery diarrhoea is associated with a reduction in luminal SCFAs and a decrease of net water and sodium absorption in the colon [14].

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