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Special Article Dietary fiber type reflects physiological functionality: comparison of grain fiber, inulin, and polydextrose Kaisa Raninen, Jenni Lappi, Hannu Mykkänen, and Kaisa Poutanen Dietary fiber is a nutritional concept based not on physiological functions but on defined chemical and physical properties. Recent definitions of dietary fiber differentiate inherent plant cell wall-associated fiber from isolated or synthetic fiber. For the latter to be defined as fiber, beneficial physiological effects should be demonstrated, such as laxative effects, fermentability, attenuation of blood cholesterol levels, or postprandial glucose response. Grain fibers are a major natural source of dietary fiber worldwide, while inulin, a soluble indigestible fructose polymer isolated from chicory, and polydextrose, a synthetic indigestible glucose polymer, have more simple structures. Inulin and polydextrose show many of the same functionalities of grain fiber in the large intestine, in that they are fermentable, bifidogenic, and laxative. The reported effects on postprandial blood glucose and fasting cholesterol levels have been modest, but grain fibers also show variable effects. New biomarkers are needed to link the physiological functions of specific fibers with long-term health benefits. nure_358 9..21 © 2011 International Life Sciences Institute INTRODUCTION The current interest in dietary fiber stems from observations that populations consuming diets high in dietary fiber have a lower incidence of the chronic diseases that are common in Western countries with low-fiber diets.1 As reviewed recently in this journal, high consumption of dietary fiber has been associated with reduced risk of cardiovascular disease, diabetes, hypertension, obesity, and gastrointestinal disorders.2 The physiological mechanisms underlying the health-protective properties of dietary fiber have been studied intensively, and a range of functions throughout the gastrointestinal tract have been elucidated.A large part of the research to date has concentrated on the effects of soluble viscous fiber with regard to lowering cholesterol or attenuating glucose responses, as reviewed by Wood in relation to oat b-glucan.3 Epidemiological evidence of the health-protective effects of dietary fiber have been considered convincing enough for the consumption of dietary fiber to be promoted in dietary recommendations,4 and has led to industrial interest in developing fiber ingredients to increase the fiber content of processed foods.5,6 However, there is no evidence from long-term studies about the efficacy of these supplements for reducing disease risk, especially if substituted for whole grain or vegetables. Whole grains and vegetables, in addition to being naturally rich in fiber, contain other nutritive substances such as vitamins and phytochemicals.7 In addition to carbohydrate fermentation, concurrent liberation of phenolic compounds has been suggested as an underlying mechanism for the protective effects of cereal dietary fiber, in particular.8,9 Dietary fiber as a concept is based on nutritional effects and physiological functions; it is not a single chemical compound but a range of compounds that are not digestible in the small intestine. Consequently, it has been difficult to reach agreement on the definition of Affiliations: K Raninen, J Lappi, and H Mykkänen are with the Food and Health Research Centre, Department of Clinical Nutrition, University of Eastern Finland, Kuopio, Finland. K Poutanen is with the VTT Technical Research Centre of Finland; and Food and Health Research Centre, Department of Clinical Nutrition, University of Eastern Finland, Kuopio, Finland. Correspondence: K Poutanen, Food and Health Research Centre, Department of Clinical Nutrition, University of Eastern Finland, POB 1627, FI-70211 Kuopio, Finland. E-mail: kaisa.poutanen@vtt.fi, Phone: +358-40540-3326. Key words: dietary fiber, functionality, grain fiber, inulin, polydextrose doi:10.1111/j.1753-4887.2010.00358.x Nutrition Reviews® Vol. 69(1):9–21 9 dietary fiber. The objective of this review was to compare the physiological effects of three types of dietary fiber with varying compositions, degrees of chemical and structural heterogeneity, origins, and physical properties. Grain fiber, inulin, and polydextrose are examples of different types of fibers that are highly variable in both their technical and physiological functionality; they also belong to different categories of the new dietary fiber definitions of the European Union and Codex. Grain fiber is chemically heterogenous, has both insoluble and soluble fractions, and the dietary fiber polymers are organized in a complex hierarchical structure in the cell wall. In addition, grain fiber contains many associated compounds with various biological activities. In contrast, inulin and polydextrose are industrial products for which the possession of beneficial physiological functions must be demonstrated in order for them to be classified as dietary fiber. Inulin is a soluble isolated fructose polymer that promotes the growth of bifidogenic bacteria and is efficiently fermented by gut microbes. Polydextrose is a soluble non-viscous manmade polymer that is only partially fermented by the gut microbiota. By comparing the literature on the physiological responses of grain fiber (representing the plant cell wall-associated fiber), inulin (isolated soluble fiber of plant origin), and polydextrose (synthetic soluble fiber), this review attempts to elucidate the “minimum physiological requirements” for food material to be classified as dietary fiber. A discussion is included of how differences in the properties of dietary fiber may be reflected in the subsequent health effects, which is the main reason that dietary fiber remains a topic of intensive research and strong debate. WHAT IS DIETARY FIBER? The term dietary fiber was first used by an Australian scientist, Eben Hipsley, in 1951; he used it to describe lignin, cellulose, and hemicelluloses in food.10 In the 1970s, an Irish surgeon, Dennis Burkitt, and a British physician, Hugh Trowell, contrived with their colleagues “The dietary fiber hypothesis,” which presented the relationship between lack of fiber and certain bowel-related diseases and cardiovascular diseases common in Western countries.11,12 In 1976, Trowell et al.13 further formulated the definition of dietary fiber, and this definition has been the basis of many amendments for over 30 years: Dietary fiber consists of the polysaccharides and lignin in plants that are resistant to hydrolysis by the digestive enzymes of humans. Dietary fiber has long been accepted as an essential constituent of a healthy diet, and dietary guidelines generally recommend a daily intake of 25–35 g, or 3 g/1,000 kJ. Although the dietary advice given in nutri10 tion and health education is consistent in recommending the consumption of dietary fiber-containing foods, the definition of dietary fiber and the methods of analyzing it have been targets of intense discussion, debate, and research. The demand for a global dietary fiber definition have risen as food markets have become more international and developments in food processing have made a range of new indigestible food ingredients available.5 Legislation on food labeling and nutrition and health claims is also evolving, further increasing the demand for a uniform definition of dietary fiber. Dietary fiber has been frequently classified as soluble and insoluble. The distinction is due to the chemical properties of fiber sources and analytical quantification; it does not necessarily reflect the physiological effects.14 The four main dietary fiber definitions in current use highlight the beneficial physiological functionality (Table 1). According to the definition presented in 2001 by the Institute of Medicine (IOM) of the National Academies in the USA, dietary fiber consists of the plant cell wall-associated fibers naturally occurring in fruits, vegetables, and cereal products, and of isolated fibers that are added to processed foods.1 The European Union published their definition in Commission Directive 2008/ 100/EC on 28 October 2008. This document amended Council Directive 90/496/EEC on nutrition labeling for foodstuffs as regards recommended daily allowances, energy conversion factors, and definitions.15 In June 2009, the Codex Committee on Nutrition and Foods for Special Dietary Uses (CCNFSDU) and the Codex Executive Committee published a definition that was almost identical to that of the European Commission.16 These definitions, as well as that of AACC International (formerly the American Association of Cereal Chemists), include isolated fibers and synthetic polymers, but with the condition that they possess beneficial physiological effects.17 The beneficial physiological effects mentioned are shown in Table 2. The fiber definitions also include lignin and other associated compounds that are intrinsic and intact in the plant cell wall. There is no consensus in the definitions regarding whether the non-digestible polymers with a degree of polymerization in the range of three to nine should be defined as fiber. In the EU definition these are included, but the Codex definition leaves it up to national authorities to decide about their inclusion. An unaccomplished target thus far is a generally accepted method of analysis for the quantification of dietary fiber. Traditionally, fiber has been analyzed using the AOAC Official Method of Analysis 985.29, or with other AOAC methods (AOAC methods 991.42, 991.43, 992.16, 993.19, 993.21, and 994.13), which recover nonstarch polysaccharides and lignin but not specific types of fibers such as polydextrose and inulin.18,19 A crucial point Nutrition Reviews® Vol. 69(1):9–21 Table 1 Definitions of fiber from the Codex Alimentarius Comission (Codex),the European Union (EU), the American Association of Cereal Chemists (AACC), and the Institute of Medicine of the National Academies (IOM). Codex (2009)16 EU (2008)15 IOM (2001)1 AACC (2000)17 Dietary fiber means Fibre means carbohydrate Dietary fiber consists of Dietary fiber is the edible carbohydrate polymers* polymers with three or nondigestible carbohydrates parts of plants or with ten or more more monomeric units, and lignin that are intrinsic analogous carbohydrates monomeric units,† which which are neither digested and intact in plants. that are resistant to are not hydrolysed by the nor absorbed in the human digestion and absorption in endogenous enzymes in small intestine and belong the human small intestine the small intestine of to the following with complete or partial humans and belong to the categories*: fermentation in the large following categories: intestine. Edible carbohydrate Edible carbohydrate polymers naturally polymers naturally occurring in the food as occurring in the food as consumed; consumed; Carbohydrate polymers, Edible carbohydrate Added fiber consists of Dietary fiber includes which have been obtained polymers which have been isolated, nondigestible polysaccharides, from food raw material by obtained from food raw carbohydrates that have oligosaccharides, lignin, physical, enzymatic or material by physical, beneficial physiological and associated plant chemical means and which enzymatic or chemical effects in humans. substances. have been shown to have a means and which have a physiological effect of beneficial physiological benefit to health as effect demonstrated by demonstrated by generally generally accepted accepted scientific scientific evidence; evidence to competent authorities; Synthetic carbohydrate Edible synthetic Total fiber is the sum of Dietary fibers promote polymers which have been carbohydrate polymers dietary fiber and added beneficial physiological shown to have a which have a beneficial fiber. effects including laxation, physiological effect of physiological effect and/or blood cholesterol benefit to health as demonstrated by generally attenuation, and/or blood demonstrated by generally accepted scientific glucose attenuation. accepted scientific evidence. evidence to competent authorities * Also lignin and other components such as phenolic compounds, waxes, saponins, phytates, cutin, and phytosterols are considered as fiber when closely associated with carbohydrate polymers of plant origin and extracted with the carbohydrate polymers for analysis of fiber. † Decision on whether to include carbohydrates from 3 to 9 monomeric units should be left to national authorities. in the fiber analysis is precipitation with ethanol. Because polydextrose, inulin, and some other fibers are soluble in ethanol, specific methods have been developed for analyzing these compounds.20–22 The current definitions of dietary fiber do not recommend a specific method of analysis but they acknowledge the lack of uniform recommendations.1,16,17 The Codex committee has established a working group to evaluate the methods for analyzing fiber to correspond with the present definition.23 Chemically, dietary fiber includes non-digestible poly- and oligosaccharides and compounds with which they are closely associated in plants (Table 3). The chemical heterogeneity is reflected in the difficulty associated with using a single method for quantification, and Nutrition Reviews® Vol. 69(1):9–21 chemically different fibers also produce variable physiological effects. PHYSIOLOGICAL EFFECTS OF DIETARY FIBER The current definitions of dietary fiber allow substances other than plant cell wall polymers to be classified as fiber provided they have been shown to have beneficial physiological functions. Usually only one beneficial effect is needed to fulfill the definition, but the criteria for its substantiation is not defined. Improved bowel function (e.g., altered fecal frequency and quality of feces), fermentability by colon microbiota, and attenuated blood glucose and cholesterol levels are the physiological effects most commonly associated with increased use of dietary 11 Table 2 Beneficial physiological effects of dietary fiber mentioned in the dietary fiber definitions from the Institute of Medicine of the National Academies (IOM), AACC International (AACC) and the European Union (EU). Authority Beneficial physiological effect IOM (2001)1 Improved laxation Attenuation of blood cholesterol Attenuation of postprandial blood glucose Influence on immune function Fermentability and production of SCFAs Laxation AACC (2000)17 Attenuation of blood cholesterol Attenuation of blood glucose Decreasing of intestinal transit time EU (2008)15 Increasing of stool bulk Fermentability by colonic microflora Attenuation of blood total cholesterol Attenuation of blood LDL cholesterol Attenuation of postprandial blood glucose Attenuation of blood insulin levels fiber (Table 2). Only the IOM definition (2002) mentions immune function as one of the target functions of dietary fiber. It remains to be seen whether the current research on the immunomodulatory and anti-inflammatory properties of dietary fiber as potential mediators of health effects24–28 will produce useful information for the assessment of fiber functionality in the future. The physiological effects of dietary fiber result from its chemical and physical properties.29 Degradability, molecular weight, viscosity, particle size, cation exchange properties, organic acid absorption, and water-holding capacity are examples of such properties.30,31 Degradability enables the utilization of fiber by colonic bacteria in intestinal fermentation. Fermentation decreases the cecal pH32 and increases the bacterial biomass leading to an increase in fecal output33 and the production of gases and short-chain fatty acids (SCFAs).22–34 SCFAs are the main energy source of the gut epithelial cells.35 Decreased pH promotes the growth of beneficial bacteria,36 such as Bifidobacteria and Lactobacilli,37 improves absorption of some minerals38,39 and inhibits the conversion of primary bile acids to carcinogenic secondary bile acids.40 Viscous fibers form a gel by binding water and thereby decrease the gastric emptying rate and rate of absorption of glucose, triglycerides, and cholesterol.14 Large particle size and water holding capacity decrease transit time by increasing fecal bulk, which prevents constipation and dilutes carcinogenic compounds in the alimentary tract. Physicochemical properties are specific 12 Table 3 Chemical constituents of dietary fiber. Nonstarch polysaccharides Analogous carbohydrates and resistant oligosaccharides Cellulose Indigestible dextrins Hemicellulose Resistant maltodextrins Arabinoxylans Synthesized carbohydrates Arabinogalactans Polydextrose Polyfructoses Methyl cellulose Inulin Hydroxypropylmethyl Oligofructans cellulose Galacto-oligosaccharides Indigestible (resistant) Gums starches Mucilages Pectins Lignin Substances associated with the nonstarch polysaccharide and lignin complex in plants Waxes Phytate Cutin Saponins Suberin Tannins Phenolic compounds to different fibers and can change during cooking or digestion.30 PROPERTIES OF GRAIN FIBER, INULIN, AND POLYDEXTROSE Grain fiber, inulin, and polydextrose are different types of dietary fiber with different physico-chemical properties, as summarized in Table 4. The key difference is that grain fiber contains polysaccharides, which, in association with other compounds, compose a complex cell-wall architecture, while inulin and polydextrose are isolated or synthesized compounds of remarkable chemical homogeneity and much smaller DP. Grain fiber (i.e., cereal fiber) includes edible plant cell wall carbohydrate polymers that occur naturally in the food as consumed. Good sources of grain fiber include whole grains and bran. The outer part of the grain consists of three kernel layers named pericarp, testa, and aleurone.41 Pericarp contains insoluble grain fiber and antioxidants bound to the cell walls; aleurone contains soluble and insoluble fiber, antioxidants, vitamins, and minerals; and testa is composed of alkylresorcinols. The main dietary fiber components in grain fiber are cellulose, arabinoxylans, and b-glucan. Barley and oat are especially Nutrition Reviews® Vol. 69(1):9–21 Table 4 Constituents and characteristics of grain fiber, inulin, and polydextrose. Character Grain fiber Inulin Carbohydrate constituents Arabinoxylans, b-D-glucans, Inulin cellulose, glucomannans Associated components Lignin, proteins, phenolic Disaccharides and compounds oligosaccharides Structure Complex plant cell wall Linear fructose polymer architecture composed of usually with terminal different biopolymers glucose, DP 3–60 Source Water solubility Viscosity (gel-forming) Fermentability Grains, cereal foods Partial Variable Partial Isolated usually from chicory Variable Low Rapid Polydextrose Polydextrose – Glucose polymer with sorbitol end groups, randomly branched chains, average DP 12 Synthesized High – Slow –, not detected. rich in b-glucan,42 and the major dietary fiber constituent in wheat and rye is arabinoxylan.43 Various specific dietary fiber preparations have been prepared by fractionation of grains, such as b-glucan44 and arabinooligosaccharides.45 It is obvious that due to the large chemical heterogeneity, grain fiber is not a well-defined term, and cereal foods that are processed differently and based on different grains can produce a variety of functionalities. Inulin is a linear fructose polymer belonging to the group of fructans.46 It is naturally found in dicotyledonous plants, such as onions, Jerusalem artichoke, and chicory root, and is industrially isolated so it may be added to food items to increase their fiber content and for use as a prebiotic. It is also used as a replacement for fat and sugar.47 Inulin is well tolerated at doses lower than 20 g/day, but it might cause gastrointestinal symptoms such as flatulence at higher doses.48 Inulin consists of monomeric fructose units with glucose as the terminal group.49 The number of fructose units (DP) varies from 3 to 60; the fructans with a DP of less than 10 are named oligofructose. Isolated inulin preparations often contain 5–10% monosaccharides. Inulin is widely accepted as a dietary fiber because of its fermentability and bulking properties.49,50 Polydextrose is a synthesized indigestible glucose polymer that was invented in the 1960s and patented in 1973 in the United States.51 It was developed as a sugar replacement to lower the energy value of food. Polydextrose has randomly polymerized branched chains and varied types of glycosidic bonds that are not hydrolyzed by the human alimentary enzymes and thus give it the properties of dietary fiber. Polydextrose is well tolerated, and the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and the European Commission Scientific Committee for Food (EC/SCF) have concluded the mean laxative threshold of polydextrose is 90 g/day (1.3 g/kg bw) or 50 g as a single dose.52 Polydextrose consists of glucose units and has an average DP of 12.53 Nutrition Reviews® Vol. 69(1):9–21 Polydextrose has been approved as a dietary fiber in a growing number of countries including Japan, Argentina, Belgium, Finland, Norway, France, Australia, and New Zealand. EFFECTS OF GRAIN FIBER, INULIN, AND POLYDEXTROSE ON BLOOD GLUCOSE AND LIPIDS Fibers can affect glucose metabolism by decreasing the glycemic load (GL, defined in54) of a meal, or by interfering with release and absorption of glucose in the small intestine. Soluble viscous fibers are especially related to attenuated absorption of glucose,14 but insoluble cereal fiber has also been shown to improve insulin sensitivity.55 Fiber may additionally influence lipid metabolism. Soluble viscous fibers are hypocholesterolemic, reducing serum cholesterol by about 5–10% for a 5–10 g dose in subjects with hypercholesterolemia, whereas insoluble fibers do not show this effect.32,56 Intake of barley and oat foods decreases total- and LDL-cholesterol.57,58 A few studies in healthy subjects have shown attenuated glucose responses to a single meal containing 4 g or more of soluble fiber59–63 and after a standardized breakfast following an evening test meal that was high in indigestible carbohydrates.64–66 In addition to glucose responses, beneficial effects on postprandial insulin responses and early insulin secretion have been shown with intakes of whole-meal rye bread both in healthy subjects67,68 and in subjects with the metabolic syndrome,69,70 though no effect on glucose responses was present. For oat beta-glucan, the effects on postprandial glucose responses have been shown to be due to high viscosity, but with respect to lipid metabolism, this demand is not as clear.3 Inulin, especially oligofructose, has been reported to affect lipid metabolism in animal studies,71 but the findings in humans are not as marked.72,73 Attenuated levels of serum triglycerides74–77 or total cholesterol74 were reported in some but not in all studies78–81 (Table 5). The 13 Table 5 Effects of inulin and polydextrose on blood glucose and cholesterol levels. Source of fiber Amount of Study Effect added fiber duration Inulin 20 g/d 3 weeks Total cholesterol → LDL, HDL → Postprandial glycemia → Inulin (6–10% monosaccharides) 15 g/d 3 weeks Total cholesterol → LDL, HDL → Postprandial glycemia → Inulin (average DP 9, max.10% 9 g/d with 4 weeks Total cholesterol ↓ LDL, HDL → monosaccharides) rice cereals Postprandial glycemia → Inulin (average DP 9, max.10% 22–34 g/d 64 days Total cholesterol → LDL, HDL → monosaccharides) Inulin (average DP > 23, 0% 10 g/d 8 weeks Total cholesterol → LDL, HDL → monosaccharides) Glucose → Insulin ↓ Inulin (average DP 10, 0.5% 14 g/d 4 weeks Total cholesterol → LDL, HDL → monosaccharides) Polydextrose 16 g/d 12 weeks Total cholesterol → LDL → HDL ↑ Glucose → Postprandial glycemia → Polydextrose 4/8/12 g/d 4 weeks Postprandial glycemia ↓ (4/8 g →) Polydextrose 15 g/d 2 months Total cholesterol → LDL→ HDL ↓ (HDL2 ↓, HDL3 ↑) Reference Causey et al. (2000)*75 van Dokkum et al. (1999)79 Brighenti et al. (1999)74 Kruse et al. (1999)81 Jackson et al. (1999)†76 Pedersen et al. (1997)78 Schwab et al. (2006)‡83 Jie et al. (2000)82 Saku et al. (1991)84 * Subjects had hypercholesterolemia. † Subjects had slightly raised total plasma cholesterol and triglyceride levels. ‡ Subjects had abnormal glucose metabolism. Symbols: ↑, increase; ↓, decrease; →, no effect. discrepancy between results from human and animal studies has been explained by the lower inulin dosage and/or higher dietary fat level in human studies.72 Equivalent doses of inulin cannot be used in humans because of gastrointestinal side effects at intake levels >30 g/d. It is speculated that inulin inhibits de novo fatty acid synthesis in the liver, which leads to attenuation of serum triglycerides, but in humans who have greater fat intake, the synthesis is naturally low and effects cannot be seen. Inulin has not been shown to influence glucose metabolism in humans,74,75,77,79 except for lowering the postprandial insulin response.76 Polydextrose has an excellent water-holding capacity,82 but it does not form viscous solutions. It has no or little effect on fasting glucose and cholesterol levels82–84 (Table 5), but a lowering of the glucose response was reported after a 4-week diet containing 12 g/d of polydextrose.82 Shimomura et al.85 observed attenuated postprandial serum triglycerides when chocolate or fat emulsion contained polydextrose and lactitol instead of sucrose, and Vasankari and Ahotupa86 reported a 25% reduction in postprandial serum triglycerides when a 14 hamburger meal was supplemented with 12.5 g of polydextrose. EFFECTS OF GRAIN FIBER, INULIN, AND POLYDEXTROSE ON BOWEL FUNCTION Dietary fiber affects bowel function by increasing fecal volume and weight (bulking effect), improving stool consistency, decreasing transit time, and increasing stool frequency, all of which eases defecation and prevents upsets in the stomach and gut.34 The bulking effect is mainly due to nonfermentable fiber, but fermentable fiber can increase the bacterial mass and therefore increase stool weight and frequency. It is well demonstrated that grain fiber affects large bowel function. A review87 and individual studies with healthy subjects consuming 11–30 g/day of fiber from bran88–91 or whole grains92–96 (Table 6) showed decreased transit time, increased stool weight, increased stool frequency, and improved stool consistency, and no gastrointestinal tract disorders were reported. Nutrition Reviews® Vol. 69(1):9–21 Table 6 Effects of consumption of grain fiber on bowel function. Source of grain fiber Amount of grain fiber* Study Amount of fiber duration in control diet† High-amylose-WG 23 g/d 4 weeks 21 g/d barley Wholemeal wheat 11 g/d WG rye 18 g/d 4 weeks 19 g/d WG wheat 18 g/d WG barley Total fiber 28.6 g/d 4 weeks 19.2 g/d WG wheat Total fiber 28 g/d 6 weeks 17.8 g/d WG rye 17.4 (women)/ 24.2 (men) g/d 4 weeks 12.7 (women) /15.2 (men) g/d Wheat bran 19 g/d 2 weeks 17 g/d Wheat bran Oat bran 11 g/d 14 g/d 4 weeks 18 or 14 g/d Oat bran 24–29 g/d 4 weeks Wheat bran 11 or 30 g/d 3 weeks 16 (women) /21 (men) g/d 2.1 g/d Effect Reference Stool weight ↑ Bird et al. (2008)92 Stool frequency → Stool weight ↑ Stool volume ↑ Stool frequency → Stool frequency ↑ Stool consistency → Stool frequency ↑ Stool weight ↑ Transit time ↓ (in men) Stool weight ↑ Stool frequency → Stool weight ↑ Stool frequency → Transit time → Stool frequency ↑ Stool weight → Stool weight ↑ Stool consistency ↑ Transit time ↓ McIntosh et al. (2003)95 Li et al. (2003)94 Pereira et al. (2002)96 Gråsten et al. (2000)93 Jenkins et al. (1999)89 Hosig et al. (1996)88 Noakes et al. (1996)120 Lampe et al. (1992)90 * If not reported, total fiber content of diet is mentioned. † Control diet includes low-fiber grain test products. Abbreviation and symbols: WG, wholegrain; ↑, increase; ↓, decrease; →, no effect. Inulin has no insoluble fractions and it is rapidly degraded in the gut; therefore, it has less potential to influence bowel function than grain fiber. However, inulin has been shown to increase stool frequency97–99 and it exerts a mild bulking effect by increasing bacterial mass33 (Table 7). Polydextrose also has beneficial effects on bowel function. It has been shown to increase stool bulk82,100,101 and frequency,82,100 to improve stool consistency,82,101 and to make defecation easier82,102 (Table 7). Due to its branched chain structure, polydextrose is only partially degraded in the gut. Therefore, increased stool weight is due to increased indigestible mass in the gut, but also to increments of total bacterial mass.103 Because of their ability to increase stool frequency, both inulin and polydextrose could prevent constipation. FERMENTABILITY OF GRAIN FIBER, INULIN, AND POLYDEXTROSE Fermentation of fiber by colon microbiota under anaerobic conditions results in the production of SCFAs, such as acetate, propionate, and butyrate. To be fermentable, fiber must have a chemical structure that intestinal microbes can degrade.104 Solubility, type of linkages, degree of polymerization, and transit time are factors that influence fermentability.105 Typically, the shorter the chain length, the more rapidly the fiber is fermented, but the chemical Nutrition Reviews® Vol. 69(1):9–21 structure also plays an important role. Highly fermentable fibers are fermented predominantly in the proximal colon, whereas less fermentable fibers reach the distal colon.106 Hence, inulin is fermented in the proximal colon, grain fiber throughout the large intestine, and polydextrose mainly in the distal colon. The overall slow rate of fermentation of grain fiber also contributes to the prolonged release of associated antioxidative phenolic compounds, such as ferulic acid, into the bloodstream.9 Grain fiber contains fermentable dietary fiber in variable amounts. The main fermentable dietary fiber components of grain are fructans, arabinoxylans, and b-glucans.107 Rye, wheat, and oat bran are fermented in vitro at a slower rate than inulin, with fermentation continuing up to 24 h. Fermentation of oat bran is slightly faster than that of wheat and rye brans. As a soluble and viscous fiber, inulin is efficiently fermented by the gut microbes,107 while polydextrose is fermented more slowly and gradually throughout the intestine108 and the major portion is excreted in the feces.109 In vitro fermentation studies performed with human fecal inocula (a mixed culture of human fecal bacteria gained from healthy donors) confirm that fermentation of grain fiber,107,110–114 inulin,105,107,115 and polydextrose105,115–117 produces SCFAs. Measurement of SCFAs in human studies is challenging,118 since most of the SCFAs produced by bacterial metabolism are instantly absorbed from the colon, and consequently the 15 Table 7 Effects of inulin and polydextrose in the large intestine. Source of fiber Amount of added fiber Study Effect duration Inulin (average DP 10, 10 g/d 3 weeks Fecal pH → Fecal SCFA → 8% monosaccharides) Bifidobacteria ↑ Inulin (average DP > 23, 15 g/d 1 week Stool frequency ↑ Transit time → 0% monosaccharides) Stool weight → Inulin 20 g/d 3 weeks Transit time → Stool frequency → Stool weight → Inulin ( 6–10% 15 g/d 3 weeks Transit time → Stool weight → monosaccharides) Fecal pH → Fecal SCFA ↑ Inulin (average DP 9, 22–34 g/d 64 days Fecal SCFA → Bifidobacteria ↑ <10% monosaccharides) Inulin (average DP 10) 20 g/d 8 days, gradually 19 days Stool frequency ↑ Stool consistency ↑ increased (3 days) to Ease of defecation ↑ 40 g/d for 8 days Stool weight → Fecal pH → Fecal SCFA → Bifidobacteria ↑ Inulin (average DP 10) 15 g/d 15 days Stool frequency ↑ Stool weight ↑ Transit time → Bifidobacteria ↑ Polydextrose 8 g/d 3 weeks Transit time ↓ Ease of defecation ↑ Stool consistency → Stool weight → Fecal pH ↓ Fecal SCFA → Polydextrose 4/8/12 g/d 4 weeks Stool frequency ↑ Stool consistency ↑ Ease of defecation ↑ Stool weight ↑, (4 g →) Fecal pH ↓ Fecal SCFA ↑, (4 g →) Bifidobacteria ↑ Polydextrose 30 g/d 4 weeks Transit time → Stool weight → Polydextrose 15 g/d 2 weeks Stool frequency ↑ Stool weight ↑ Fecal pH ↓ Polydextrose 10/30 g/d 10 days Transit time → Stool frequency → stool consistency ↑(10 g →) Stool weight ↑ Ease of defecation → Reference Ramirez-Farias et al. (2009)121 Den Hond et al. (2000)98 Causey et al. (2000)*75 van Dokkum et al. (1999)79 Kruse et al. (1999)81 Kleessen et al. (1997)†99 Gibson et al. (1995)97 Hengst et al. (2008)102 Jie et al. (2000)82 Achour et al. (1994)135 Endo et al. (1991)100 Tomlin (1988)101 * Subjects had hypercholesterolemia. † Subjects had constipation. Symbols: ↑, increase; ↓, decrease; →, no effect. 16 Nutrition Reviews® Vol. 69(1):9–21 production of SCFAs is not directly associated with the concentration of SCFAs in feces.119 In human studies, grain fiber either has had no effect on total SCFA concentration89,92,93,95,120 or it has increased the concentration of butyrate in feces.92,93,95 Increased fecal SCFA concentration has been reported only in one human study on inulin79 and one on polydextrose.82 However, reduced fecal pH in human studies has been detected following consumption of grain fiber88,90,92,95,120 and polydextrose82,100,102 but not with inulin,79,99,121 which is rapidly fermented in the proximal colon. CONCLUSION Dietary fiber has recently been redefined in ways that are wide-ranging and allow many different types of food components to be classified as fiber. The definitions of both the EU and Codex point out that isolated fibers must have beneficial physiological effects in order to fulfill the definition. Nonetheless, the definitions do not specify these effects. Thus, the physiological effects must be considered individually for different substances. Although the definitions created by the EU and Codex are similar, there is a key difference: the EU definition allows polymers within the DP range of 3–10 to be classified as fiber, but Codex assigns responsibility for assigning this designation to national authorities. In the case of inulin and polydextrose this is an important point, because these substances contain oligomers that are both below and above DP 10. The evidence reviewed in this article and summarized in Table 8 shows clearly that inulin and polydextrose can exert some beneficial gastrointestinal effects that are similar to those of grain fibers and thus fulfill both of the definitions of dietary fiber in this respect. However, based on the available evidence, it is not possible to conclusively determine their role in glucose and cholesterol attenuation. In this field, the literature pertaining to grain fibers spans a wide range of preparations and matrixes. Several epidemiological studies and meta-analyses show that increased intakes of cereal dietary fiber or whole-grain foods is associated with reduced risk of type 2 diabetes122–128 and cardiovascular diseases.129–131 Furthermore, recent studies concluded that high intakes of cereal fiber and whole-grain products reduce the risk of colon or colorectal cancer,132,133 though opposite results have also been reported.134 Epidemiological studies, however, show only the associations of dietary components with chronic disease risk; they do not give information on the causal relationships. It is thus difficult to prove that inulin and polydextrose have the same kinds of effects on chronic diseases without carrying out wellcontrolled intervention studies that explore the role of specific dietary components on risk factors of chronic diseases. It is likely that many more health-promoting mechanisms of dietary fiber will emerge in the future, including effects on immunomodulation and antiinflammatory properties. The accumulating information might help establish a more detailed set of criteria for dietary fiber with regard to its role in different diseases. Since the functions of different fibers vary, it is recommendable to eat a varied diet that provides dietary fiber from different food sources. A range of fiber ingredients can be used to increase the fiber content of processed foods that appeal to consumers, but it is not likely that a single dietary fiber source will produce all of the potential health outcomes anticipated for dietary fiber consumption. The baseline for demonstration of the physiological functionality of a fiber should be its effects on intestinal functions, such as fermentability, and its effects on stool frequency and consistency.Other physiological effects,such as effects of glucose and lipid metabolism or the oxidative defense Table 8 Summary of physiological functionalities demonstrated for grain fiber, inulin, and polydextrose. Physiological functionality Grain fiber Inulin Polydextrose Blood glucose and cholesterol Attenuation of postprandial glucose ⫾* ⫾ Attenuation of blood total cholesterol ⫾* Promotion of blood HDL cholesterol ⫾ Attenuation of blood LDL cholesterol ⫾* Large intestine + Attenuation of fecal pH + +† + Production of SCFA + +† Increase of Bifidobacteria ⫾* + ⫾ Increase of stool frequency + + + Decrease of intestinal transit time + ⫾ Increase of stool weight + ⫾ Improvement of stool consistency + + + * Results vary depending on type of grain fiber. † Shown only in in vitro studies. Nutrition Reviews® Vol. 69(1):9–21 17 system, may then link various fibers to specific health outcomes. Progress is foreseen in this area, but in order for it to occur, the performance of a series of mechanistic intervention studies of both natural and man-made fibers is required. Acknowledgments Funding. Financial support was provided by Danisco Ltd with a grant to Kaisa Raninen, and by the Raisio Foundation with a grant to Jenni Lappi. Additional financial support was provided by the NCoE on “HELGA: Nordic Health – Wholegrain Food” (Hannu Mykkänen), Academy of Finland (Hannu Mykkänen and Kaisa Poutanen), and by the European Commission in the Communities 6th Framework Programme, Project HEALTHGRAIN (FOOD-CT-2005-514008) (Hannu Mykkänen and Kaisa Poutanen). The present publication reflects the authors’ views and the Community is not liable for any use that may be made of the information contained in this publication. Declaration of interest. 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