Dietary Fibre Functionality in Food and Nutraceuticals E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Food Science and Technology
- Sprache: Englisch
Increasing fiber consumption can address, and even reverse the progression of pre-diabetes and other associated non-communicable diseases. Understanding the link between plant dietary fiber and gut health is a small step in reducing the heavy economic burden of metabolic disease risks for public health. This book provides an overview of the occurence, significance and factors affecting dietary fiber in plant foods in order to critically evaluate them with particular emphasis on evidence for their beneficial health effects.
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Table of Contents
Cover
Title Page
Copyright
List of Contributors
Preface
Chapter 1: Do the Physical Structure and Physicochemical Characteristics of Dietary Fibers Influence their Health Effects?
1.1 Influence of the Chemical and Physical Structure on the Metabolic Effects of Fibers
1.2 Influence of the Physicochemical Properties of Fibers on their Metabolic Effects
1.3 The Effect of Fiber Structure on Fermentation Patterns and Microbiota Profiles: Slowly versus Rapidly Fermented Fiber
1.4 Conclusions
References
Chapter 2: Interaction of Phenolics and their Association with Dietary Fiber
2.1 Introduction
2.2 Phenolic Compounds
2.3 Bioactivities of Phenolics
2.4 Dietary Fiber
2.5 Antioxidant Dietary Fiber
2.6 Protein–Phenolic Interactions
2.7 Starch–Phenolic Interactions
2.8 Phenolic Compounds and Starch Digestibility
2.9 Interactions of Phenolic Compounds
2.10 Phenolics and Dietary Fiber
2.11 Conclusion
References
Chapter 3: Dietary Fiber-Enriched Functional Beverages in the Market
3.1 Introduction
3.2 Dietary Fiber Definition and Classification
3.3 Fiber-Enriched Non-Dairy Beverages
3.4 Suitable Dietary Fiber Types for Fortifying Non-Dairy Drinks
3.5 Contributions of Beverages in Dietary Studies
3.6 The Functional Beverage Market
3.7 Fiber-Enriched Dairy Products
References
Chapter 4: Dietary Fiber as Food Additive: Present and Future
4.1 Dietary Fiber: Definition
4.2 Chemical Nature of Dietary Fiber Used as Food Additive
4.6 Conclusions
References
Chapter 5: Biological Effect of Antioxidant Fiber from Common Beans (Phaseolus vulgaris L.)
5.1 Introduction
5.2
Phaseolus vulgaris
Generalities
5.3 Composition of Common Bean Antioxidant Fiber
5.4 Biological Potential of Antioxidant Fiber of Common Bean
References
Chapter 6: In Vivo and In Vitro Studies on Dietary Fiber and Gut Health
6.1 Introduction
6.2 Research into Dietary Fiber and Health
6.3
In Vivo
Studies on Intestinal Function
6.4
In Vitro
Studies
6.5 Current Trends and Perspectives
6.6 Conclusion
References
Chapter 7: Dietary Fiber and Colon Cancer
7.1 Introduction
7.2 Physiological Action and Function of Dietary Fiber in Colon Cancer
7.3 Colon Cancer Chemopreventive Bioactivities
7.4 Future Directions: Food Designs New Structures for Colon Cancer Prevention
7.5 Conclusions
References
Chapter 8: The Role of Fibers and Bioactive Compounds in Gut Microbiota Composition and Health
8.1 The Influence of Gut Microbiota in Health and Disease
8.2 Bioactive Substances and Fiber Promoting a Healthy Gut
8.3 Survey of Epidemiological Studies
8.4 Diabetes
8.5 Infertility
8.6 Mental Health and Gut Microbiota
8.7 Cancer of the Gastrointestinal Tract and Extragastrointestinal Organs
8.8 Conclusion
References
Chapter 9: Effect of Processing on the Bioactive Polysaccharides and Phenolic Compounds from Aloe vera (Aloe barbadensis Miller)
9.1
Aloe vera
9.2 Effect of Processing on the Main Bioactive Compounds from
Aloe vera
9.3 Conclusions
References
Index
End User License Agreement
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Guide
cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: Do the Physical Structure and Physicochemical Characteristics of Dietary Fibers Influence their Health Effects?
Figure 1.1 Chemical, physical and physicochemical properties of dietary fiber, their digestive fate and potential health effects via the gut.
Chapter 2: Interaction of Phenolics and their Association with Dietary Fiber
Figure 2.1 Chemical structures of hydroxybenzoic acids.
Figure 2.2 Chemical structures of hydroxycinnamic acids.
Figure 2.3 Chemical structures of flavonoids.
Figure 2.4 Chemical structures of ferulic acid cross-linked compounds.
Figure 2.5 Structures of proanthocyanidins reported in sorghum.
Figure 2.6 Antioxidant capacity of individual phenolic acids and mixtures containing two (a) and three (b) phenolic acids. (A) Gallic acid, (B) protocatechuic acid, (C) chlorogenic acid, (D) vanillic acid.
Source:
Data from Saura-Calixto (2012).
Chapter 4: Dietary Fiber as Food Additive: Present and Future
Figure 4.1 Composition of dietary fibers and associated substances.
Source
: Thebaudin
et al.
(1997). Reproduced with permission from Elsevier.
Figure 4.2 Polymers of fructose. (a) Inulin; (b) agavin.
Figure 4.3 Chitin structure.
Figure 4.4 Pectin (polygalacturonic acid) structure.
Figure 4.5 Cellulose (β1–4 glucose)
n
.
Figure 4.6 Hemicellulose structure.
Chapter 5: Biological Effect of Antioxidant Fiber from Common Beans (Phaseolus vulgaris L.)
Figure 5.1 Biological potential of common bean antioxidant fiber.
Chapter 6: In Vivo and In Vitro Studies on Dietary Fiber and Gut Health
Figure 6.1 Distal colon tissue stained with hematoxylin and eosin. (a) Control; (b) cooked bean; (c) non-digestible fraction; (d) azoxymethane; (e) cooked bean + azoxymethane; (f) non-digestible fraction + azoxymethane. Magnification × 20. The aberrant crypt foci (ACF) (indicated by arrows) increased staining intensity of epithelial cytoplasm and presented irregular elongation of the ducts. Some ACF presented conical shape of the focus. Source: Vergara-Castañeda
et al.
(2010). Reproduced with permission of the Royal Society of Chemistry.
Figure 6.2 Expression of (a) apoptosis-related proteins, (b) cell cycle-related proteins, and (c) expression of MSH2, NFκB, and HDAC1-related proteins in HT29 cells after 24 hours of treatment with LC
50
/FP (fermented polysaccharide)-hgf and SCFAs mixture found in the LC
50
/FP-hgf. Expression was analyzed by Western blot using specific antibodies. Control: protein expression in cells without any treatment. The blot was tested with anti-actin antibody to confirm equal protein loading. The protein expression was normalized to β-actin. Data are the mean ± standard errors of three independent experiments (
p
< 0.05 vs. control). Source: Campos-Vega
et al.
(2012). Reproduced with permission of the American Chemical Society.
Figure 6.3 Dietary fiber effects on gut health:
in vitro
and
in vivo
evidence. Short-chain fatty acids (SCFAs), metabolic products from dietary fiber (DF), play a role in cell differentiation and growth, have anti-inflammatory effects by promoting recruitment of neutrophils, while butyrate is a natural inhibitor of the histone deacetylases. SCFAs confer protection again chronic diseases such colon cancer. Dietary fiber increases mineral absorption, mainly Ca and Mg. It has also been related to a decrease in proinflammatory cytokine levels and may acutely reduce inflammatory activity. Certain types of fibers stimulate growth of intestinal bacteria during fermentation. Dietary fiber has a key role in the gut in producing a variety of enteroendocrine-derived peptides that control and modulate miscellaneous metabolic and physiological processes and create a link between the gut and the brain. Their health benefits on attenuation of colonic inflammation is mediated through beneficial effects on intestinal microbiota and by increasing the colonic SCFAs concentration. Prevention of intestinal inflammation may be achieved with fermentable prebiotic fibers that enhance beneficial bifidobacteria or with soluble fibers that block bacterial–epithelial adherence (contrabiotics). The dysbiosis (perturbations to the structure of complex commensal communities) observed in inflammatory bowel diseases, such as ulcerative colitis, irritable bowel syndrome, and dietary fiber can modulate Crohn's disease. Weight management effects are linked to SCFAs, which are substrates for the host and induce the release of satiety hormones, and regulate microbiota in obesity. Fiber-enriched diets improve insulin sensitivity and glucose tolerance and increase butyrate-producing bacteria. Postprandial serum glucose is lowered by dietary fiber through at least three pathways: increasing the viscosity of the small intestinal content and slowing the diffusion of glucose; adsorbing glucose and preventing its diffusion; and inhibiting the activity of α-amylase and postponing the release of glucose from starch. Interestingly, dietary fiber improves microbiota dysbiosis, which predicts acute cardiovascular events in a large general population. Finally,
in vitro
and
in vivo
evidence supports health claims about the role of dietary fiber in cancer prevention.
Chapter 8: The Role of Fibers and Bioactive Compounds in Gut Microbiota Composition and Health
Figure 8.1 The gut microbiota is composed of a diverse number of bacterial species. Source: Sartor (2008). Reproduced with permission of Elsevier.
Figure 8.2 Environmental factors can cause inflammation and dysbiosis in the gut. Source: de Vos
et al.
(2013). Reproduced with permission of Elsevier.
Figure 8.3 Immunosenescence in aging causes inflammaging, leading to gut microbiota dysbiosis, which further increases inflammation and promotes disease in older adults. Source: Data from Biagi
et al.
(2012).
Figure 8.4 There are some similarities and some differences in microbiota composition across different countries. Source: Data from Bisanz
et al.
(2015), Blaut
et al.
(2002), De Filippo
et al.
(2010), Gu
et al.
(2016), Hisada
et al.
(2015), Lin
et al.
(2013), Raymond
et al.
(2015), Yatsunenko
et al.
(2012).
Figure 8.5 The gut microbiota influences health balance in our bodies, including mental health and behavior. Source: Foster and McVey Neufeld (2013). Reproduced with permission from Elsevier.
Figure 8.6 Environmental influences such as a poor diet can lead to gut microbiota dysbiosis, increasing inflammation and cancer risk. However, intake of dietary fiber and bioactive compounds may help return homeostasis, leading to a decreased risk. Source: Data from Keibel
et al.
(2009), Ostan
et al.
(2015).
Figure 8.7 Polyphenols may play an important role in reducing inflammation and cancer risk and improving cancer prognosis through several molecular pathways. Source: Ostan
et al.
(2015).
Chapter 9: Effect of Processing on the Bioactive Polysaccharides and Phenolic Compounds from Aloe vera (Aloe barbadensis Miller)
Figure 9.1
Aloe vera
(
Aloe barbadensis
Miller) plant.
Figure 9.2 Chemical structure of acemannan polymer. Source: Adapted from Chokboribal
et al.
(2015).
Figure 9.3 Schematic representation of the pectin structure. Source: Adapted from McConaughy
et al.
(2008a).
Figure 9.4 Structure of some phenolic compounds from
Aloe vera
. Source: Adapted from Olennikov
et al.
(2013) and Sun
et al.
(2015).
List of Tables
Chapter 2: Interaction of Phenolics and their Association with Dietary Fiber
Table 2.1 Total phenolic content (TPC) of methanolic extracts of tropical fruits and reduction of TPC by adding fruit dietary fiber
Chapter 3: Dietary Fiber-Enriched Functional Beverages in the Market
Table 3.1 The chemical composition and sources of dietary fiber
Table 3.2 β-Glucan-enriched beverage intake studies and related health benefits
Table 3.3 Inulin-enriched beverage intake studies and related health benefits
Table 3.4 Flaxseed-enriched beverage intake studies and related health benefits
Table 3.5 Industrial fibers used in beverages (www.preparedfoods.com)
Table 3.6 Fiber-enriched dairy beverage intake studies and related health benefits
Chapter 4: Dietary Fiber as Food Additive: Present and Future
Table 4.1 Classification of dietary fiber according to its water solubility properties
Table 4.2 Effect of various dietary fibers added in bread making
Chapter 5: Biological Effect of Antioxidant Fiber from Common Beans (Phaseolus vulgaris L.)
Table 5.1 NDF composition of different
Phaseolus vulgaris
varieties
Table 5.2 Biological effects of antioxidant fiber of
Phaseolus vulgaris
L. on colon cancer models
Table 5.3 Protein levels summary of colon cancer models in response to the antioxidant fiber of
Phaseolus vulgaris
L
Table 5.4 Differential gene expression summary of colon cancer models in response to the antioxidant fiber of
Phaseolus vulgaris
L
Chapter 6: In Vivo and In Vitro Studies on Dietary Fiber and Gut Health
Table 6.1 Regulated genes in treated (human gut flora-FE-hgf, fermented NDF-cv. Bayo Madero) compared with untreated HT29 cells
Table 6.2
In vivo and in vitro
dietary fiber effects on gut health
Functional Foods Science and Technology Series
Functional foods resemble traditional foods but are designed to confer physiological benefits beyond their nutritional function. Sources, ingredients, product development, processing, and international regulatory issues are among the topics addressed in Wiley's Functional Food Science and Technology book series. Coverage extends to the improvement of traditional foods by cultivation, biotechnological, and other means, including novel physical fortification techniques and delivery systems such as nanotechnology. Extraction, isolation, identification, and application of bioactives from food and food processing by-products are among other subjects considered for inclusion in the series.
Series Editor: Professor Fereidoon Shahidi, PhD, Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada.
The books under the series are as follows,
Dried Fruits: Phytochemicals and Health Effects
by Cesarettin Alasalvar (Editor), Fereidoon Shahidi
Bio-Nanotechnology: A Revolution in Food, Biomedical and Health Sciences
by Debasis Bagchi (Editor), Manashi Bagchi, Hiroyoshi Moriyama, Fereidoon Shahidi
Cereals and Pulses: Nutraceutical Properties and Health Benefits
by Liangli L. Yu (Editor), Rong Tsao (Editor), Fereidoon Shahidi (Editor)
Functional Food Product Development
by Jim Smith (Editor), Edward Charter (Editor)
Nutrigenomics and Proteomics in Health and Disease: Food Factors and Gene Interactions
by Yoshinori Mine (Editor), Kazuo Miyashita (Editor), Fereidoon Shahidi (Editor)
Dietary Fiber Functionality in Food and Nutraceuticals
From Plant to Gut
Edited by Farah Hosseinian, B. Dave Oomah and Rocio Campos-Vega
This edition first published 2017 © 2017 by John Wiley & Sons Ltd
Registered office:
John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, P019 8SQ, UK
Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK The Atrium, Southern Gate, Chichester, West Sussex, P019 8SQ, UK 111 River Street, Hoboken, NJ 07030-5774, USA
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Library of Congress Cataloging-in-Publication Data
Names: Hosseinian, Farah, 1960, Oomah, B. Dave, and Campos-Vega, Rocio, editors
Title: Dietary fiber functionality in food & nutraceuticals : from plant to gut / [edited] by Farah Hosseinian.
Other titles: Dietary fiber functionality in food & nutraceuticals | Dietary fibre functionality in food and nutraceuticals | Functional food science and technology series.
Description: Chichester, UK ; Hoboken, NJ : John Wiley & Sons, 2017. | Series: Functional foods science & technology series | Includes bibliographical references and index.
Identifiers: LCCN 2016039780 (print) | LCCN 2016053505 (ebook) | ISBN 9781119138051 (cloth) | ISBN 9781119138075 (pdf) | ISBN 9781119138082 (epub)
Subjects: LCSH: Food–Fiber content–Analysis. | Fiber in human nutrition. | Functional foods.
Classification: LCC TX553.F53 D54 2017 (print) | LCC TX553.F53 (ebook) | DDC 613.2/63–dc23
LC record available at https://lccn.loc.gov/2016039780
A catalogue record for this book is available from the British Library.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Cover images: Alaettin YILDIRIM/Shutterstock (center); Science Photo Library/Shutterstock (bottom-right); SCIEPRO/gettyimages (bottom-left)
List of Contributors
Maritza Alonzo-Macías
Escuela de Ingeniería y Ciencias
Tecnologico de Monterrey
Querétaro
Mexico
Nawal Alsadi
Faculty of Medicine
University of Ottawa
Ottawa
Ontario
Canada
Rocio Campos-Vega
Programa de Posgrado en Alimentos del Centro de la República (PROPAC)
Universidad Autónoma de Querétaro
Querétaro
Mexico
Anaberta Cardador-Martínez
Escuela de Ingeniería y Ciencias
Tecnologico de Monterrey
Querétaro
Mexico
Anoma Chandrasekara
Department of Applied Nutrition
Wayamba University of Sri Lanka
Makandura
Gonawila
Sri Lanka
María Teresa Espino-Sevilla
Universidad de Guadalajara
Centro Universitario de la Ciénega
Ocotlán
Mexico
Anthony Fardet
INRA, UMR 1019, UNH, CRNH Auvergne, Clermont-Ferrand and Clermont Université
Université d'Auvergne
Unité de Nutrition Humaine
Clermont-Ferrand
France
Antoni Femenia
Department of Chemistry
University of the Balearic Islands
Balearic Islands
Spain
Marcela Gaytan-Martinez
Programa de Posgrado en Alimentos del Centro de la República (PROPAC)
Research and Graduate Studies in Food Science, School of Chemistry
Universidad Autónoma de Querétaro
Querétaro
Mexico
Émilie A. Graham
Faculty of Health Sciences
University of Ottawa
Ottawa
Ontario
Canada
Aynur Gunenc
Food Science and Nutrition
Department of Chemistry
Carleton University
Ottawa
Ontario
Canada
Farah Hosseinian
Food Science and Nutrition
Department of Chemistry
Carleton University
Ottawa
Ontario
Canada
Majed Jambi
Faculty of Medicine
University of Ottawa
Ottawa
Ontario
Canada
Guadalupe Loarca-Piña
Programa de Posgrado en Alimentos del Centro de la República (PROPAC)
Research and Graduate Studies in Food Science, School of Chemistry
Universidad Autónoma de Querétaro
Querétaro
Mexico
Diego A. Luna-Vital
Programa de Posgrado en Alimentos del Centro de la República (PROPAC)
Research and Graduate Studies in Food Science
School of Chemistry
Universidad Autónoma de Querétaro
Querétaro
Mexico
Maria Elena Maldonado
Escuela de Nutrición y Dietética
Universidad de Antioquia
Medellín
Colombia
Jean-François Mallet
Faculty of Medicine
University of Ottawa
Ottawa
Ontario
Canada
Sandra T. Martín del Campo
Escuela de Ingeniería y Ciencias
Tecnologico de Monterrey
Querétaro
Mexico
Chantal Matar
Faculty of Health Sciences and Faculty of Medicine
University of Ottawa
Ottawa
Ontario
Canada
José Rafael Minjares-Fuentes
Department of Chemistry
University of the Balearic Islands
Balearic Islands
Spain
Luis Mojica
Department of Food Science and Human Nutrition
University of Illinois at Urbana-Champaign
Urbana
USA
B. Dave Oomah
Retired
Formerly with Pacific Agri-Food Research Centre
Agriculture and Agri-Food Canada Summerland
British Columbia
Canada
Aurea K. Ramírez-Jiménez
Programa de Posgrado en Alimentos del Centro de la República (PROPAC)
Research and Graduate Studies in Food Science, School of Chemistry
Universidad Autónoma de Querétaro
Querétaro
Mexico
Fereidoon Shahidi
Department of Biochemistry
Memorial University of Newfoundland
St. John's
NL
Canada
Luz Amparo Urango
Escuela de Nutrición y Dietética
Universidad de Antioquia
Medellín
Colombia
Haydé A. Vergara-Castañeda
Nucitec
S.A. de C.V.
Querétaro
Mexico
Preface
Dietary fiber is an essential component of most dietary guidelines and regulations although the vast majority of the population consume less than the recommended amount. Individuals with total fiber intake of over 26 g per day have an 18% lower risk of developing diabetes compared to those consuming less than 19 g total fiber per day according to an 11-year diabetes study. Moreover, every daily 10 g increase in overall fiber intake reduces the risk of dying by 15% over the 9 year follow-up period. A high intake of whole grains (210–225 g/day as a fiber source) has also been associated with reduced risk of coronary heart disease, cardiovascular disease, total cancer, and all-cause mortality, as well as mortality from respiratory disease, infectious disease, diabetes, and all non-cardiovascular, non-cancer causes. Regular cereal fiber consumption can reduce the risk of all-cause (19%), heart disease-related (up to 18%), and cancer (15%) mortality. The importance of dietary fiber intake on gut health has been demonstrated in the Belgian Flemish Gut Flora Project (FGFP), where individuals preferring low fiber bread as the major carbohydrate source had reduced microbiome diversity. This is in line with the marketing focus of the function and advantages of dietary fibers in gastrointestinal health benefits, cancer prevention, diabetes risk reduction, cholesterol-lowering effects, and weight management.
It is estimated that the global dietary fiber market volume will reach 465 128.3 metric tons by 2019, with a projected cumulative annual growth rate of 10.4% from 2014 to 2019. Novel fibers are projected to drive this growth, although conventional fibers still dominate the market. Thus, developments and technology of unearthing new fiber sources may boost the demand for dietary fibers, create opportunities for the novel fiber segment, and extend end-use beyond established sectors, particularly for certain segments of the population.
This book presents a large volume of new data. The reader is directed to the table of contents, which illustrates the wide coverage of subjects related to dietary fibers. Knowledge of the physical structure and physicochemical characteristics of dietary fiber on human health (Chapter 1) is essential in providing key restraints/requirements for novel dietary fiber end-use. Phytochemicals often associated with fiber and their interactions (Chapters 2 and 8) may account for the various protective roles of dietary fiber in health. Niche applications are explored (Chapter 3) in the ever-expanding beverage sector with both conventional and novel fiber types. The various fiber fortified foods (Chapter 4) provide a proactive approach towards increasing daily dietary fiber intake in improving human health. The mechanisms of the purported benefits of dietary fibers (Chapters 5, 6, 7, and 8) are explored for specific diseases. Novel fibers (Chapters 5 and 9) may be the launching pad for new types of products that cross over various market segments.
This book should give researchers, nutritionists, health professionals, chemists, and industry professionals interested in dietary fibers useful and up-to-date information to advance the field.
Farah HosseinianB. Dave OomahRocio Campos-Vega
Chapter 1Do the Physical Structure and Physicochemical Characteristics of Dietary Fibers Influence their Health Effects?
Anthony Fardet
INRA, JRU 1019, UNH, CRNH Auvergne, F-63000 Clermont-Ferrand & Clermont Université, Université d'Auvergne, Unité de Nutrition Humaine, BP 10448, F-63000, Clermont-Ferrand, France
Studies on humans, animals, and in vitro have clearly shown that the way dietary fiber is degraded and fermented throughout the digestive tract depends on both its physical and chemical structure (intrinsic properties such as crystallinity and particle size) and its interaction with the closed environment of the gut (i.e., physical–chemical properties such as porosity, water-holding capacity, and solubility) (Guillon and Champ, 2000). For example, cellulose, which has a compact structure, is only partially fermented whereas soluble pectin is fully fermented, due to its much greater porosity (Fardet et al., 1997; Salvador et al., 1993). Thus, a greater porosity enables enzymes to access their substrate and degrade it more efficiently. This illustrates the interaction between factors such as porosity, solubility, and water-holding capacity.
Although much is known about factors influencing the fermentation of dietary fiber, less is known about the influence of a change in fiber structure, either isolated or within a complex food matrix, on human health. For example, is an increase in the porosity of fibers in a food beneficial? What are the consequences of higher fiber porosity on the short-chain fatty acid (SCFA) profiles generated during fermentation in the colon? Increasing porosity probably increases the rate of fermentation within the colon, yielding a more rapid and massive surge of SCFAs. But does the way the SCFAs are released have any effect on human physiology and health? Do the exact location where SCFAs are released (transverse, ascending, or descending colon) influence human health? These questions are of great interest in terms of the important physiological roles of the main SCFAs: butyric (Blouin et al., 2011), propionic (Hosseini et al., 2011), and acetic (Kondo et al., 2009) acids.
Although today we cannot fully answer these questions, this review will attempt to discuss the physicochemical parameters of fiber that can be modified and their relationship with their effects on human physiology and/or health (e.g., glycemia, cholesterolemia, satiety, microbiota, and fecal bulking). In a recent publication, Monro notably reviewed and discussed the impact of polysaccharide-based structures on nutritional properties in the foregut, focusing on complex foods containing such fiber-based structures (Monro, 2014). This review is more focused on isolated fibers and their structural features; some of the best studied being crystallinity, particle size, solubility, porosity, water-holding capacity, and the ability to adsorb bile acids, complex minerals, and trace elements (Guillon and Champ, 2000).
1.1 Influence of the Chemical and Physical Structure on the Metabolic Effects of Fibers
The intrinsic properties of fibers, their chemical and physical structure, are fundamental to their biological actions. The chemical structure of a fiber greatly influences the rate and extent of its fermentation in the colon. Thus, pectins, hemicelluloses, cellulose, lignin, and resistant starch (all included in the definition of fiber) are not all fermented at the same speed and the same extent. Cellulose has a compact structure, whereas hemicellulose is much more porous and more accessible to bacterial enzymes. Hence, hemicelluloses are almost completely degraded in the colon, but cellulose is only partially fermented and is excreted in the feces. Lignins are almost undegraded in humans (Holloway et al., 1978; Slavin et al., 1981).
Interestingly, Eastwood et al. (1986) showed that there is no obvious correlation between the chemical composition, structure, molecular size, shape, and physical properties of a fiber and its physiological effects in humans. For example, wheat bran and gum tragacanth have very different chemical structures but they have similar physiological effects. However, these findings are only valid for the physiological properties tested: the weight of stool, serum cholesterol levels, and the excretion of hydrogen. From this study, other physiological parameters have been tested.
It is hardly surprising that the chemical structure of a fiber influences its physiological effects, as each type of fiber is a complex mixture of carbohydrates (including pentoses and hexoses). A review has focused on the relationships between the molecular structure of cereal fibers and their physiological effects in humans (Gemen et al., 2011). There appears to be a clear link between the chemical structure of a fiber and blood glucose and insulin responses and satiety. However, the authors emphasize that information on the molecular structure are rarely given in the literature and there are no obvious trends in the relationship between the molecular structures of fibers and their fermentation profiles in humans (Gemen et al., 2011).
1.1.1 Changing the Molecular Weight
Some of the results appear contradictory. Some studies have shown that reducing the molecular weight of a fiber, and hence its potential viscosity in vivo, has no significant effect on the glycemic response (Ellis et al., 1991; Gatenby et al., 1996). These authors concluded that low molecular weight guar gum can be used in bread instead of a high molecular weight guar gum that is more viscous but less palatable (Ellis et al., 1991). Another study showed that reducing the molecular weight of β-glucan in muffins tended to increase the blood glucose and insulin responses in humans (Tosh et al., 2008). Immerstrand et al. (2010) showed that β-glucans with different molecular weights all had the same effect on the plasma cholesterol of mice. However, Kim and White (2010) found that low molecular weight β-glucan from oats produced more volatile fatty acids that did the β-glucan with a higher (4.4 times) molecular weight after fermentation for 24 hours in vitro.
An exhaustive review of the literature on cereal fiber suggests that the molecular weight of the fiber must be above a certain value to significantly increase the viscosity of the digestive effluents and to have a significant effect on postprandial glycemic and insulinemic responses. The authors even suggest that the thresholds value should be above 100 kDa for β-glucans and above 20 kDa for arabinoxylans. However, although low molecular weight fibers are more rapidly fermented, just how the molecular characteristics of a fiber influence its fermentation profile remains unclear (Gemen et al., 2011). Nevertheless, viscosifying fibers with high molecular weights increase the viscosity of the digesta more than do lower molecular weights fibers that tend to be fermented faster (Gemen et al., 2011). It has been shown that the molecular weights of fungal β-glucans significantly influence the secretion of interleukin-8 (IL-8) by HT29 cells in vitro, with lower molecular weight β-glucans producing more secretion than those of high molecular weight (Rieder et al., 2011). Finally, the prebiotic effect of wheat arabinoxylans increases inversely with their molecular weight in the presence of human feces in vitro (Hughes et al., 2007).
The fermentation and prebiotic properties of arabinoses from arabinoxylo-oligosaccharides (AXOS) have also been tested with respect to the degree of polymerization and substitution. Low molecular weight AXOS (average MW <3) produced more acetic and butyric acid and also stimulated an increase in the concentrations of bifidobacteria, whereas the fermentation of higher molecular weight (average MW = 61) AXOS resulted in a lack of the branched volatile fatty acids that are considered to be markers of protein fermentation and had no effect on the production of acetic and butyric acids or on bifidobacteria (Van Craeyveld et al., 2008). The authors used an experimental design that varied both the molecular weight and degree of substitution of arabinose in AXOS and concluded that AXOS with an average molecular weight of 5 and a degree of substitution of 0.27 produces the best effects on intestinal health (Van Craeyveld et al., 2008).
1.1.2 Changing the Degree of Crystallinity
Changes in the crystalline structure of a fiber are best illustrated in cellulose, the most abundant fibrous compounds on Earth. Indeed, like starch, cellulose has a crystalline structure, and by modifying it, it is possible to alter its digestibility/fermentation. This was clearly demonstrated in rats fed celluloses having degrees of crystallinity from 6 to 81%. As expected, the more crystalline the cellulose, the less it was fermented (from 9 to 20%) and the lower the fecal water content (Hsu and Penner, 1989). The degree of crystallinity influenced the accessibility of the cellulose to its cellulase enzyme because of the way it altered the porosity of the substrate (Jeoh et al., 2007).
1.1.3 Modifying Particle Size
Intensive grinding of a fiber can influence the speed at which it passes through the gastrointestinal tract, and may promote hydrolysis of its constituent polysaccharides and, ultimately, their hydration and water-holding capacity (Lewis, 1978). Heller et al. (1980) showed that coarse wheat bran had a shorter transit time in humans, with more excreted daily in the feces, which had a higher water content, whereas the degradation/digestibility of the cellulose was low. In contrast, the fibrous components of fine bran were more digestible, probably due to its longer retention time in the colon. These results were later confirmed in humans by comparing coarse and fine wheat bran: the authors state that grinding the bran reduced the amount of feces excreted by reducing the water-holding capacity of the fibrous matrix (Wrick et al., 1983). This effect was called the destruction of “the spongy action of fibrous matrix” by van Dokkum et al. (1983), who tested bread composed of fine and coarse brans in humans. The integrity of the fibrous matrix therefore appears to significantly influence the weight of the stool (van Dokkum et al., 1983).
However, others found that the size of wheat bran particles (0.5 or 2 mm) had no effect on the morphology or function of rat's intestine (e.g., fat digestion, water content of feces, or cecum length), except that the fiber in the coarser bran was better digested (Kahlon et al., 2001). In another study, the size of wheat bran particles had no effect on the fermentability of fiber in rats (Nyman and Asp, 1985). Similarly, there was no significant difference in the production of volatile fatty acids from coarse and fine wheat bran in the large intestine of pigs (Ehle et al., 1982).
More recently, a Taiwanese team compared increasing intensity of micronization on some physicochemical properties of the fiber-rich fractions extracted from orange peel and cellulose, and showed that the micronized fibers were able to adsorb glucose and reduce the activities of α-amylase and lipase, which could slow down glucose uptake and reduce the serum concentration of glucose (Chau et al., 2006). They tested the reduction of particle size of orange insoluble fiber in hamsters and concluded that micronized fibers would have a positive effect on the health of hamster intestines by decreasing the amount of harmful ammonia produced, increasing the dry weight of stools, and decreasing the activities of β-d-glucuronidase (associated with a lower incidence of colorectal tumors) and mucinase (leading to increased mucins that protect against bacterial invasion) (Wu et al., 2007).
The apparent differences between the results of these studies may be due to differences in the particle sizes tested. Perhaps large differences in particle size (at least 10-fold) are needed to obtain significant differences in physiological effects. Controlling the size of the fiber particles could therefore help improve the health of the digestive tract, particularly the colon. But further studies are needed to confirm these results in humans.
1.2 Influence of the Physicochemical Properties of Fibers on their Metabolic Effects
The physicochemical properties of fibers determine the way they interact with their environment, in this context, the digestive tract, either the small intestine or the colon. But little is known about the long-term influence of changes in the physicochemical properties of fibers on human health. The most studied effect is probably the influence of the viscosity of fibers such as β-glucans and arabinoxylans on the digestion and metabolic fate of other nutrients (glucose or cholesterol). Viscosity is generally modified by changing the molecular weight of the fiber (Chillo et al., 2011; Regand et al., 2011; Wolever et al., 2010). Thus, incorporating soluble, viscous fiber into starchy products significantly reduces their glycemic index (Fardet, 2015). The main actions of the added fiber are to encapsulate the starch (Brennan and Tudorica, 2008), slow the rate at which α-amylase diffuses to its substrate, and/or the movement of glucose to its intestinal absorption site due to increased viscosity and/or delayed transit (Hlebowicz et al., 2008). Some fibers may also slow the rate of gastric emptying (Hlebowicz et al., 2007; Mastropaolo et al., 1986). In a previous recent review I also discussed the implication of pre-hydrolyzing fiber, either soluble or insoluble, on some physiological functions (e.g., cholesterolemia and glycemia) (Fardet, 2015).
1.2.1 Modifying the Degree of Solubility
The solubility of a fiber depends on the conformation of its polysaccharide components (linear or branched) and its crystallinity, and may be affected by grinding, cooking, and other processes (Lewis, 1978). Thus, increasing the proportion of insoluble fiber from wheat bran (0, 200, and 400 g/kg diet) decreases the retention times of both solid and liquid phases in the small intestine and colon of pigs (Wilfart et al., 2007). Increasing the proportion of soluble fiber in the diet increases the viscosity of the digestive effluent, so slowing intestinal transit and the rates of diffusion and absorption of nutrients by the gastrointestinal mucosa.
In general, soluble fibers such as soluble arabinoxylans and β-glucans are rapidly fermented, whereas insoluble fibers such as cellulose and insoluble arabinoxylans are fermented more slowly (Williams et al., 2011). Each type of fiber (e.g., insoluble barley fiber and soluble beet fiber; Fardet et al., 1997) produces a specific profile of volatile fatty acids that may have different metabolic effects. However, the degree of polymerization of a soluble fiber such as the β-glucans or arabinoxylans, and thus their viscosity, does not significantly influence their rate of fermentation or the amounts of butyric, acetic, and propionic acids produced by fermentation (Williams et al., 2011).
1.2.2 Changing the Water-Holding Capacity
There have been very few studies in humans in vivo on the influence of changing the water-holding capacity of a fiber on its digestive and fermentative fate. An early study on potato fibers with different water-holding capacities found that the water-holding capacity had no effect on the stool weight, but that the type of fiber, potato or wheat bran, had a significant effect, with wheat bran producing heavier stools (Eastwood et al., 1983). Another study in the same year found that 12 subjects produced significantly heavier stools after consuming a diet that included bread with coarse bran (>0.35 mm) than they did after eating bread containing fine bran. The authors ascribed this observation to the ability of the larger bran particles to retain water and suggested that the “spongy activity of fibrous matrix” is the main factor involved (van Dokkum et al., 1983). A more recent study in rats fed insoluble fibers of tossa jute (Corchorus capsularis) and shiitake fungus (Lentinula edodes) found that the viscosity of the rat digesta was negatively correlated with its free water content, which was reduced by fibers that held water and swelled (Takahashi et al., 2009). The authors suggested that insoluble fiber may increase the viscosity of the digesta. Similar changes in the colonic digesta of piglets were obtained when they were fed insoluble fiber such as wheat bran (Molist et al., 2009). Such results are important for human nutrition because of the key influence of viscosity on the rate at which nutrients like glucose and cholesterol are absorbed in the intestine and on the physiology of satiety. For example, human subjects fed two liquid meals with identical compositions that differed only in their viscosities, containing oat bran β-glucans with different molecular weights, experienced different degrees of satiety and hormone-related responses (Juvonen et al., 2009).
It is therefore possible to use the water-holding capacity of a fiber, and hence the rheological properties of ingested foods, to control the absorption of nutrients by the human gastrointestinal tract.
1.2.3 Changing Fiber Porosity
Porosity is another important physicochemical parameter of fibers that determines the surface area of a fiber that is accessible to the enzymes responsible for its fermentation (Chesson et al., 1997). Clearly, the greater the porosity, the easier it will be for hydrolytic enzymes to access their substrate and degrade it, as was shown with cellulose under steam explosion (Wong et al., 1988). Thus, digestion in the small intestine can also increase the porosity of a beet fiber matrix by causing a loss of pectin, resulting in faster fermentation in vitro (Fardet et al., 1997). Another in vitro study found that fermentation was directly correlated with the porosity of beet fiber, indicating that the pore volume accessible to bacteria controlled fermentation (Guillon et al., 1998). How more rapid fiber fermentation influences metabolism and the resulting effects on health remain to be explored.
1.2.4 Adsorption of Bile Acids
Another property of fibers that has been extensively studied is their ability to bind bile acids, and so influence cholesterol metabolism by reducing blood cholesterol. Thus, low molecular weight oat β-glucan binds more bile acids (4.4 times) than do higher molecular weight oat β-glucans (Kim and White, 2010). The ability of various cereal brans (rice, oats, wheat, and maize) to bind bile acids in vitro does not appear to be proportional to their soluble fiber content. This suggests that soluble fiber is probably not involved in this property (Kahlon and Chow, 2000). At first glance, these results seem to contradict the finding that viscosifying soluble fiber can reduce plasma cholesterol. However, while soluble fibers bind less bile acid (precursors of cholesterol) than do insoluble fiber in vitro, it is possible that the two act in synergy in vivo, with insoluble fiber fixing bile acids and viscosifying soluble fiber decreasing the diffusion of ingested cholesterol.
A study tends to confirm these results. Zacherl et al. (2011) studied three types of fiber – cellulose, psyllium, and oat fiber – that had been digested to the same degree as when they arrived in the colon and found that the capacity to bind bile acid was mainly, but not solely, correlated with the viscosity of the digested chyme. Heat damage that caused oat fibers to lose their viscosity did not reduce their capacity to bind bile acids, which was higher than that of cellulose. Binding forces other than viscosity (e.g., hydrophobic interactions) are therefore involved. These other binding forces might be responsible for the capacity of insoluble fiber to bind bile acids, as discussed above.
There is therefore good evidence that the hypocholesterolemic capacity of a fiber can be modified by altering its structure.
1.2.5 The Ability to Complex Minerals and to Increase their Extent of Absorption
The properties of fibers are seemingly paradoxical vis-à-vis mineral absorption: they can both form complexes with them (Bergman et al., 1997; Lopez et al., 2002) and promote their absorption by the intestine. The fermented fibers increase the area for their absorption by causing hypertrophy of colon cells and increasing length of the small intestine (Faraldo Correa et al., 2009; Lopez et al., 2000, 2001a), or by promoting the hydrolysis of phytic acid via increased fermentation and stimulating bacterial enzymes (Lopez et al., 2001b; Callegaro et al., 2010). Phytic acid is well known for its ability to complex minerals (Lopez et al., 2002).
The cation-exchange capacity of fiber is due to the presence of negative charges at their surface. These affect the viscosity of the digesta, but the exact mechanisms involved are still not known (Takahashi et al., 2009).
Again, it is possible to manipulate the quality of dietary fiber to promote mineral absorption to a greater or lesser degree. However, the ability of some fibers to increase mineral absorption in humans remains to be demonstrated.
1.2.6 Fiber Structure and Hindgut Health
Monro and colleagues have extensively studied the influence of fiber structure on hindgut functions (Monro, 2014). They report that beyond providing essential fermentable substrates for bacteria, from a physical viewpoint, polysaccharide-based structures that survive fermentation also make a major contribution to fermentation and large bowel function. They act as supports on which societies (“consortia”) of bacteria proliferate as biofilms, in which metabolic interactions between species of bacteria determine the metabolic products, such as the type of short-chain fatty acid produced from fermentable substrates (Macfarlane and Dillon, 2007).
One of the most important effects of fiber within hindgut is notably its fecal bulking effect. In fact, “persistent plant structure in the form of robust cells occupies volume and provides water-bearing cavities” (Monro and Mishra, 2010), leading to removal of stagnant fecal water, reducing the chemical activity of toxins, promoting fecal softening, and distributing pressure (Monro, 2014). In other words, (insoluble) fiber with remaining unfermented structure keeps its ability to hold water, participating in very important health effects such as those described by Monro (i.e., potentially being able to protect from constipation, hemorrhoids, diverticular disease, colitis, and colorectal cancer) (Monro, 2014; Rose et al., 2007). In contrast, fermentable fiber has other health benefits within hindgut more in association with fecal microbiota and production of SCFAs. These different behaviors of fiber, depending on their fermentability, illustrate well the dual characteristics of fiber (i.e., insoluble (that I call lente fiber)) and soluble (that I call rapid fiber) fibers with different health effects.
Interestingly, Monro and colleagues further developed a fecal bulking index in relation to fiber, and expressed in wheat bran equivalents (Monro, 2001). Briefly, “wheat bran equivalents for fecal bulking are defined as the gram quantity of wheat bran that would augment fecal bulk to the same extent as a given quantity of a specified food” (Monro, 2001).
Finally, minimally processed fibers such as those of swede, broccoli head, broccoli rind, and asparagus exhibited a much higher fecal bulking effect (around 2- to 4-fold) than highly processed or unstructured fibers that are generally either added as isolated ingredients in foods or come from ultraprocessed foods (Monro, 2014). These data showed that processed fiber partially lost their ability to hold water via alteration of their original complex physical structure.
Consequent to the fecal bulking effect, there is also a relation between fiber physicochemical properties and transit time. Thus, Cherbut et al. (1991) showed that the water-binding capacity of fibers from wheat bran, sugarbeet, maize, pea hulls, and roasted cocoa might affect the orofecal transit time in healthy volunteers. Fibers were found to act through a mechanical effect if they were not fermented, and the partly degradable fibers also changed the transit time via their products of fermentation (i.e., a large production of propionic and butyric acids).
1.3 The Effect of Fiber Structure on Fermentation Patterns and Microbiota Profiles: Slowly versus Rapidly Fermented Fiber
In vitro data from the literature clearly show that fibers impact SCFA fermentation patterns and microbiota profiles differently, depending on their type or origin and their structure. In addition, dietary fiber fermentation profiles are important in determining optimal fibers for colonic health, and may be a function of structure, processing conditions, and other food components. A greater understanding of the relationships between fermentation rate and dietary fiber structure would allow for development of dietary fibers for optimum colonic health. (Rose et al., 2007)
1.3.1 Fiber Structure and Fermentation Patterns
In their review Rose et al. (2007) examined parameters of the fiber chemical and physical structure that may play a role on their fermentation rate and patterns. Briefly, they cited numerous studies emphasizing the importance of arabinoxylan cross-linking (through oxidative dimerization of ferulic acid moieties that are esterified to the arabinoxylan polymer), pectin degree of methylation or polymerization, fiber glycosidic linkages and molecular packing, native versus isolated fiber, resistant starch type, and particle size on fermentation patterns (Rose et al., 2007). However, they underlined that the physical inaccessibility of colon renders such analyses difficult.
In a recent study, Rumpagaporn et al. (2015) tried to elucidate the structural properties of cereal arabinoxylans that drive the rate of fermentation. They used predigested residues of arabinoxylan isolates from corn, wheat, rice, and sorghum brans, and showed, using in vitro human fecal bacteria, that there was no relationship between molecular mass, arabinose/xylose ratio, or degree of substitution to fermentation rate patterns. However, interestingly, slow fermenting wheat and corn arabinoxylans had much higher amount of terminal xylose in branches than fast fermenting rice and sorghum arabinoxylans. The slowest fermenting wheat arabinoxylan additionally contained a complex trisaccharide side chain with two arabinoses linked at the O-2 and O-3 positions of an arabinose that is O-2 linked to the xylan backbone. (Rumpagaporn et al., 2015)
They concluded that the major structural factor that related to slow fermentation was the type of linkage of the branch constituents, and large amounts of branches with single xylose units. Simpler structures were associated with a rapid initial rate of fermentation that was comparable to that of the fast fermenting fructo-oligosaccharides.
Similarly, with cereal arabinoxylans, Karppinen (2003) divided fiber polysaccharides of rye bran into three groups: (1) fermentable, soluble polysaccharides that are rapidly fermented, (2) fermentable cell wall-associated polysaccharides that are gradually released from the cell wall matrix and then fermented, and (3) polysaccharides and cell wall structures that are not fermented at all. However, in the study by Van Nevel et al. (2006) fiber water-holding capacity was surprisingly not correlated with fermentability within contents of pig cecum: it was highest for chicory roots, followed by wheat bran and sugar beet pulp; water-holding capacity was very high for sugar beet pulp (10.05 g H2O/g dry matter), whereas the lowest value was obtained with wheat bran (3.00 g H2O/g dry matter) (Van Nevel et al., 2006). Similar results were obtained with oat hull fiber, gum arabic, carboxymethylcellulose, soy fiber, and psyllium (Bourquin et al., 1993a), and also with fibers from broccoli, carrot, cauliflower, celery, cucumber, lettuce, onion, and radish (Bourquin et al., 1993b) for which their water-holding capacity – an indirect measure of fecal bulking potential – was not correlated with SCFA production and organic matter disappearance.
The results of these studies seems to show that water solubility is not a major, or at least not the only, determinant of fermentability and that structural characteristics at the molecular level of fiber would be particularly involved. Other physicochemical features have been shown to be involved in fiber fermentability such as gross porosity, microporosity, particle size, or crystallinity (Guillon et al., 1998). Two sources of sugar beet fibers were submitted to various chemical and then dehydration treatments, resulting mainly in the removal of pectic polysaccharides (9–49% recovery) at the expense of cellulose (80–100% recovery). Following chemical extraction, harsh drying induced a noticeable decrease in the total pore volume (from 14.9 to 6.1 mL/g) and especially in the pore volume accessible to bacteria (from 10.4 to 3.2 mL/g). Drying following chemical extraction did not affect the crystallinity of cellulose in the fiber. Main results showed that neither the particle size, nor the crystallinity of cellulose were major determinant factors in degradability of sugar beet fibers, but that pore volume accessible to bacteria in sugar beet fibers was highly correlated (r = 0.88) with its fermentability. Authors concluded that such results “illustrate the importance of matrix physical structure (especially porosity) in the control of the physicochemical behavior of fiber.” This conclusion was also supported by the results of the study by Mortensen and Nordgaardandersen (1993), showing with cellulose and dietary fiber in common clinical use that the amounts of soluble nonstarch polysaccharides in the fiber were closely associated with the mean productions of SCFAs after in vitro incubation with human fecal homogenates, but also that the mean production of ammonia was inversely related to the soluble fraction of the fiber. The authors concluded that their “findings support that the water solubility determines the degree of fermentability of dietary fiber and thereby the corresponding bacterial assimilation of ammonia.”
However, most studies do not go as far as recording the physicochemical properties of fibers with fermentation profiles in the analysis, and they only describe SCFA production patterns according to fiber type. For example, in batch cultures of pig intestinal digesta, while β-glucan-grown cultures yielded the highest level of lactate, flaxseed or fenugreek gum-containing cultures generated a significant amount of acetate, propionate, and butyrate (Lin et al., 2011). In another study, 20 soluble fibers (alginate, apple pectin, arabinogalactan, carrageenan, carboxymethylcellulose, citrus pectin, gellan gum, guar gum, gum arabic, gum ghatti, gum karaya, hydrolyzed guar gum, konjac flour, locust bean gum, methylcellulose, oat β-glucan, psyllium, tomato pectin, tragacanth gum, and xanthan gum) were tested in vitro for their fermentation profile using three human fecal inocula (Hussein et al.,2008). Although all are soluble, and therefore supposed to be quite highly fermentable, significant differences were observed after 24 hours for dry matter disappearance (between 20% and more than 91%) and gas production, with some fiber having no gas produced. In the same vein, Lu et al. (2000) examined the effects of an arabinoxylan-rich fiber extracted from a byproduct of wheat flour processing in the rat colon compared with well-characterized soluble/rapidly fermentable (i.e., guar gum) and insoluble/slowly fermentable (i.e., wheat bran) fibers. The SCFA pool was particularly high with arabinoxylan and guar gum fibers. Otherwise, arabinoxylans fiber was a good source for acetate, whereas guar gum and wheat bran favored propionate and butyrate production, respectively. Finally, fecal output was 7-, 6-, and 5-fold higher, respectively, in the arabinoxylan, guar gum, and wheat bran groups of rats than in the nonfiber groups (p < 0.01). Authors concluded that these results suggested that arabinoxylan fiber behaves like a rapidly fermentable, soluble fiber in the rat colon.
In another study, Monsma et al. (2000) used ileal digesta collected from swine fed oat or wheat bran fermented for 0–96 hours in an anaerobic in vitro system using inocula prepared from ceca of rats fed the same fiber sources. As in the studies described above, the authors distinguished between slow and rapid fiber. Fermentation of wheat bran digesta was significantly slower than fermentation of oat bran digesta, and oat bran digesta fermentation produced a significantly greater molar proportion of SCFAs as propionate, these latter being produced during fermentation of β-glucan. With regard to particle size, in rats coarse wheat bran gave significantly higher fecal butyrate concentrations than rice brans and fine wheat bran (Folino et al., 1995).
More generally, Salvador et al. (1993) assessed the relationship between the disappearance of dietary fiber sugars and the production of individual SCFAs by studying in vitro using a human fecal inoculum the bacterial degradation of five dietary fibers whose sugars were quantified. Their results confirm that the nature and associations between the fiber sugars were key variables in the fermentability, and that the nature and the amounts of SCFAs produced were closely related to the in vitro fermentation of the main sugars available. Thus, as they concluded: uronic acids seemed to be principally involved in the production of acetic acid whereas the production of propionic acid could be promoted by the fermentation of glucose and, to a lesser extent, by that of xylose and arabinose. Xylose tend to have a greater impact than uronic acids and glucose on the production of butyric acid.
Such results, together with those mentioned previously, suggest that one should be able to predict which SCFA would be specifically produced if the chemical composition and structure of the fiber are known (Salvador et al., 1993).
1.3.2 Fiber Structure and Fecal Microbiota Profiles
Some studies showed that fibers differing in their structure may impact differently on the bacterial community structure. For example, oat β-glucan, flaxseed gum, and fenugreek gum significantly influenced bacterial community structure in batch cultures by pig intestinal digesta (Lin et al., 2011). Significant differences in bacterial species were also observed with fiber from chicory roots, sugar beet pulp, wheat bran, and corn cobs incubated with contents of pig cecum (Van Nevel et al., 2006). In addition, bacterial mass increased and was maintained longer during fermentation of oat bran digesta than the wheat bran digesta in an anaerobic in vitro system using inocula prepared from ceca of rats (Monsma et al., 2000).
Figure 1.1 Chemical, physical and physicochemical properties of dietary fiber, their digestive fate and potential health effects via the gut.
1.4 Conclusions
The results presented here clearly show that the intrinsic and physicochemical properties of fibers determine the rates at which they are fermented and their consequent health impacts (see brief summary in Figure 1.1). It is therefore no exaggeration to say that, there are slow and rapid (fermented) fibers, just as there are slowly and rapidly digested carbohydrates (Englyst et al., 2003), fats (Keogh et al., 2011), and proteins (Boirie et al., 1997). What we do not know is how the kinetics of absorption of volatile fatty acids thus modified in the colon impacts the physiology and modifies the health effects over the long term. Nevertheless, the structure of a fiber can be modified to improve colon health, especially by altering the speed of fermentation, the site of fermentation and then butyrate production, which helps protect against carcinomas and colonic inflammation (Rose et al., 2007). However, although many studies have compared the fermentative fate of different types of fibers, few have investigated the relationship between changes in physicochemical parameters of a single given fiber type (i.e., of equal chemical composition) and its implications for human physiology and health. Nevertheless, the development of the fecal bulking index is a promising step in this direction.
It is also worth emphasizing that fibers may act as vectors, delivering compounds associated with their structure in the gastrointestinal tract, notably at colonic level as shown in vitro; fruit and vegetable fibers release significantly more polyphenols than cereal fibers, for example (Tabernero et al., 2011). Thus, most of the antioxidants in the colon, such as cereal phenolic acids, are bound to them (Vitaglione and Fogliano, 2010; Vitaglione et al., 2008). Indeed, Vitaglione et al
