Download Dietary fiber type reflects physiological functionality

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
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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. The authors have no relevant
interests to declare.
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