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The Interaction Between Insoluble and Soluble Fiber

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Dietary fiber has been extensively studied in last few decades for their physiological health benefits. Depending on its solubility, dietary fibers are classified in two groups-soluble and insoluble dietary fibers. Soluble dietary fibers include β-glucan, galactomannan, pectin, psyllium, inulin, and resistant starch, whereas insoluble fibers include cellulose, hemicellulose, chitosan, lignin, etc. Dietary fibers are characterized with some physicochemical properties, such as solubility, fermentability, viscosity, water absorption, binding ability and so on. These properties are responsible for the functional behavior of dietary fibers. In this chapter, classification, physicochemical properties, and interactions of soluble and insoluble dietary fibers are discussed with respect to their specific health benefit.
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Dietary Fiber for the Prevention of Cardiovascular Disease
http://dx.doi.org/10.1016/B978-0-12-805130-6.00003-3
Copyright © 2017 Elsevier Inc. All rights reserved. 35
CHAPTER 3
The Interaction Between
Insoluble and Soluble Fiber
Deepak Mudgil
1 INTRODUCTION
In 1953, Hispley coined the term “Dietary fiber” for first time, which
included the cell wall portions, such as cellulose, hemicelluloses, and
lignin [1]. In 430 BC, Hippocrates explained the laxative action of coarse
wheat against refined wheat. In 1920, J. H. Kellogg reported attributes
of bran, such as increased stool weight and enhanced laxation. After this,
very little or no research work was carried out on dietary fiber until the
1970s [2]. Generally, dietary fibers are indigestible in human small intes-
tine and are considered as “roughage” materials that are resistant to diges-
tion by the secretions of the human digestive tract. If these nondigestible
carbohydrates are isolated from the plant sources and have physiological
health benefits, then these are known as functional fibers. Classification
of dietary fiber can be done on the basis of their solubility, fermentability,
and physiological effects. The term dietary fibers generally include non-
starch polysaccharides (NSP), oligosaccharides, lignin, and associated plant
substances (such as cutin, suberin, phytic acid, and so on) [3]. Resistant
starches (RS) are also included in the definition of dietary fiber as they
also resist digestion and not absorbed in small intestine of healthy individ-
uals [4]. Whole grains include partly milled grains, breakfast cereals, such
as All Bran or Fiber One. In the last decade, dietary fibers are extensively
studied for their participation in regulation of physiological functions,
such as lowering of blood cholesterol, improvements in gastrointestinal
mobility(GI), regulation of glucose and lipid metabolism, stimulation of
bacterial metabolic activity, detoxification of colon luminal content, and
integrity of intestinal mucosa. When dietary fiber components are con-
sumed in the diet, they also interact with the other food components and
can potentially interfere with metabolism and absorption. List of dietary
fiber components along with their food sources are given in Table 3.1.
Dietary Fiber for the Prevention of Cardiovascular Disease
36
2 CLASSIFICATION OF DIETARY FIBERS
Several classification systems have been suggested for the classification of
dietary fiber components, which are based on their role, fiber constituents,
type of polysaccharide, GI solubility, site of digestion, and products of
digestion and physiological classification. However, none of these systems
are solely satisfactory because they do not cover all the aspects completely.
Among these systems of classification are the two most accepted classifica-
tions, which are based on the concept of solubility and on the concept
of fermentability of dietary fiber in an in-vitro system using an aqueous
enzyme solution representative of human alimentary enzymes. Generally,
it has been considered that fermented fibers are soluble in water, whereas
nonfermented or poorly fermented fibers are insoluble in water [5].
According to its solubility, there are two different categories of fibers
present in dietary constituents: insoluble dietary fibers and soluble dietary
Table 3.1 Dietary fiber components from food sources
Food source Dietary fiber component
Cereals Cellulose
Xylan
Arabinoxylan
β-d-Glucan
Fructans
Galactans
Lignin and lignan
Phenolic ester
NSP
Legumes Galactomannans
Glucomannan
Xyloglucans
Pectin
Lignin
Cellulose
NSP
Fruits and vegetables Cellulose
Pectin
Xyloglucans
Lignins
Cutin
Waxes
Gums and mucilages
NSP, Nonstarch polysaccharides.
The Interaction Between Insoluble and Soluble Fiber 37
fibers. Both types of fibers have different actions and influence on the nor-
mal gut activity. Soluble fiber dissolves in water whereas insoluble fiber does
not dissolve in water. Soluble dietary fibers bypass the digestion process of
the small intestine and are easily fermented by the microflora of the large
intestine. Sources of soluble dietary fiber include pectin, gums, starches, oats,
psyllium, fruit, vegetables, pulses, etc. [6]. Soluble fibers upon dissolution in
water may or may not have viscosity effects, which means that after dissolu-
tion in water it forms a viscous gel. This characteristic of soluble dietary fiber
further classifies them into two categories—viscous fibers and nonviscous
fibers. Viscous soluble fiber includes pectin, galactomannan, glucomannan,
β-glucan, psyllium, etc. whereas nonviscous soluble fiber includes fructooli-
gosaccharides (FOS), resistant dextrin, inulin, etc. (Tables 3.2 and 3.3).
Table 3.2 Different classes of dietary fiber
Dietary fibers
Insoluble dietary fibers Soluble dietary fibers
Poorly fermentable or non-
fermentable fibers Fermentable fibers
Celluloses
Hemicelluloses
Lignins
Resistant starches
Arabinoxylans
NSP
Viscous Nonviscous
Pectin Resistant dextrin
β-d-Glucan Resistant starch
Galactomannans Polydextrose
Glucomannans Inulin
Psyllium FOS
FOS, Fructooligosaccharides; NSP, nonstarch polysaccharides.
Table 3.3 Classification of dietary fiber based on solubility/fermentability
Property Component Sources
Water solubility/
fermentability
Pectin Fruits and vegetables, legumes, etc.
Gums Guar seeds, locust bean, seaweeds
extract, microbial gums, etc.
Mucilages Vegetables, plant extracts, etc.
Water insolubility/little
or no fermentability
Cellulose Cereal brans, such as rice bran,
wheat bran, root vegetables,
peas, beans, legumes, etc.
Hemicellulose Cereal whole grains, cereal bran,
pulses, dried beans, etc.
Lignin Cereals, stone fruits, edible seeds,
vegetable filaments, flaxseed, etc.
Dietary Fiber for the Prevention of Cardiovascular Disease
38
Soluble fibers generally increase the transit time through human diges-
tive tract, which results in delayed gastric emptying, which ultimately results
in slower glucose absorption. In the human GI tract, insoluble dietary fibers
are not soluble in water and do not form gels due to their water insolubility
and no or very limited fermentation of insoluble fiber occurs in the large
intestine. Cereal brans, such as All Bran, Fiber One, and rice bran and whole
grains are major sources of insoluble dietary fiber in the form of cellulose
and lignins. Wheat bran generally contains up to 50% dietary fiber while oat
bran reported to have about 20% dietary fiber [4]. Insoluble fibers generally
increase the fecal bulk and excretion of bile acids, while decreasing intestinal
transit time. Insoluble and soluble fibers are present in different concentra-
tion in various foods and have different properties; hence it is important to
include a variety of fiber-containing foods in daily dietary plan. Besides ben-
eficial physiological effects, such as cholesterol lowering, diabetes control,
and digestive system improvements, dietary fibers also improves the growth
and activity of beneficial bacteria in human digestive tract by acting as food
for the beneficial human intestinal microflora. This activity of dietary fiber
is known as prebiotic activity and the fibers showing this activity are known
as prebiotics. Prebiotic are generally defined as the indigestible food compo-
nent that beneficially influences the host organism by selective stimulation
of growth and activity of beneficial bacteria (such as Lactobacilli and Bifi-
dobacteria) in the human digestive tract which leads to improvement in the
host health. Examples of prebiotic substance includes guar gum, gum acacia,
tragacanth gum, FOS, and galactooligosaccharides which act as food for the
human colonic bacteria and helps in growth and activity of these bacteria
[7]. Insoluble dietary fiber is more abundant in foods as compared to soluble
dietary fiber. Most of the fiber containing foods generally contains approxi-
mately one-third soluble and two-third insoluble fiber [8].
3 PROPERTIES OF DIETARY FIBER
The physiological effects caused by dietary fibers are dependent on their
physicochemical properties, such as solubility, fermentability, viscosity, water
absorption and water-holding capacity, adsorption and binding ability, and
particle size [9].
3.1 Solubility
Dietary fibers are of two types depending on their solubility in water:
soluble and insoluble dietary fiber. The soluble and insoluble nature of
The Interaction Between Insoluble and Soluble Fiber 39
dietary fibers influences their physiological effects and technological func-
tions [10]. When ingested in diet, soluble fibers increase the viscosity of
the aqueous phase and leads to reduction in the glycemic response and
plasma cholesterol [11,12]. Insoluble fibers are designated by their porosity
and low density. Due to these properties, insoluble dietary fiber results in
increase in fecal bulk and decrease in intestinal transit [13]. In functional
foods market, development of soluble fiber fortified food products is more
beneficial as it provides viscosity and ability to form gels, as compared to
insoluble fiber [6].
3.2 Fermentability
Fermentability is an important characteristic of the dietary fiber as it can
contribute as substrate for fermentation. It has been reported that the
average excretion of dry weight and energy on low fiber diet are 50 g/day
and 800 kJ/day, respectively and 88 g/day and 1700 kJ/day, respectively
on a high fiber diet [14]. Fermentation of the fiber is highly variable
including little or no fermentation (lignins) to almost complete fermen-
tation (pectins). Among dietary fibers, soluble fibers are more prone to
fermentation by colonic bacteria as compared to insoluble fibers [15].
Fermentability of dietary fiber is directly related with its effect on bowel
function, such as fecal mass, stool frequency, maintenance of colonic pH,
and retrieve energy from nondigestible foods are directly related to their
fermentation pattern [16].
Depending on the fermentability, dietary fibers are classified as rapidly
fermented, slowly fermented, and nonfermentable dietary fibers. Fruits,
such as apples and vegetables, such as beans are considered as rapidly fer-
mented proportions and these may contribute less to fecal bulking than
other fibers. Psyllium and wheat bran are considered as slow fermenting
proportions and contribute to fecal mass through fermentation. Dietary
fibers with poor fermentation capacity contribute toward increased bulk
in the large intestine, which results in reduced risk of constipation and
colonic cancer. Dietary fibers with high fermentability are associated with
physiological actions on colonic mucosa via production of fermentation
metabolites [16,17]. Highly fermented fibers are also associated with posta-
bsorptive actions on the liver and other tissues [18].
In addition to these functions, some fiber sources (FOS) can selectively
stimulate the growth and activity health-promoting bacteria, such as bi-
fidobacteria and lactobacilli which are responsible for the protective bar-
rier function and for stimulating healthy immune response in adults [19].
Dietary Fiber for the Prevention of Cardiovascular Disease
40
The access of bacteria to substrate is linked with the physical structure of
the fiber matrix and the chemical structure of dietary fiber component,
which ultimately describe the rate and extent of fermentation [9]. Fiber
components, which contain high amounts of secondary tissues, such as hull
and bran are poorly fermented compared to fiber products rich in paren-
chyma tissue, such as fruits and vegetables. Soluble polysaccharides exhibits
higher fermentation rate than equivalent polysaccharides within cell walls.
Blocked branched polysaccharides exhibit more fermentation as compared
to highly and randomly branched polysaccharides. The course of fermen-
tation depends on the microflora composition of the individual. It may
vary from one individual to another individual and also within the cell
population densities of the principal taxonomic groups [20]. The examples
of bacteria that can hydrolyze polysaccharides include Bacteroides, Bifidobac-
terium, Ruminococcus, and some species of Eubacterium and Clostridium. Some
substrates, such as FOS and inulin can promote selectively the growth and/
or the activity of one or a limited number of bacteria in the colon and are
known as prebiotics. End products of fermentation include short chain fatty
acids (SCFA), mainly acetate, propionate, and butyrate and gases.
3.3 Viscosity
Viscosity is one of the significant properties associated with soluble dietary
fibers [21]. Some soluble dietary fiber, such as pectins, gums, psyllium, and
β-glucan may form viscous solutions when interact with aqueous phase.
Viscosity or gel-formation is associated with the ability of dietary fiber to
absorb water, which results in the formation of gelatinous mass. Inclusion of
these dietary fibers in daily diet can enhance the volume, as well as viscosity
of the contents of the GI tract [13]. This increase in volume and viscosity
of digesta leads to delay gastric emptying in the stomach, which ultimately
can enhance satiety. This enhanced viscosity also reduces the emulsification
of dietary lipids in the acid medium of the stomach, which results in lower
extent of lipid assimilation. Viscosity caused by the action dietary fiber can
resist the effects of GI motility in the lumen of small intestine. Because of
the enhanced viscosity, gut content behaves like gel and responds more like
solids than liquids in the GI tract. This phenomenon can describe the de-
layed gastric emptying often associated with the ingestion of fibers due to
the reduced diffusion of digestive enzymes toward their substrates, which
ultimately slows down the digestion. Enhanced viscosity of the digested
contents can also retard the release and transit of the hydrolysis products
toward the absorptive surface of the mucosa. Chemical structure of the
The Interaction Between Insoluble and Soluble Fiber 41
dietary fiber component and its interaction with other macromolecules are
responsible for the viscosity. Volume occupied by the dietary fiber compo-
nents are generally characterized by its intrinsic viscosity [22].
This viscosity property of the dietary fiber components can be en-
hanced or reduced by certain treatments. Hydrolysis treatments of dietary
fiber components, such as gums, pectins, and beta glucans into lower mo-
lecular weight molecules reduce the viscosity enhancement capacity of the
digestive contents by these fiber components. Certain treatments, such as
extrusion cooking can enhance the amount of water-soluble dietary fiber
component and ultimately the viscosity caused by dietary fiber.
Viscosity of the digestive contents containing dietary fiber, do not
remain the same throughout the gut. Dietary fiber components are gener-
ally not digested by enzymes in upper gut, but they can undergo signifi-
cant degradation, for example, pectin can be solubilized via breakdown of
calcium bridges in stomach at acidic pH or via beta elimination in small
intestine at about neutral pH. The extent of this degradation of dietary
fiber component could have a nutritional impact due to reduced viscosity
action which is further associated with physiological actions. Solubility
and viscosity of polysaccharides can also be influenced via any change in
ionic environment and pH throughout the GI tract. This phenomenon
is generally observed in case of polyelectrolytes, such as alginate, which
forms a gel in the stomach but solubilizes in the small intestine. The vis-
cosity of fluid digesta may vary within the gut, which means that the mea-
sured viscosity of a dietary fiber source may have little relationship to the
viscosity in the digestive segments of interest depending on the conditions
in the gut and type of dietary fiber. Concentration, structure, and molecu-
lar weight of the polymer under consideration are important to study the
effect of particular dietary fiber source. Viscosity value for dietary fiber
sources should be reported at a range of values of the shear rates as most
of the dietary fibers show shear thinning behavior. Hence, viscosity value
at a single shear rate cannot explain the complete rheological behavior of
the dietary fiber [23].
3.4 Water Absorption and Water-Holding Capacity
Insoluble type of dietary fibers, such as lignin and cellulose are mostly
unfermentable by colonic microflora and increase fecal bulk by swelling
and water-holding capacity [15]. Water holding or water retention capac-
ity is described by the amount of water that is retained by known weight
of dry fibers under specified conditions of temperature, time of soaking,
Dietary Fiber for the Prevention of Cardiovascular Disease
42
and duration of centrifugation and centrifugation speed. A portion of the
soluble fiber is generally lost during measurement, which affects the water-
holding capacity; hence the amount of water measured by centrifugation is
generally higher than the amount of water absorbed [24].
Polysaccharide components of dietary fibers are generally hydrophilic
in nature and the water molecule is adhered on the hydrophilic sites of the
fiber component or within the intercellular void spaces in the molecu-
lar structure. Hence, insoluble dietary fibers can absorb, swell, and entrap
water within its porous matrix in its molecular structure, which results
in the bulking effect of fiber in the colon. Insoluble dietary fiber com-
ponents can reduce the potency of cytotoxic substances via dilution of
such substances in the large intestine. The hydration properties of dietary
fiber components depend on the chemical nature of the components,
their arrangement in the cell walls and the anatomy, and particle size of
the dietary fiber. The dietary fiber components, which are composed of
primary cell walls shows higher hydration capacity as compared to the
fiber components, composed of secondary cell walls [25,26]. Hydration
properties of fiber components containing charged groups, such as pectin
are influenced by pH, ionic strength, and nature of the ions. Processing
treatments, such as heating, grinding, extrusion, and so on given to food
components containing fiber components affects the hydration properties
of dietary fiber component via modification of composition and physical
properties of fiber matrix.
3.5 Adsorption and Binding Ability
Trapping of bile acids or adsorption of bile acids has been suggested as a
potential mechanism by which certain dietary fiber components may in-
crease the excretion of bile acids in feces. Soluble fiber forms gel matrices,
which are eventually excreted in the feces entrap some of the bile acids
released from the gallbladder. This physical entrapment of bile acids appears
to be more pronounced in the terminal region of ileum where reabsorp-
tion of bile acids from the digesta usually occurs [15]. Enhanced excretion
of bile acids leads to higher cholesterol turnover from the body. The exact
mechanisms of binding of bile acids by dietary fiber components are still
not known whereas hydrophobic and ionic interactions have been sug-
gested in support of the mechanism [25]. Dietary fiber components com-
posed of lignified or coarse tissues (e.g., rice straw) are reported to have
binding properties [27]. Adsorption of the components can be measured
by methods which are similar to those generally used for water retention
The Interaction Between Insoluble and Soluble Fiber 43
capacity. Retention of the components is caused by their absorption and
entrapment in cell matrix. Physiologically reliable measurement of binding
capacity of dietary fiber components is based on certain conditions, such as
chemical environment of the small intestine and behavior of dietary fiber
components in small intestine.
3.6 Particle Size
Particle size of dietary fiber is an important characteristic, which indi-
rectly control the physiological function in the digestive tract, such as
transit time, fermentation, and fecal excretion. The rate of fermentation
of dietary fibers is directly related to surface area of the fiber, which is in
direct contact with bacteria [28]. Wheat bran with coarser particle size
is more effective in regulating transit time as compared to wheat bran
with fine particle size. Dietary fiber components reduce the intestinal
transit time, which is beneficial in terms of protection of colon from the
extended exposure to cytotoxic substances, which may be harmful for hu-
man health. Particle size of dietary fiber depends on processing treatments
on the fiber product. Mechanical treatment, such as grinding, as well as
chewing reduces the particle size of the dietary fiber. Approximate com-
plete disintegration of the particles can be achieved by degradation of the
fiber matrix by colonic bacteria.
4 SOLUBLE FIBER INTERACTIONS
4.1 β-Glucan
β-Glucan is a water-soluble dietary fiber obtained from oats, barley, bacte-
ria, yeast, algae, and mushrooms [29]. Cell wall of the baker’s yeast, that is,
Saccharomyces cerevisiae is most abundant in β-glucan. β-Glucan is a water-
soluble polysaccharide consists of glucose units. Glucose monomers are
linked via β-(13) glycosidic bonds in bacteria and algae whereas glucose
monomers are linked via β-(13) and β-(16) glycosidic bonds in yeast
and mushrooms. In oats and barley, glucose monomers are linked via β-
(14) and β-(13) glycosidic bonds. β-Glucan obtained from bacteria
and algae shows a linear structure whereas β-glucan extracted from yeast,
mushrooms, oats, and barley exhibits branched structure. β-Glucan synthesis
in cell wall is a complex process because of the identification of large num-
ber of different classes of glucans. Several classes of enzymes are involved in
synthesis of β-glucan [30]. β-(16) Glycosidic side chains interconnect the
β-(13) glucan chains to create a rigid network [31]. β-Glucan and chitin
Dietary Fiber for the Prevention of Cardiovascular Disease
44
components are often linked by β-(14) linkages. No sharp distinction lies
between the insoluble and soluble fractions of β-glucan, however the water
solubility of β-glucan is dependent on its structure [32].
Extraction conditions of β-glucan are highly responsible for the ratio
of soluble and insoluble fraction [33]. β-Glucans having β-(13) link-
ages and high degree of polymerization are completely insoluble in water
which allows interactions between glucan molecules and interaction be-
tween glucan molecules and water molecules [34]. Water-solubility and
molecular weight of β-glucan is considered to control its hypocholester-
olemic effect. It has been reported in literature that viscosity of β-glucan
in the gut is mainly responsible for its cholesterol (bad) lowering effects.
Viscosity of β-glucan is directly related to its molecular weight, molecular
structure, solubility in water, and food matrix [35]. High molecular weight
and high solubility in water of β-glucan have high capacity of reducing
serum cholesterol as compared to low molecular weight and low soluble
β-glucan [36]. This may be due to the higher intestinal viscosity of β-
glucan, which reduces the reabsorption of bile acids and leads to higher
excretion of bile acids [37]. Higher excretion of bile acids enhances the
synthesis of bile acids from cholesterol, which ultimately increase the cho-
lesterol uptake and thus reduces the LDL serum cholesterol. Viscosity of
digesta is controlled by β-glucan concentration consumed in the diet and
the molecular weight of β-glucan, hence the glycemic response is reported
to have significant correlation with concentration and molecular weight
of β-glucan [38,39]. β-Glucan with high molecular weight at low con-
centration forms viscous and pseudoplastic solutions whereas β-glucan
with low molecular weight forms softer gel at high concentration [40].
β-1,3-Glucan molecules are almost resistant to the acidic secretions in
human stomach. After ingestion, β-glucans gradually passes into the first
section of small intestine (duodenum) and are trapped by macrophage
receptors located on intestinal wall. These receptors are protein in nature
and are produced by bone marrow [41]. When glucan molecule comes in
a contact with glucan receptors, it is activated and generates bactericidal
compounds, such as lysozyme, reactive oxygen radicals, and oxides. After
that cells commence to yield numerous cytokines, which activate the sur-
rounding phagocytes and leukocytes that lead to specific immunity [42].
4.2 Galactomannan
Galactomannan is an endosperm hetero-polysaccharide present in most le-
guminous seeds and is composed of galactose and mannose [43]. Common
The Interaction Between Insoluble and Soluble Fiber 45
sources of galactomannan include guar gum [44], locust bean gum [45],
fenugreek, and alfalfa [46]. The galactomannan molecules are resistant to
human digestive secretions in small intestine and hence functions as dietary
fiber. Even though the difference in sources of extraction, galactomannan
shows similar basic structure but shows variations in molecular weight,
mannose/galactose ratio, and galactose distribution as side chains on the
mannose chain which ultimately affect the physicochemical properties of
gum, such as thermal stability, solubility, and viscosity [47].
Guar gum is the most common galactomannan used as dietary fiber
source. Guar gum molecule is composed of a linear backbone chain of
β-(14) linked mannose units to which galactose units are attached as
side chains via α-(16) linkage. Guar gum is obtained from the seed
endosperm of Cyamopsis tetragonolobus (cluster bean) and generally used
as stabilizer and thickener in various foods due to its high viscosity [48].
The ratio of mannose to galactose units has been reported as 2:1 which
means galactose units are attached at alternate mannose unit [44]. Native
guar galactomannan shows molecular weight of about 900 kDa and vis-
cosity of its 1% solution is about 5500 cps. Whereas partially hydrolyzed
guar gum (PHGG), which is used as soluble dietary fiber has low viscos-
ity (4 cps) and low molecular weight of about 8 kDa. PHGG is obtained
via partial enzymatic hydrolysis of guar gum at control conditions of pH,
temperature, and time. PHGG has similar structure as native guar but
with reduced degree of polymerization (DP29). PHGG when incor-
porated in the food products do not affect the color, taste, and viscosity
of that product.
PHGG is fermented over a longer duration in large intestine due to
its medium chain length (DP29) and provides a better prebiotic effect
with the production of high amounts of SCFA when compared to other
soluble dietary fibers [49,50]. PHGG is reported to have effects in lower-
ing blood glucose level [51] and serum cholesterol [52]. It is also reported
that guar gum reduce the absorption of glucose within rat jejunum due
to its gel forming action and by influencing the mucus barrier function
[53]. Intake of dietary fiber generally hinders the absorption and utiliza-
tion of other nutrients whereas PHGG is reported to have no such effects
[54,55]. Clinical studies suggest that PHGG is effective in reduction of
colonic transit time (CTT), induction of satiety hormone cholecystoki-
nin (CCK) and increased perception of postmeal satiety. The intake of
diet incorporated with PHGG slowed the CTT and increased the release
of CCK, which may be due to the structure and function of PHGG as it
Dietary Fiber for the Prevention of Cardiovascular Disease
46
produces huge amounts of SCFA, such as butyrates and propionates that
can cause slower CTT and increased release of CCK [56].
4.3 Pectin
Pectin is a type of structural fiber found in the primary cell wall and in-
tracellular layer of plant cells mainly in fruits, such as apples, oranges, lem-
ons, and so on. Citrus fruit contains 0.5%–3.5% pectin which is largely
present in peel portion of the fruit. During the ripening process, pectins
change to a water-soluble material (ripened fruit) from an insoluble sub-
stance (unripe fruit). Pectin is a polymer with linear structure in which
few hundred to thousand galacturonic acid monomer units are linked via
α-(14)-glycosidic bond forming a backbone. The average molecular
weight of pectin ranges between 50 and 150 kDa. The backbone of pectin
molecule is substituted at certain regions with α-(12) rhamnopyranose
units from which side chains of galactose, mannose, glucose, and xylose may
occur. Methyl esterification of galacturonic acid occurs in pectin. On the
basis of methyl esterification, there are two different types of pectin—high
methoxyl and low methoxyl pectin. High methoxyl pectins are character-
ized with more than 50% esterified galacturonic acid residues whereas low
methoxyl pectins are characterized with less than 50% esterified galact-
uronic acid residues [57].
Pectins are reported to have hypocholesterolemic properties but due
to its high gelling capacity, it cannot be incorporated in food products at
higher concentration as it negatively affects the product sensory character-
istics. Pectin is water soluble in nature and bypass the enzymatic digestion
process of human small intestine but is easily degraded by the microflora
of the colon. In human GI tract, pectin is capable of holding water and
forming gel, which ultimately leads to binding of ions and bile acids. Gel
forming ability of pectin is considered as possible mechanism of its ben-
eficial health effects, such as improved cholesterol and lipid metabolism,
improved gastric emptying, and improved glucose metabolism [58–60].
Pectins are also reported to have some unique abilities for prevention or
treatment of diseases, such as intestinal infections, atherosclerosis, cancer,
and obesity. In a human study, 24 healthy subjects consumed biscuit forti-
fied with pectin (15 g/day) for 3 weeks. The results of the study suggest
that pectin consumption (15 g/day) led to a reduction in total cholesterol
by 5% [61]. After the recommendations of Keys et al. (1961), many of the
studies have been carried to study the hypocholesterolemic effect of pectin
and confirmed the same. In hyperlipidemic men, pectin is associated with a
The Interaction Between Insoluble and Soluble Fiber 47
decrease in tensile strength of fibrin and increase in permeability of fibrin.
Fibrin is a fibrous, nonglobular protein involved in the clotting of blood.
The quality of fibrin is considered as an important risk factor associated
with atherosclerosis, stroke, and coronary heart disease. Pectin produces
acetate in the human colon, which is assumed to enter in the peripheral
circulation and amend the fibrin structure [62]. Pectin is also thought to
have a potential role in cancer prevention.
It has been reported in the literature that pectin is capable to bind and
decrease tumor growth and cancerous cell migration in rats, which were
fed with modified pectin obtained from citrus fruit. The exact mechanism
is unclear but it is believed to be a result of galectin inhibitory activity of
pectin [63,64]. Pectin performs like a natural prophylactic substance against
noxious effect of toxic cations. It is potent in binding and removal of lead
and mercury from GI tract [65]. Intravenous injection of pectin reduces
the blood coagulation time and control hemorrhage or bleeding. Pectin
is also effective in the treatment of diarrheal diseases due to their bacteri-
cidal action [57]. Pectin retards the rate of digestion of food components
in the intestine via immobilization of food components, which results in
low absorption of food. The viscosity of the pectin layer affects the absorp-
tion of food components by constraining the contact between the intestinal
enzyme and food components [66].
4.4 Psyllium
Psyllium is water-soluble gel-forming mucilage obtained from the seed husk
of Plantago ovate plant, which is a native herb from regions of Asia, Europe,
and North Africa. Mucilage is an adhesive gelatinous substance similar to
natural gums. Seed husk of Plantago ovate is a rich source of water-soluble
fiber, known as psyllium hydrocolloid or psyllium seed gum. The bioactive
component of psyllium is composed of a highly branched arabinoxylan. The
main backbone chain consists of xylose units, to which arabinose and xylose
are attached as side chains. Arabinoxylans from cereal grains are largely fer-
mented by colonic microflora, however arabinoxylans from psyllium exhibit
unrecognized structural characteristic that hinders its fermentation by co-
lonic microflora [67]. Presently, psyllium fortification in various foods, such
as ready-to-eat cereals is carried out due to its cholesterol-lowering effects.
Several clinical studies in literature reported that psyllium consumption in
human reduces 7%–9% serum total cholesterol concentrations [68]. It is also
reported that effect of psyllium is independent of fat content and cholesterol
content of diet consumed during the clinical study. It is reported that total and
Dietary Fiber for the Prevention of Cardiovascular Disease
48
LDL cholesterol (LDL-C) levels are reduced by 0.028 and 0.029 mmol/L by
each gram of water-soluble fiber from psyllium. However, no effect on serum
HDL cholesterol concentration is reported [58].
Psyllium is reported to exhibit a greater tendency to decrease LDL-C
levels and significant improvement in levels of total serum cholesterol. Low-
ering of total serum cholesterol and LDL-C are consistently observed in the
subjects with type II diabetes who consumed psyllium-fortified diet. Exact
mechanism of action of psyllium fiber is not clear like other soluble fibers.
However, the cholesterol lowering action of psyllium is described by two
fundamental hypotheses. According to first hypothesis, psyllium fiber has a
tendency to bind or absorb bile acids when they pass through the intestinal
lumen and thus prevent their normal reabsorption, which leads to increase
in fecal bile acid content and reduce the cholesterol pool. According to sec-
ond hypothesis, psyllium fiber physically disturbs the intraluminal forma-
tion of micelles, which leads to reduction in absorption of cholesterol and
reabsorption of bile acids. In both hypotheses, bile acids bound to psyllium
fiber are passed to the colon, which leads to higher hepatic conversion of
cholesterol into bile acids. This conversion of cholesterol leads to an up-
regulation of the LDL receptor and results in higher uptake of LDL-C from
the plasma. Overall result is a decrease in serum LDL-C level and hence
in total cholesterol level [69]. In 1998, FDA has approved health claim for
psyllium that “A food product containing water-soluble fiber from psyllium
seed husk, consumed as part of a diet low in saturated fat and cholesterol,
may reduce the risk of heart disease.
4.5 Inulin
Inulin is a polymer composed of fructose monomers linked via β-(2-1)-
d-frutosyl fructose bonds. It is indigestible in human small intestine due
to presence of β-configuration of C-2 and can be fermented by intestinal
microflora in large intestine [70]. Nearly 90% of the inulin passes to the
colon and digested by colonic bacteria [71]. In chicory inulin, degree of
polymerization or the number of monomer unit vary from 2 to 60 showing
a combination of both oligomers and polymers [72]. Molecular weight and
degree of polymerization (103–105) of bacterial inulin is very high and more
branched as compared to plant inulin [73]. Sources of inulin include onions,
garlic, wheat, artichokes, and bananas. Caloric value of inulin is low, that is,
1.5 kcal/g which may be due to its indigestible nature. Inulin is converted
to SCFA (e.g., acetate, propionate, and butyrate), lactates and gases [74].
Only the SCFA and lactates are responsible for the caloric value of inulin.
The Interaction Between Insoluble and Soluble Fiber 49
When fermented, inulin has a tendency for propionate production, which
leads to decrease in acetate to propionate ratio, which ultimately results in
decreased total serum cholesterol and LDL [75].
Inulin is beneficial in reducing the risk of many GI tract diseases, such
as irritable bowel diseases and colon cancer. Inulin has also been reported
to contribute to the optimal health of the human colon as a prebiotic [76].
It is reported that inulin accelerate the growth of bifidobacteria while ham-
pering the growth of potential pathogenic bacteria, such as Escherichia coli,
Salmonella, and Listeria. This is helpful in disorders, such as ulcerative colitis
and Clostridium difficile infections. It is also reported that inulin reduced the
biological compounds associated with colonic cancer, including reduced
colorectal cell proliferation and water induced necrosis.
Functionality of inulin also includes increased mineral absorption, for
example, increased calcium absorption (20%) was reported in adolescent
girls who consumed food supplemented with inulin [77]. Inulin intake is
also evidenced with increased bone mineral density when compared to
the control subjects. This may be attributed to increased calcium absorp-
tion from the colon or an increased solubility in the lumen of the GI
tract because of SCFA. It may also enhance mineral absorption through
an enrichment of vitamin D. Inulin may have a function in prevention
and treatment of obesity. It is reported that inulin increased satiety in
adults which led to a decrease in total energy intake. This may be due to
the ability of SCFA to increase appetite-suppressing hormones, such as
glucagon-like peptide 1 [78].
Inulin is water soluble in nature and is not digested by human digestive
enzymes. It produces peculiar results on the effectiveness of the gut, such
as reduced pH of intestine; provide support in relieving constipation; and
increasing stool volume due to bulking effect. Bulking effect of inulin is simi-
lar to bulking effect of other soluble fiber, such as pectin and guar gum [79].
4.6 Resistant Dextrin
Resistant dextrins (RD) are short chain glucose polymers which are
strongly resistant to human digestive enzymes and do not have sweet taste
[80]. Resistance of RD toward digestive enzymes are due to presence of
(12)-, (13)-, (16)-α-, and β-glycoside bonds which is not present in
native starch [81]. Dextrinization of starch is observed when starch is heat-
ed at high temperature with or without catalyst. Dextrinization involves
depolymerization, transglucolyzation, and repolymerization. During the
process of dextrinization, isomerization, or formation of new bonds is also
Dietary Fiber for the Prevention of Cardiovascular Disease
50
observed along with cleavage of bonds. Hydroxyl groups at C-2, C-3, or
C-6 glucose unit act on free radicals and undergoes transglycolization,
which is based on formation of (12)-, (13)-, and (16)-bonds which
leads to formation of branched dextrins [82]. In next phase reversion and
recombination reactions occurs which leads to formation of β-(16)-
glycosidic bonds. RD are associated with all the health benefits caused
by dietary fiber, such as PHGG and inulin. Similar to soluble fiber resis-
tant dextrin undergo fermentation in human colon and produces SCFA
mainly acetate which accounts for 50% of the SCFA. It is reported that
in an in-vitro fermentation study, wheat dextrin produced considerably
more total SCFA, propionate and butyrate than PHGG. Hence, RD have
higher fermentability by colonic microflora as compared to PHGG [83].
The SCFAs produced in large intestine by soluble fermentable fibers are
moderately strongly acidic in nature and hence they reduce the colonic
pH and produce acidic environment there. In the large intestine, acidic pH
may support the growth of beneficial bacteria, such as bifidobacteria and
lactobacilli because they have a strong intrinsic resistance to acid. How-
ever, acidic pH prevents the growth of pathogenic bacteria (e.g., Clostridia),
which is pH sensitive in nature [84]. The formation of SCFAs is associated
with better laxation and regularity and also with bulking effect caused by
dietary fiber. RD can stimulate pancreatic insulin release via the SCFAs
production and affect liver control of glycogen breakdown and ultimately
leads to reduction in blood glucose and insulin levels [85]. RD are also
associated with reduction in cholesterol levels in humans. The SFCAs pro-
duced in large intestine via fermentation of resistant dextrin suppresses
the cholesterol synthesis by liver and may reduce LDL-C and triglycerides
levels in serum [86].
4.7 Resistant Starch
RS are defined as the starches, which are resistant to the digestion process
in human small intestine [87]. RS behave like soluble fiber due to their
water solubility and indigestibility without losing their mouth feel and pal-
atability. Hence, resistant starches are important class, which gives benefits
of fiber without affecting the sensory characteristics. RS have been classi-
fied into four fundamental classes, such as RS1, RS2, RS3, and RS4. RS1
type is physically inaccessible starch composed of starch granules, which are
entrapped by indigestible plant material. RS2 type belongs to raw starch
granules occur in its natural form, such as in raw potato and high amylose
maize. RS3 are retrograded or crystallized starches made by unique cooking
The Interaction Between Insoluble and Soluble Fiber 51
and cooling processes of unmodified starch or by food processing opera-
tion. RS4 belongs to the starches, which are chemically modified and the
modified form becomes resistant to enzymatic digestion. RS are reported
to show a reduction in postprandial blood glucose and insulin levels. Among
these four classes of resistant starches RS4 type has been reported to have
greater glucose lowering effect [88].
It is reported that long-term consumption of the diet containing resis-
tant starch may decrease fasting cholesterol and triglyceride levels. Serum
triglyceride levels increases as a result of interactions between sucrose, fruc-
tose, and saturated fatty acids. The mechanism behind the reduction of cho-
lesterol level by resistant starches includes increased intestinal viscosity of
digesta by resistant starches, which retarded the interactions between sugars
and fatty acids [89]. RS increase the intestinal absorption of minerals in rats
and humans. It is reported that enhanced intestinal absorption of calcium,
magnesium, zinc, iron, and copper in rats observed who were fed with diets
rich in resistant starch [90].
5 INSOLUBLE FIBER INTERACTIONS
5.1 Cellulose
Cellulose is an important structural component of primary cell walls in
vegetables, green plants, algae, and some bacteria. Cellulose is a linear chain
of glucose monomers, which are linked via β-(14) glycosidic linkage.
It is insoluble in water and resistant to the action of digestive enzymes in
the human small intestine and may be fermented by colonic bacteria in
the large intestine following production of SCFA. Natural cellulose can be
classified into two classes: natural and modified. The crystalline cellulose is
made up of intra- and intermolecular noncovalent hydrogen bonds which
make it insoluble in water. In last few decades, process have been developed
for the preparation of modified celluloses, such as powdered cellulose, mi-
crocrystalline cellulose, and hydroxypropylmethyl cellulose which are used
as food additives and ingredients. Natural and modified celluloses differ
with each other in the amount of crystallization and hydrogen bonding.
When these hydrogen bonds are rattled and the crystallinity of cellulose is
lost which results in the water solubility of cellulose derivative [91]. Several
studies have been reported that studied the influence of cellulose on blood
glucose and insulin levels in various models. The results obtained from these
studies are extremely contradictory and may be dependent on the sub-
ject, cellulose type, and other unexplored factors. Studies using modified
Dietary Fiber for the Prevention of Cardiovascular Disease
52
cellulose reported more consistent data as compared to studies with natural
cellulose. Modified cellulose has been reported to influence lipid metabo-
lism. Hypercholesterolemic adults who were consuming 5 g of Hydroxy-
propylmethylcellulose per day for a duration of 4 weeks showed a valuable
reduction in total and LDL-C [92]. Modified celluloses perform better and
more beneficial than native cellulose. These modified celluloses increased
the viscosity of the content in GI tract, which is thought to delay nutrient
absorption and increase in bile acid excretion.
5.2 Hemicellulose
Hemicelluloses are the polysaccharide and are component of plant cell
wall. Hemicelluloses are comprised of different types (hetero) of monomer
units. Hemicelluloses show both linear and branched molecules. Hemi-
celluloses molecules contain 50–200 monomer units and are somewhat
smaller than cellulose molecules. Monomer units present in hemicellu-
lose may include pentose monomers (xylose and arabinose) and hexose
monomers (glucose, galactose, mannose, rhamnose, glucuronic, and ga-
lacturonic acids). As its name indicates, hemicellulose describes a hetero-
geneous group of chemical structures. Hemicelluloses may exhibit water
soluble and insoluble nature [93].
Arabinoxylan is an important type of hemicellulose, which is classified
as insoluble dietary fiber. Arabinoxylan is composed of xylose and arabinose
monomers. Xylose units are linked via β-1,4-xylosidic linkage to form the
backbone to which arabinose units are attached as side chains via α-1,3-
linkage. Arabinoxylan forms a major portion of insoluble dietary fiber in
whole grains. It forms a major portion of NSP in wheat. It is present in both
endosperm and bran portion of the wheat however bran portion is densely
loaded with arabinoxylan as compared to endosperm of wheat kernel [94].
Major portion of arabinoxylan is removed as by-product during processing
of wheat kernel to wheat flour. In human GI tract, arabinoxylan is immedi-
ately fermented by colonic microflora.
Inverse relationship between the levels of intake of arabinoxylan rich
bread and postprandial glucose response in healthy adult subjects has been
reported [95]. Breads supplemented with arabinoxylan also thought to con-
trol blood glucose and insulin in adults having imperfect glucose tolerance
[96]. The mechanism of action by which arabinoxylan improve glucose tol-
erance is not clear. However, a hypothesis is given, according to which high
viscosity of the arabinoxylan in the lumen of GI tract reduced the rate of
glucose absorption.
The Interaction Between Insoluble and Soluble Fiber 53
5.3 Chitin and Chitosan
Chitin and chitosan are biopolymers, which consist of glucosamine and
N-acetylated glucosamine monomer units attached via β-1-4-glycosidic
bonds. Sources of chitin and chitosan include shells of arthropods, such
as crabs and shrimps [3]. Some fungi and brown algae can also pro-
duce chitin and chitosan as exopolysaccharide. Chitin and chitosan differ
with each other in terms of acetylation and solubility. Chitin molecule
is highly acetylated whereas chitosan molecule is greatly deacetylated.
Chitin is insoluble in water and acid whereas chitosan is analogously
water-insoluble but soluble in acid. It is reported that chitosan inges-
tion efficiently reduces serum cholesterol in humans. In a clinical study,
chitosan was ingested as biscuits fortified with chitosan by healthy adult
males at a dose of up to 6 g/day showed a significant reduction in total
serum cholesterol and showed higher levels of serum HDL-cholesterol
[97]. Chitosan molecules consisting of more than six units of glucos-
amine with moderate degree of deacetylation are efficient in inhibit-
ing the cholesterol and lipids absorption in the human intestinal tract.
The mechanism behind the reduction in cholesterol absorption is its gel
forming ability in the intestinal tract, which binds cholesterol and lipids
[98,99]. It is also reported that ascorbic acid enhances gel formation ca-
pacity of chitosan and hence increases lipid binding capacity and plasma
cholesterol lowering action of chitosan.
A significant unwanted effect of chitosan is the reduction in the absorp-
tion of minerals and fat-soluble vitamins, such as vitamin A, D, E, and K.
Along with the entrapping lipids and cholesterol; viscous gel of chitosan
also binds minerals and fat-soluble vitamins. It is reported that chitosan
ingestion results in a symbolic enhancement of fecal excretion of bile acids,
such as cholic acid and chemodexoycholic acid in healthy adult males [97].
Chitin may be attributed to disrupt tumor metastasis, as it is a constituent
of extracellular matrix.
5.4 Lignin, Suberin, and Cutin
Lignin, suberin, and cutin are complex polymers that occur in cell walls of
some specific type of cell. These are present in very small amount in food
plants but have significant role in protection against colorectal cancer [100].
Lignin is a highly branched polymer, which is composed of phenylpro-
panoid units and it is covalently bound to fibrous polysaccharides within
plant cell walls. The magnitude of cells having lignified walls often in-
creases with plant maturity and reduces their palatability. Lignin is generally
Dietary Fiber for the Prevention of Cardiovascular Disease
54
extracted from wood in different ways. Lignin is not a carbohydrate but due
to its association with dietary fiber component, it affects the physiological
effects of dietary fiber and hence classified as dietary fiber. Suberin is an
extracellular biopolymer, which consists of a polyaliphatic domain with a
polyaromatic domain, which is obtained from ferulic acid. It occurs in the
walls of cork cells that form the skins of many root vegetables and tubers,
such as potato tubers. Processed potato peels have been utilized as food
additive with enticing baking characteristics in muffins and cookies [101].
Cutin is polyester, which forms the cuticle along with related waxes. This
cuticle occurs inside and outside of the outer epidermal wall of leaves and
fruits of plants.
Lignin, suberin, and cutin in plant cell walls are considered to safe-
guard the cell wall polysaccharides from degradation by colonic bacterial
enzymes [102]. Plant cell walls become hydrophobic due to the presence of
lignin, suberin, and cutin and are potent in vitro adsorbers of hydrophobic
carcinogens [103].
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... In this mechanism, various hormonal responses associated with satiety and insulin sensitivity, which are relevant factors contributing to diabetes, can be affected by viscous soluble fibers. Moreover, soluble fibers can be easily fermented in the gut, resulting in the production of various metabolites, especially SCFAs, and changes in the gut microbiome [39,96]. SCFAs can be absorbed via GPR41/43 metabolism in the gut and used as an energy source [41]. ...
... Unlike soluble fibers, insoluble fibers with non-viscous properties are mostly poorly fermented in the gut and thus produce fewer SCFAs than soluble fibers [92]. However, accumulated insoluble fibers in the gut decrease gut transit time and increase fecal bulk because of their water-holding and swelling capacities [96]. Decreased gut transit time and increased fecal bulk due to insoluble fibers interfere with the absorption of glucose and cholesterol, resulting in the reduction of blood glucose and cholesterol levels [94,97]. ...
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... Vivaspin Turbo 15, Membrane 100.000 MWCO Ultrafiltration Centrifugal [26] 50-150 kDa [27] Gum arabic (E414) Three fractions: Arabinogalactan (main component), Arabinogalactan-protein complex Glycoprotein containing galactose, rhamnose, arabinose, glucoronic acid and proteins [21,28] ~ 380 kDa (from Acacia senegal) ~ 850 kDa (from Acacia seyal) [21] Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
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