IS DIETARY FIBER AND WHY IS IT SO IMPORTANT?
Dietary fiber (also known as bulk or roughage) includes
all the parts of plant food that our bodies cannot
digest or absorb. Yet it is an important part of a
healthy diet. It
adds bulk to your diet and makes you feel full faster,
helping you control your weight.
aids digestion, helps prevent constipation, and is
sometimes used for the treatment of diverticulosis,
diabetes, and heart disease.
ARE THE DIFFERENT TYPES OF FIBER?
fiber was defined to be the components of plants that
resist human digestive enzymes, a definition that includes
lignin and polysaccharides. The definition was later changed
to also include resistant starches, along with inulin
and other oligosaccharides.
are two types of fiber:
type of fiber dissolves in water to form a gel-like material.
It can help lower blood cholesterol and glucose levels.
Soluble (prebiotic, viscous) fiber is readily fermented
in the colon into gases and physiologically active byproduct.
Soluble fiber is found in oats, peas, beans, apples, citrus
fruits, carrots, barley and psyllium.
FIBER-- This type of fiber promotes the movement of
material through your digestive system and increases stool
bulk, so it can be of benefit to those who struggle with
constipation or irregular stools. It is fiber that is
metabolically inert, absorbing water throughout the digestive
system and easing defecation . It acts by changing
the nature of the contents of the gastrointestinal tract,
and by changing how other nutrients and chemicals are
absorbed. Whole-wheat flour, wheat bran, nuts and many
vegetables are good sources of insoluble fiber.
DO YOU GET FIBER FROM IN YOUR DIET?
fiber is found in varying quantities in all plant
foods, including: legumes (peas, soybeans, lupins and
other beans) oats, rye, barley, some fruits and fruit
juices (including prune juice, plums, berries, bananas,
the insides of apples and pears) certain vegetables such
as broccoli, carrots, and Jerusalem artichokes, root tubers
and root vegetables such as sweet potatoes and onions
(skins of these are sources of insoluble fiber.
of insoluble fiber include: whole grain foods, wheat
and corn, bran, nuts and seeds, potato skins, vegetables
such as green beans, cauliflower, zucchini (courgette),
celery, and some fruits including avocado, and bananas,
the skins of some fruits, including tomatoes
plants contain significant amounts of both
soluble and insoluble fiber. For example, plums (or
prunes) have a thick skin covering a juicy pulp. The
plum's skin is an example of an insoluble fiber source,
whereas soluble fiber sources are inside the pulp.
ARE THE HEALTH BENEFITS OF FIBER?
fiber has been scientifically proven to lower cholesterol,
which can help prevent heart disease. In addition, soluble
fiber lowers total and LDL cholesterol, regulates blood
sugar, balances intestinal pH and stimulates intestinal
fermentation production of short-chain fatty acids, speeds
the passage of foods through the digestive system, and,
adds bulk to your diet, making you feel full faster.
fiber-- appears to speed the passage of foods through
the stomach and intestines and adds bulk to the stool.
is a table which shows the function and benefits for soluble
and insoluble fiber.
fiber functions and benefits
water and turns to gel during digestion, trapping
carbohydrates and slowing absorption of glucose
variance in blood sugar levels
total and LDL cholesterol
risk of heart disease
reduce onset risk or symptoms of metabolic syndrome
and stimulates intestinal fermentation production
of short-chain fatty acids
reduce risk of colorectal cancer
the passage of foods through the digestive system
bulk to the stool
bulk to your diet, making you feel full faster
does not bind to minerals and vitamins and therefore
does not restrict their absorption, but rather evidence
exists that fermentable fiber sources improve absorption
of minerals, especially calcium. Some plant foods can
reduce the absorption of minerals and vitamins like
calcium, zinc, vitamin C, and magnesium, but this is
caused by the presence of phytate
(which is also thought to have important health benefits),
not by fiber.
ARE SOME SIDE-EFFECTS OF TOO MUCH FIBER?
a large amount of fiber in a short period of time can
cause intestinal gas (flatulence), bloating, and abdominal
cramps. You should add fiber to your diet slowly. Increasing
dietary fiber too quickly can lead to gas, bloating and
cramps. This usually goes away once the natural bacteria
in the digestive system get used to the increase in fiber
in the diet. Adding fiber gradually to the diet, instead
of all at one time, can help reduce gas or diarrhea. Too
much fiber may interfere with the absorption of minerals
such as iron, zinc, magnesium, and calcium. However, this
effect usually does not cause too much concern because
high-fiber foods are typically rich in minerals.
MUCH FIBER IS NEEDED IN THE DAILY DIET?
average American now eats 10 - 15 grams of fiber per day.
The recommendation for older children, adolescents, and
adults is 20 - 35 grams per day. Younger children will
not be able to eat enough calories to achieve this, but
it is a good idea to introduce whole grains, fresh fruits,
and other high-fiber foods. To ensure that you get enough
fiber, eat a variety of foods, including: cereals, dried
beans and peas, fruits, vegetables and whole grains. Add
fiber gradually over a period of a few weeks to avoid
abdominal discomfort. Water aids the passage of fiber
through the digestive system so drink plenty of fluids
(approximately 8 glasses of water or noncaloric fluid
a day). Peeling can reduce the amount of fiber in fruits
and vegetables. Eating fiber-containing food is beneficial,
whether it is cooked or raw.
ARE THE SOME OF THE MOLECULES IN FIBER?
dietary fiber consists of non-starch polysaccharides
such as arabinoxylans, cellulose and many other
plant components such as resistant dextrins, inulin,
lignin, waxes, chitins, pectins, beta-glucans and
oligosaccharides. The term "fiber" is something
of a misnomer, since many types of so-called dietary
fiber are not fibers at all. See source:
-- Chemically defined as oligosaccharides occurring
naturally in most plants, inulins have nutritional
value as carbohydrates.
are a group of naturally occurring polysaccharides
produced by many types of plants. They belong to
a class of fibers known as fructans. Inulin is used
by some plants as a means of storing energy and
is typically found in roots or rhizomes. Most plants
that synthesize and store inulin do not store other
materials such as starch. Inulins are polymers composed
mainly of fructose units, and typically have a terminal
- Pectins are a family of complex polysaccharides
that contain 1,4-linked a-D-galactosyluronic acid
residues. Three pectic polysaccharides have been
isolated from plant primary cell walls and structurally
acid is a sugar acid, an oxidized form of D-galactose.
It is the main component of pectin, in which it
exists as the polymer polygalacturonic acid
ß-Glucans -- (beta-glucans) are polysaccharides
of D-glucose monomers linked by ß-glycosidic bonds.
ß-glucans are a diverse group of molecules that
can vary with respect to molecular mass, solubility,
viscosity, and three-dimensional configuration.
They occur most commonly as cellulose in plants,
the bran of cereal grains, the cell wall of baker's
yeast, certain fungi, mushrooms and bacteria.
- Cellulose is an organic compound with the
formula (C6H10O5)n, a polysaccharide consisting
of a linear chain of several hundred to over ten
thousand ß- linked D-glucose units.
ß-glucan (insoluble version) --Glucans
are polysaccharides that contain only glucose as
structural components, and are linked with ß-glycosidic
Yeast and medicinal mushroom derived ß-glucans
are notable for their ability to modulate the immune
system. Research has shown that insoluble (1,3/1,6)
ß-glucan, has greater biological activity than that
of its soluble (1,3/1,4) ß-glucan counterparts.
The differences between ß-glucan linkages and chemical
structure are significant in regards to solubility,
mode of action, and overall biological activity.
- Lignin or lignen is a complex chemical compound
most commonly derived from wood, and an integral
part of the secondary cell walls of plants and some
A hemicellulose is any of several heteropolymers
(matrix polysaccharides), such as arabinoxylans,
present along with cellulose in almost all plant
cell walls. Hemicelluloses include xylan
(shown to left), glucuronoxylan, arabinoxylan, glucomannan,
OF SOLUBLE FIBER
is now well-established that dietary fiber reaches the
large intestine and is fermented by the colonic microflora
with the production of short chain fatty acids (SCFA),
hydrogen, carbon dioxide and biomass. This fermentative
process dominates human large bowel function and provides
a means whereby energy is obtained from carbohydrates
not digested in the small bowel, through absorption of
of fiber in the colon
cannot penetrate in the bacterial cells. They are first
hydrolyzed in monosaccharides, by membranous or extra-cellular
enzymes secreted by bacteria. Metabolism of these monomeric
sugars continue in the bacterial cells using the Embden-Meyerhoff
pathway which leads to pyruvate. Pyruvate does not appear
in the large bowel because it is immediately converted
in end-products. These are SCFA, mainly acetate, propionate
and butyrate, and gases: carbon dioxide (CO2),
hydrogen (H2), and methane (CH4).
fermentation is an efficient digestive process since starch
is almost totally degraded, as well as lactose, alcohol-sugars
and fructans if the intake of these sugars is not too
high. More than half of the usually consumed fibers are
degraded in the large intestine, the rest being excreted
in the stool (see Table 9). A number of factors are likely
to affect the utilization of fermentable carbohydrates
in the colon. Among these is solubility. The more soluble
substrates, being more accessible to hydrolytic enzymes,
are likely to be degraded more rapidly. Nevertheless,
some soluble fibers such as alginates or carragheenans
are poorly fermented. Other factors involving digestive
motility and individual differences in microflora could
also modulate fermentation. Furthermore, certain metabolic
pathways can be modified by the repeated occurrence of
some sugars (lactose, lactulose, fructans) in the colon.
The mechanisms and the physiological consequences of this
adaptation are not completely identified.
Effects of Dietary Fibre
Food and Agriculture Organization of the United Nations
to a 2002 journal article, fibers compounds with partial
or low fermentability include:
a group of phytoestrogens
compounds with high fermentability include:
a group of polysaccharides
a group of heteropolysaccharides
gums, a group of polysaccharides
a group of polysaccharides
a group of short-chained or simple sugars
or kilojoules (as used on nutrition labels) are intended
to be a measure of how much energy is available from
the food source. This energy can be used immediately,
for example allowing the body to move during exercise,
or to make the heart beat. Energy that is not used immediately
is stored as sugars in the short term and later converted
to fats, which act as energy reserves.
is extracted from food in a chemical reaction. Because
of the principle of conservation of energy, energy can
only be extracted when the chemical structure of food
particles is changed. Since insoluble fiber particles
do not change inside the body,
 the body should not absorb any energy (or
Calories/kilojoules) from them.
soluble fiber is changed during fermentation, it could
provide energy (Calories/kilojoules) to the body. As
of 2009 nutritionists have not reached a consensus on
how much energy is actually absorbed, but some approximate
around 2 Calories (8.5 kilojoules) per gram of soluble
of the type of fiber, the body absorbs fewer than 4
Calories (16.7 kilojoules) per gram of fiber, which
can create inconsistencies for actual product nutrition
labels. In some countries, fiber is not listed on nutrition
labels, and is considered 0 Calories/gram when the food's
total Calories are computed. In other countries all
fiber must be listed, and is considered 4 Calories/gram
when the food's total Calories are computed (because
chemically fiber is a type of carbohydrate and other
carbohydrates contribute 4 Calories per gram). In the
US, soluble fiber must be counted as 4 Calories per
gram, but insoluble fiber may be (and usually is) treated
as 0 Calories per gram and not mentioned on the label.
occurs by the action of colonic bacteria on the food mass,
producing gases and short-chain fatty acids. It is these
short-chain fatty acids butyric, acetic (ethanoic), propionic,
and valeric acids that scientific evidence is revealing
to have significant health properties.
soluble fiber is fermented, short-chain fatty
acids (SCFA) are produced. SCFA are involved in
numerous physiological processes promoting health,
blood glucose levels by acting on pancreatic
insulin release and liver control of glycogen
gene expression of glucose transporters in the
intestinal mucosa, regulating glucose absorption
nourishment of colonocytes, particularly by
the SCFA butyrate
cholesterol synthesis by the liver and reduce
blood levels of LDL cholesterol and triglycerides
responsible for atherosclerosis
colonic pH (i.e., raises the acidity level in
the colon) which protects the lining from formation
of colonic polyps and increases absorption of
production of T helper cells, antibodies, leukocytes,
cytokines, and lymph mechanisms having crucial
roles in immune protection
barrier properties of the colonic mucosal layer,
inhibiting inflammatory and adhesion irritants,
contributing to immune functions
that are absorbed by the colonic mucosa pass through
the colonic wall into the portal circulation (supplying
the liver), and the liver transports them into the general
SCFA affect major regulatory systems, such as blood
glucose and lipid levels, the colonic environment, and
intestinal immune functions.
major SCFA in humans are butyrate, propionate, and acetate,
where butyrate is the major energy source for colonocytes,
propionate is destined for uptake by the liver, and
acetate enters the peripheral circulation to be metabolized
by peripheral tissues.
short-chain fatty acids produced through the fermentation
of soluble fiber in the large intestine serve to stabilize
blood glucose levels, lower low-density lipoproteins
(LDL) or "bad" cholesterol in the blood, increase the
production of immune cells, and promote colon health.
Soluble fiber prevents the formation of intestinal polyps
or inflammation by maintaining a healthy pH in the intestine,
aids in the absorption of certain minerals, and increases
the production of helpful bacteria in the colon.
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Fiber: An Essential Part of a Healthy Diet
--Department of Energy
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