sweet to taste prove in digestion sour"
(from "The Winter's Tale, III:2)
Topics briefly covered:
II Importance Of Fibre
III Kinds Of Fibre:
IV What Fibre Does:
V How Does Cooking Affect Fibre?
VI How The Gastrointestinal Tract (Gi) Is Affect
VIII Potential Problems With Too Much Fibre
IX Fibre Intake And Vegetarians
X Dietary Fibre Of Some Common Foods
Some recent queries about fibre have arose. To help sort things
out a bit, I hope you will find the following information
II. IMPORTANCE OF
An adequate intake of fibre has great importance for health
indicated by its demonstrated physiologic effects. Among these
- the hypoglycemic
effect of soluble fibre (reduces high blood
- the hypolipidemic
effect of soluble fibre (reduces high
levels, i.e., those lipoproteins like LDL
- the lowering
of serum cholesterol levels. Such
as we know,
presently appears to have a significant
the prevention of atherosclerosis
- slowing the
absorption of carbohydrate can be very useful to
in regulating blood sugar levels.
- anti-toxic effects.
Most international epidemiological
an inverse relationship between colon cancer
and fibre content of diet. While these
to disentangle the known effects of fat and
on colorectal cancer, some studies have
a inverse relationship after these factors
statistically adjusted for. Besides
discussed below, the reduced intestinal
is also thought to be a key factor.
- apparent reduction
or control of gastrointestinal disorders
diverticular disease, gallstones, irritable-
inflammatory bowel disease and
- the satiety
effect that can help *some* individuals better
ideal body weight (also helps a little with
dietary utilization of some sugars and
It is important to recognize that various kinds of fibre
different function and therefore a variety of fibre containing
foods should be contained in one's diet. Eating oat bran alone
is simply a bad way to get fibre. Indeed, there is some folly to
the careless practice of adding large amounts of a single
of purified fibre to the diet. Varied whole plant foods is still
the best course to take.
Dietary fibre is derived from solely from plant cells, mostly
from the plant cell wall.
It is NOT to be found in any animal
product. Dietary fiber is not a single entity but in fact
consists of many substances, as we shall see, and each has
different properties. Important properties of fibre include
- literally, "water-guarding" which
expands and holds onto water as it dissolves
things into a stable soup or thin, viscous
gel. Thus they are used as "stablizers"
helps to keep ingredients dissolved and at an
consistence or texture.
Fibre differ in their absorption of
like that of digestive biles.
Fiber varies in how well it can feeds the
in the human gut.
Increasing particle size helps to
the absorptivity of fiber to absorb. The
a fiber particle becomes, the better for
- All gums are soluble in water and produce
solution, but only a few gums have the ability
rigid textures that do not flow. As
helps to slightly reduce the absorption
Fecal bulk producing
- Considered a desirable thing so as to
intestinal transit time. Highly fermentable
produce less fecal bulk.
Now that we have briefly describe some of the properties
fiber, it now time to bite the bullet and learn about the
different types of fibre.
Below are the major dietary "fibre"
substances to be found.
consists of a polymer chain of glucose units.
the only fibre component with a truly fibrous
A major component in vegetable and legume
Also found in most fruits. Fermentability:
cereals and moderate in legumes
- these sugar containing substances are more
to bacterial enzymes than is cellulose. A
of cereal fibre. Wheat bran in
has a large amount of hemicellulose.
moderate-high, very low in raw corn
gums - these
are hydrocolloids secreted by the plant at
sites. They are composed of various
They also can be highly soluble and
E.g., guar gum. Fermentability: high
pectin - these
polysaccharides are water soluble and gel
. Found in fruits and to a lesser extent in
algal polysaccharides - agar and carrageenan are
of algal polysaccharides. Agar is
Because of their "hydrophylic" (literally,
water-guarding) properties they are used
which is a mucilage, is in fact secreted by
to protect the seed endosperm from
lignin - this
is the primary noncarbohydrate component of
is very inert. Highest in mature root
like carrots or fruits with edible seeds
Sorry about introducing so many new terms, but it is important
understand that there are DIFFERENT kinds of fibres and
not all play the same physiologic and nutritional role. Just as
not all fats are equal (or even saturated fats for that
so too with fibre. And
just as we need to eat a variety of
vegetables, so too with dietary fiber.
If you are now a little confuse about fibre, you are not
In fact, nutritionists themselves disagree as to which
substances should be consider dietary "fiber"
and there is no
"universally accepted definition for this food component"
Goff, 1990). At
least not quite yet. Some nutritionists
consider lignin to be fiber, hence the differences one might
between estimates of total fiber in common foods (e.g.,
Southgate method includes lignin, the Englyst method does
Generally speaking, fibre has been considered is foodstuff
resists digestion, at least until they enter the large intestine
unchanged. But which
"foods" and what kind of changes?
the most widely accepted definition was proposed by Trowell
and lignin which are resistant to
the digestive enzymes of man"
One problem even with this definition is that it doesn't
all the indigestible residues from food that may reach the
Some potentially digestible starches may reach the colon
unaltered state. This
can occur in varying amounts. Yet,
considers such starches to be fiber (because they are
"potentially" digestible before reaching the colon),
yet they fit
the above definition as it stands.
Another problem with the above definition is that it is
predicated on the idea of "undigestibility" as
a criterion, but
some so-called "undigestible" foods (e.g., nonstarch
polysaccharides) can undergo fermentation by colonic bacteria
thereby producing short-chain fatty acids that can be used
energy by the host. This
are considered to be fiber, yet they
are "digestible" and so we must now note that
no longer can the
potential energy in fibre be considered totally unavailable
the human body. An
In sum, most researchers believe that materials such as
starch and man-made ingredients should not be considered
components of dietary fibre -- Styrofoam packing "peanuts"
not to be considered fibre even if they do provide bulk ;-)
Joking aside, it is should be obviously that one must eat
variety of *plant* foods in order to get each of these important
fiber substances well represented in your diet.
PLANT CELL WALL AND DIETARY FIBRE
plant | lipids
cell ---| inorganic constituents
wall | lignin
| cellulose |
| hemicellulose |--- dietary
|- pectins | fibre
CATEGORIES OF FIBER
insoluble non- |-insoluable
cellulosic | fibre
-| | fibre
| (non-starch | fibre
soluble non- -|
cellulosic |-soluble | saccharides) | carbohydrate)
-| fibre -|
Fibre has an effect throughout the gastrointestinal tact,
beginning in the mouth.
Insoluble fibre components (lignin,
cellulose and most of the hemicellulose) necessitate greater
chewing which in turn stimulates saliva secretion, together
serves as a tooth cleaner. Eat some fruit if you forgot
Some of the more important gastrointestinal responses to
ingestion of fibre include:
- increased fecal
- decreased intraluminal
- greater frequency
- reduced intestinal
- delayed gastric
- increased postprandial
- reduced glucose
- changes in
pancreatic and intestinal enzyme activity
- increased bile-acid
- possible alteration
in mineral balances
Different fibre components will, of course, produce these
in different degrees.
V. HOW DOES COOKING
While cooking and kitchen processing is not going to decrease
increase the total amount of major fibres, heat from cooking
make certain "indigestible starches" more digestible.
Conversely, what is called "Maillard products can occur
resistant linkages between amino acids of proteins and the
carboxyl groups of reducing sugars), particularly from baking
frying. Of course,
there is debate as to whether or not include
such Maillard compounds as components of dietary fibre. Most
researchers prefer not to consider as components of fibre
the resistant starch or Maillard compounds.
It is also the case that the size of the particles and/or
of processing of the foods providing fibre appear to influence
the GI response to ingested fibre. For example, coarsely ground
bran has a higher hydration capacity than that which is
ground. Hence, coarsely
ground bran increases fecal volume by
its water-holding capacity, and it also speeds up fecal
time through the colon. With respect to emptying food from
stomach, these larger particles slow it down rather than
VI HOW THE GASTROINTESTINAL TRACT (GI) IS AFFECT BY FIBRE
Important characteristics of dietary fibre with respect
physiologic role in the GI includes:
absorptive attraction for organic
A. THE UPPER GI
The upper GI is affected more by the gellation effect of
and hydrocolloids (i.e. the gums, mucilages and algal
polysaccharides) than by the hydration effects of cellulose
hemicellulose, irrespective of particle size.
Hydrocolloids and pectin reduce the rate of glucose absorption,
and also decrease the rate of absorption and/or availability
fats and proteins. The
reduction in "apparent protein
digestibility" is likely nutritionally insignificant.
hydrocollids are natural components in beans and certain
(e.g., oats and barley), most enter the food supply as additives
used in processed food.
This decrease on lipid absorption by fibre is not well
understood. Some general effects of fibre on nutrient absorption
that have been proposed that could in part account for this
decreased absorption (e.g.., blunting of villi in the small
intestine, decreased secretion of GI and pancreatic hormones,
direct reduction of pancreatic enzyme activity, decreased
diffusion rate in the proximal intestine due to an increased
thickness of the unstirred water layer, and decreased solute
movement within the lumen of the intestine). More specific
mechanisms include the lowering of bile acid concentration
their absorption into the fibre.
Pectin and guar gum (12-30 g/daily) have been shown to lower
serum cholesterol by 6-15% in normal volunteers. A number of
mechanism have been proposed for the blood cholesterol lowering
effects of fibre. For instance, when fibre absorbs bile
thereby removes some bile from circulation. A decrease in bile
acids returned to the liver would cause diversion of some
cholesterol from lipoprotein synthesis to the synthesis
acids, thereby lowering serum cholesterol. Another proposed
mechanism involves the fibre stimulated shift of bile acid
toward chenodexoycholic acid -- which inhibits cholesterol
synthesis. It is
thought that the chenodeoxycholate alerts the
liver through inhibition of a key enzyme that no more cholesterol
is needed for bile acid synthesis. Still, neither of these
proposed mechanisms fully explains the degree to which fibre
lower serum cholesterol.
Another effect of fibre is its influence on cation aDsorption,
particularly calcium, zinc and iron. Not only do the cation
bridges formed by fibre serve as a mechanism for the adsorption
of bile acid and fats, but also of minerals. This can ultimately
help or hinder mineral absorption, depending upon the
fermentability (or its accessibility to bacterial enzymes)
fibre when it enters the lower GI.
B. THE LOWER GI
It is here where most of the signification action of dietary
fibre occurs. Fermentation
of food by colonic anaerobes make
available to the body much of the energy of undigested foods
reaching the cecum. This has indeed been an overlooked source
energy. For instance,
as much as 10% to 15% of the carbohydrates
we eat in the West may be fermented in the colon. In general,
from 40-95% of dietary fibre is fermented by intestinal
Certain fibres, like the plant gums (and any starch that
passed undigested into the cecum), are rapidly fermented
various anaerobic bacteria residing in the colon. The main
metabolites produced by this rapidly fermentable fibre are
short-chain fatty acids (acetic, butyric & propionic
products of this fermentation are hydrogen, carbon dioxide
methane. Keep matches
away! These gases are excreted as
or are expired by the lungs.
These fatty acids produced by fermentation are rapidly absorbed
or are used by the epithelial cells of the colon for energy.
propionic acid produced from fibre may also contribute to
cholesterol lowering effect of certain fibres by acting
inhibit a rate-limiting enzyme (HGG CoA reductase) in the
synthesis of cholesterol in the liver.
The more slowly fermentable or non-fermentable fibres than
gums are particularly helpful for overcoming constipation
increasing fecal bulk (1) water absorption and/or (2) promotion
of microbe proliferation.
Slowly fermentable fibres, like cereal
fibres, are particularly valuable in causing microbial
cells form part of the fecal mass and
provide moisture. The
volatile fatty acids produced by the
bacteria acidify the colonic content, act on the musosa
following absorption, modify the lipid metabolism. Due to
two factors, it has been shown that for every extra gram
cereal fibre stool weight gains an extra 2 to 9 grams!
Wheat bran, for instance, can absorb 3 times its weight
thereby producing a much softer, bulkier stool. The large wheat
bran particles take a curly shape on fermentation, constituting
microenvironments in the distal colon, and providing a physical
resistance against the removal of interstitial water and
dispersed gases, thus counterbalancing the absorptive capacity
the colon. The resulting decrease in fecal density prevents
impaction and constipation.
The threshold volume is rapidly
attained in the rectum triggering defecation, thus limiting
opportunity for reabsorption and hardening of the intestinal
It should be noted that reducing particle size eliminates
effect since small particles retain non-solid components
effectively. Coarse bran will reduce colon segmenting activity
and intraluminal pressure, normalizes slow transit time
hours) to about 20 hours, increase fecal weight (4 times
than fine bran and 7 times more than oat bran).
Interestingly, rice bran has been found to be even more
in increasing fecal bulk, frequency of defecation and reduced
intestinal transit time.
Now only are these responses are
particularly important in the prevention of constipation,
they may be advantageous in the management of irritable
Microbial proliferation and excretion is not only important
increasing fecal volume but is thought to play an important
as a "DETOXIFICATION MECHANISM". Do not confuse this term with
what the colon irrigation quacks call "detoxification".
not the same thing.
It works as follows. Increased
microbial cell synthesis would
scavenge degradable nitrogenous substances and thereby sequester
those substances into the microbes themselves, which in
The downside to this function is that excessive microbial
proliferation may decrease mineral absorption. What is thought
to happen is that certain essential elements may become
the microbial cells themselves, to then be excreted rather
absorbed. In contrast, the more rapidly fermentable fibre
components release their calcium, zinc and iron for absorption
the colon as fermentation occurs.
Fibre from fruit and vegetables is less effective in increasing
fecal bulk since much of their fibre consists of rapidly
fermentable pectin and the less microbial promoting cellulose.
Hence, for every 1 gram extra of vegetable fibre consumed,
about a 1.9 gram increase in fecal weight occurs. In contrast to
cereal fibre, fruit and vegetable fibre which contain
considerable amounts of pectin, can delay gastric emptying
reduce glucose absorption because of its gellation quality.
It would seem that both fast and slow fermentable fibres
be consumed. Again,
it is not simply the amount of fibre that
should be important, but also that fibre from VARIOUS sources
ingested so that a varied selection of fibre components
of one's diet.
A comparison of the levels of mutagens in the faeces of
omnivores, 6 vegetarians and 6 vegans showed even with this
sample significant lower levels in the vegans and vegetarians
volunteers showed (to pick up a statistically significant
in a small sample requires that the size of the effect size
study with volunteers on 20-day experimental
diets showed that vegan diets produced the lowest concentration
of bile acids, and of course cholesterol, in their faeces.
Apparently a high concentration of bile acids or cholesterol
faeces is associated with risk of colorectal cancer.
VIII. POTENTIAL PROBLEMS
WITH TOO MUCH FIBRE
There are few reports of adverse effects on the gastrointestinal
tract directly related to fibre. Excessive intakes of
particulate fibres (e.g., cereal fibres), for instance,
reported to produce intestinal obstruction in susceptible
general, more finely ground fibre (even from
wheat bran) may cause difficult or uncomfortable defecation.
mean particle size of fibre in ready- to-eat breakfast cereals
varies from 350um to above 1mm. The number of particles less
than 150um appears to be negligible.
Excessive fibre consumption may cause a transient fluid
when the fibre consumed absorbs a lot of water.
An excessive intake of nonfermentible fibre could make for
negative mineral balance, particularly among infants, children,
adolescents, and pregnant women whose mineral needs are
relatively greater than for adult men or nonpregnant woman.
the intake of calcium, zinc and iron is marginal, then excessive
fibre could exacerbate the already low intake of these minerals.
The nutrition recommendations from the 1990 Canadian scientific
review committee concluded that "evidence of mineral
unequivocal but it is doubtful whether such effects are
nutritional importance in the context of an adequate diet".
ordinary man or woman were confronted for the first
time with the
offensive contents of a constipated large
bowel, and were
told that the body he or she thinks so much
the casket of such a jewel the shock and
almost prove fatal" ;-) :-)
[from F. A. Hornibrook, "The Culture
of the Abdomen",
IX. FIBRE INTAKE AND
We've seen that a varied selection of fibre should be ingested,
now the question is how much?
The recommendation for the general population has ranged
to 40 grams/day, and may up to 50 gram/day for
hypercholesterolemic individuals. The National Health and
Nutrition Examination Survey (1976-1980) showed that the
consumption of fibre was lower than expected. Young white males
(19 to 29 years) had the highest intake of 13 g/d, while
black males (55 to 74 years) and middle-aged black females
54 years) had the lowest intake averaging 7.4 g/d.
Presumably people are consuming more fibre since that survey
taking, but it is likely that the greater majority of people
still not consuming enough.
A recent survey (Carlson, 1985) of vegetarians has shown:
vegans 45 g/d
vegetarians in general 38 g/d
Rather than simply "adding" refined fibre to one's
the better approach is to thinks in terms of a dietary change
foodstuffs that simply include foods with more fibre and
foods (like meat or dairy products) that have none. Vegetarians
naturally do well in this respect :-)
If you think that you need more fibre in your diet, then
a dietary change that includes:
1. a greater consumption of fibre-rich legumes
2. increased consumption of fresh fruits and vegetables
3. replacement of refined cereals and flour products
Vegetarians have no problem in getting enough fibre, but
not be getting a great enough VARIETY of fibre due to an
or shortage one or two of the above three areas. A bad practice
is to simply consume large amounts of a single source of
fibre. Better to simply eat a variety of fibre by simply
variety of whole foods.
By ensuring that at least 60% of energy
is in the form of whole, complex carbohydrates the resulting
dietary patter will perforce increase present intakes of
as we have seen, do well in this regard. :-)
X. DIETARY FIBRE OF
SOME COMMON FOODS
Because definitional problems there are different ways to
fibre depending own what is being tested. Earlier tables only
measure for crude fibre (cellulose & lignin) and did
for the "noncellulosic polysaccharides like pectin,
hemicellulose, and other polysaccharides (e.g., gums, mucilages
and algal polysaccharides). The figures for total dietary
in the following table may be larger than some other tables
may have, but that may be due simply the following table
more inclusive in what is being measured as "fibre".)
SOME COMMON FOODS
DIETARY FIBER CONTENT
Total Cellulose Noncellulose Lignin
(g/100g) (g/100g) (g/100g)
white 2.72 .71 2.01 trace
whole meal 8.5 1.31 5.95 1.24
broccoli 4.10 .85 2.92 .03
beans, baked 7.27 1.41 5.67 .19
2.83 .69 1.76 .38
corn (canned) 5.68 .64 4.97 .08
lettuce 1.53 1.06 .47 trace
onions (raw) 2.10 .55 1.55 trace
peas (raw, frozen)
7.75 2.09 5.48 .18
carrots (boiled) 3.70 1.48 2.22 trace
tomato (fresh) 1.40 .45 .65 .30
apple (flesh) 1.42 .48 .94 .01
apples (peels 3.71 1.01 2.21 .49
banana 1.75 .37 1.12 .26
peach (flesh &
skin) 2.28 .2 1.46 .62
pear (flesh) 2.44 .67 1.32 .45
pear (peels) 8.59 2.18 3.72 2.67
strawberries 2.12 .33 .98 .81
strawberry jam 1.12 .11 .85 .15
1.69 6.40 1.21
peanut butter 7.55 1.91 5.64
Southgate et al., A guide to calculating
intakes of dietary
fiber. J. HUM. NUTR., 1976, 30:303-13]
To put things in more practical terms, consider again the
foods but this time in terms of the kinds of actual servings
is more likely to consume at any one meal.
SOME COMMON FOODS
DIETARY FIBER CONTENT
Serving Total dietary
size weight fiber/serving
white 1 slice 23 .63
whole meal 1 slice 23 1.96
broccoli 1/2 cup 73 2.99
beans, baked 1/3 cup 85 6.18
1/2 cup 73 2.07
corn (canned) 1/2 cup 83 4.72
lettuce 1/2 cup 55 .84
onions (raw) 2.25" onion 100
peas (raw, frozen)
1/2 cup 73 5.66
1/2 cup 75 2.78
tomato (fresh) small tomato 100 1.40
apple (flesh) 1 medium apple 141 2.00
apples (peels 1 medium apple 11 .41
banana 6" banana 100 1.75
peach (flesh &
skin) 1 medium peach 100 2.28
pear (flesh) 1/2 medium pear 87 1.12
pear (peels) 1/2 medium pear 11 .95
strawberries 10 large berries 100 2.12
strawberry jam 1 Tbsp 20 .22
Tbsp 9 .84
peanut butter 1 Tbsp 15 1.13
Southgate et al., A guide to calculating
intakes of dietary
fiber. J. HUM. NUTR., 1976, 30:303-13]
Anderson, J.(1986). Fibre
and health: An overview. Nutr. Today,
Carlson et al. (1985).
A comparative evaluation of vegan,
omnivore diets. J. PLANT FOODS, 6:89-100
Dreher, M. (1987). HANDBOOK
OF DIETARY FIBER: AN APPLIED
Health & Welfare Canada. (1990). NUTRITION RECOMMENDATIONS:
REPORT OF THE
SCIENTIFIC REVIEW COMMITTEE.
Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN
Kay & Truswell (1977).
Effect of citrus pectin on blood lipids
and fecal steroid
excretion in man. AM. J. CLIN. NUTR.,
Kuhnlein et al. (1981).
Mutagens in feces from vegetarians and
MUTATION RES., 85:1-12.
McCance & Widdowson (1991), THE COMPOSITION OF FOODS,
Southgate, D. (1987). Minerals,
trace elements and potential
J. CLIN. NUTR., 45:1256-66.
Spiller, G. (1986). CRC
HAND BOOK OF DIETARY FIBER IN HUMAN
NUTRITION. CRC Pr.
Van Faasen et al. (1987).
Bile acids, neutral steroids, and
feces as affected by a mixed, a lacto-
and a vegan diet. AM. J. CLIN. NUTR.,
Van Soest (1984). Some
characteristics of dietary fibre and
on the microbial ecology of the human colon.