From: altar@beaufort.sfu.ca (Ted Wayn Altar)
Subject: Fibre: How It Affects the GI
Organization: Simon Fraser University, Burnaby, B.C., Canada
Date: Mon, 11 Jan 1993 08:42:31 GMT




VII HOW THE GASTROINTESTINAL TRACT (GI) IS AFFECT BY FIBRE

Important characteristics of dietary fibre with respect to its
physiologic role in the GI includes:

        hydration capacity
        absorptive attraction for organic molecules
        cation exchange capacity
        fermentability


A.  THE UPPER GI

The upper GI is affected more by the gellation effect of pectins
and hydocolloids (i.e. the gums, mucilages and algal
polysaccharides) than by the hydration effects of cellulose and
hemicellulose, irrespective of particle size.

Hydocolloids and pectin reduce the rate of glucose absorption,
and also decrease the rate of absorption and/or availability of
fats and proteins.  The reduction in "apparent protein
digestibility" is likely nutritionally insignificant.  While some
hydrocollids are natural components in beans and certain cereals
(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 by
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 acids it
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 of bile
acids, thereby lowering serum cholesterol.  Another proposed
mechanism involves the fibre stimulated shift of bile acid pools
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 can
lower serum cholesterol.

Another effect of fibre is its influence on cation aDsorption,
particularly calcium, zinc and iron.  Not only do the cationic
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) of the
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 of
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 flora.

Certain fibres, like the plant gums (and any starch that has
passed undigested into the cecum), are rapidly fermented by
various anaerobic bacteria residing in the colon.  The main
metabolites produced by this rapidly fermentable fibre are some
short-chain fatty acids (acetic, butyric & propionic acids).  By-
products of this fermentation are hydrogen, carbon dioxide and
methane.  Keep matches away!  These gases are excreted as flatus
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.  The
propionic acid produced from fibre may also contribute to the
cholesterol lowering effect of certain fibres by acting to
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 the
gums are particularly helpful for overcoming constipation by
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
proliferation.  Bacterial 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 and,
following absorption, modify the lipid metabolism.   Due to these
two factors, it has been shown that for every extra gram of
cereal fibre stool weight gains an extra 2 to 9 grams!

Wheat bran, for instance, can absorb 3 times its weight in water
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 of
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 the
opportunity for reabsorption and hardening of the intestinal
contents.

It should be noted that reducing particle size eliminates this
effect since small particles retain non-solid components less
effectively.  Coarse bran will reduce colon segmenting activity
and intraluminal pressure, normalizes slow transit time (40-150
hours) to about 20 hours, increase fecal weight (4 times more
than fine bran and 7 times more than oat bran).

Interestingly, rice bran has been found to be even more effective
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, but
they may be advantageous in the management of irritable colon and
diverticular disease).



                                     ted

References:

Hunt & Groff's (1990), ADVANCED NUTRITION AND HUMAN METABOLISM,
    1990.

Dreher, M. (1987).  HANDBOOK OF DIETARY FIBER: AN APPLIED
    APPROACH.

Spiller, G. (1986).  CRC HAND BOOK OF DIETARY FIBER IN HUMAN
    NUTRITION.  CRC Pr.

Kay & Truswell (1977).  Effect of citrus pectin on blood lipids
    and fecal steroid excretion in man.  AM. J. CLIN. NUTR.,
    30:171-5.

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