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Department of Nutritional Sciences, Faculty of Medicine, and St. Michael's Hospital, University of Toronto, Toronto, ON M5S 1A8, Canada
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONThe dietary fiber hypothesis,...Colonic eventsRelation of colonic events...REFERENCES |
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KEY WORDS: • bifidobacteria • colon • inulin • carcinogenesis • ammonium
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONThe dietary fiber hypothesis,...Colonic eventsRelation of colonic events...REFERENCES |
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at the
beginning of the 1970s was the stimulus to research in this area.
Dietary fiber was initially defined as indigestible plant cell wall
material that was predominantly carbohydrate in nature but included
lignin, a polyphenol propane. Later the definition was enlarged to
include all nonabsorbable carbohydrates of plant origin (Trowell et al. 1976
). The debate continues over the status as fiber of
purified gums and pectic substances, unavailable storage
polysaccharides, resistant starch, nonabsorbable sugars of various
kinds and associated oligosaccharides. The term non-starch
polysaccharides is now preferred by some to refer to cell wall
materials that do not include lignin (Cummings and Englyst 1995
). Nevertheless, these nonabsorbable plant-derived
materials share overlapping spectra of physiologic effects and
potential preventive or therapeutic indications for incorporation into
the diet. In this respect, the dietary fiber hypothesis and associated
research continue to provide valuable insights relevant to the
potential alterations in colonic function and systemic metabolism that
these materials may induce. |
The dietary fiber hypothesis, inulin and oligofructose |
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TOPABSTRACTINTRODUCTIONThe dietary fiber hypothesis,...Colonic eventsRelation of colonic events...REFERENCES |
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). Since the time of the original hypothesis,
research has been carried out, initially defining some of the
physiologic effects of fiber (Jenkins et al. 1997
) and
recently providing more support for the disease associations than was
considered likely by most scientists at the time (DeCosse et al. 1989
, Morris et al. 1977
, Rimm et al. 1996
, Salmeron et al. 1997
). Much of the
research on colonic function, colonic microbiology and short-chain
fatty acid (SCFA) metabolism is relevant to the discussion of the
effects of inulin and oligofructose. The potential importance of the
changes they induce in colonic function led to the term
"prebiotic" to indicate the ability of supplementation with
nonabsorbed substances to induce favorable changes in colonic
microflora and hence metabolism (Gibson and Roberfroid
1995
).
Table 1 indicates some of the colonic and systemic physiologic effects
that may relate to the disease entities, both colonic and systemic,
indicated in Table 2. It is worth addressing the issues raised by Table 1
in relation
to colonic function, inulin and oligofructose with mention of how these
physiologic functions relate to the disorders in Table 2
.
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Colonic events |
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TOPABSTRACTINTRODUCTIONThe dietary fiber hypothesis,...Colonic eventsRelation of colonic events...REFERENCES |
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Possibly the most widely accepted effect of inulin and oligofructose is
to increase both the numbers and the proportion of fecal
bifidobacteria. These studies raise the question of whether the
increased vitamin B absorption associated with oligofructose results
from increased small intestinal absorption or bacterial synthesis.
However, there does not appear to be an increase in total bacterial
numbers or a change in the anaerobe:aerobe ratio (Bouhnik et al. 1996
, Gibson et al. 1995
). Furthermore, pH
reduction for the most part is not seen nor are increases in the fecal
SCFA or their proportions (Gibson et al. 1995
). However,
more work is required in this area because in vitro studies indicate
differences in SCFA proportions in relation to different substrates
fermented.
SCFA have been shown to enhance the colonic absorption of calcium
(Trinidad et al. 1997
). In the absence of a change in
fecal SCFA concentrations, the increased absorption of divalent
cations seen with inulin and oligofructose supplementation is
less readily explained. Nevertheless, the increased uptake of Ca, Mg,
Zn and Fe in laboratory animals (Delzenne et al. 1995
)
can be seen as another potential health benefit if confirmed in humans.
What is of interest, however, is the fact that the bifidobacteria
appear to displace potential pathogens by reducing their numbers
selectively (Gibson et al. 1995
). It has been implied
that this is an "antibiotic-like" effect not related to changes
in SCFA or pH. This is an obvious area for further research.
Fecal bulk and nitrogen excretion.
In most studies, there is a small increase in fecal bulk upon inulin or
oligofructose ingestion. When measured, there also appears to be an
increase in fecal nitrogen. Despite the recorded absence of a change in
total bacterial numbers (Bouhnik et al. 1996
,
Gibson et al. 1995
) with inulin and oligofructose as
assessed in log units, it is likely that an increase in the biomass
occurs that accounts for the increase in fecal bulk because all of the
fructo-oligosaccharide (FOS) is fermented in the colon.
Furthermore, in the absence of a reduction in pH to convert free
NH3 to NH4+ ion and thus
reduce nitrogen elimination in the feces, it is likely that the
increased fecal nitrogen comes from the increase in the protein
nitrogen contained in the increased fecal biomass.
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Relation of colonic events to systemic effects |
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TOPABSTRACTINTRODUCTIONThe dietary fiber hypothesis,...Colonic eventsRelation of colonic events...REFERENCES |
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Studies in rats have demonstrated that feeding inulin or oligofructose
as 10% of the diet reduced hepatic triglyceride synthesis and serum
VLDL triglyceride (Fiordaliso et al. 1995
Kok et al. 1996a
, Kok et al. 1996b
). In addition, it
prevented the increase in hepatic triglyceride synthesis normally seen
after fructose feeding. Lower levels of serum phospholipids and
cholesterol were also seen. The exact mechanism is not clear because in
the absence of small intestinal absorption of the intact oligofructose,
a colonic explanation must be sought. SCFA have been shown to modulate
cholesterol synthesis, with acetate as stimulator and propionate as
inhibitor (Thacker et al. 1981
, Wolever et al. 1989
). Thus lactulose, which produces acetate, may raise serum
cholesterol levels (Jenkins et al. 1991
). Furthermore,
antibiotic therapy, which may selectively alter microbial growth
(Morgantein et al. 1981
), may also reduce serum
cholesterol (Samuel 1979
). However, no effects have been
reported for serum triglyceride, which could be linked to altered SCFA
production. Because there is no clear evidence as yet in humans that
SCFA proportions are altered by inulin or oligofructose, detailed
studies in humans are necessary or an alternative explanation must be
sought.
Blood NH3 and serum urea.
Dietary fibers and nonabsorbable sugars such as lactulose have long
been known to reduce blood NH3 and serum urea
levels. These effects have been associated with the growth of the
colonic biomass and nitrogen fixation by colonic bacteria, coupled with
colonic acidification and conversion of diffusable
NH3 into the less diffusable NH4+
ion. Increased fecal nitrogen elimination has been reported with inulin
and oligofructose (Bouhnik et al. 1996
, Gibson et al. 1995
), and therefore these systemic effects would be
expected.
Immune function enhancement.
This has been proposed as a possible function of inulin and
oligofructose. Studies have not been undertaken to assess
systematically lymphocyte activity or other tests of immune function.
However, lactulose administration raises serum glutamine levels
(Jenkins et al. 1997
) possibly as a result of its
ability to spare glutamine as a substrate for the colonic mucosa by
provision of increased SCFA. Because glutamine is a preferred substrate
for lymphatic tissue, it is possible that this may improve immune
function under some circumstances. Such an effect may also be relevant
to inulin and oligofructose if SCFA production is increased in the
proximal, even if not in the distal colon.
Disease prevention and therapy.
Changes in colonic function induced by inulin and oligofructose may be relevant to both local and systemic disorders. Both would be expected to have preventive and treatment implications.
Colonic disorders.
Inulin and oligofructose at doses of 10–20 g/d produce a small
increase in fecal bulk. For example, in one study, oligofructose
produced a 1.3-g increase in fecal weight for every gram fed compared
with 2.0 g for inulin from chicory roots (Gibson et al. 1995
).
No studies in which butyrate deficiency has been implicated appear to
have been undertaken, for example, in patients with ulcerative colitis
(Roediger 1980
). However, in laboratory animals, studies
suggest that 10% oligofructose, possibly equivalent to an intake of
40–60 g in human terms, reduces the development of chemically induced
aberrant crypt foci and the growth of transplanted tumors (Koo and Rao 1991
, Pierre et al. 1997
, Taper et al. 1997
). Studies of fecal SCFA, nitroreductase and
ß-glucuronidase activity in humans have not provided an explanation
for these potential anticancer effects (Bouhnik et al. 1996
). Nevertheless, the data suggest that clinical trials of
the effects of oligofructose on a precursor lesion such as polyp
recurrence are appropriate in view of these data and the effect on the
colonic microflora.
Systemic disorders.
As mentioned in other articles in this proceeding, some studies have
observed that inulin and oligofructrose can reduce triglyceride and/or
cholesterol concentrations. Dietary fibers may also reduce serum urea
levels in renal disease (Reddy et al. 1994
) and
NH3 levels in hepatic encephalapathy. The effects
of inulin and oligofructose in these areas remain to be determined.
Similarly, if further evidence surfaces that inulin and oligofructose
can suppress tumor incidence and growth, it would have profound
implications.
Conclusion.
There is strong evidence in humans for an effect of inulin and oligofructose increasing fecal bulk and fecal nitrogen elimination and promoting the growth of bifidobacteria at the expense of other anaerobes. What is required now is the exploration of these changes in colonic function in relation to both colonic and systemic disorders for which laboratory studies have indicated possible benefits. These areas include colon cancer, ulcerative colitis, hyperlipidemia, renal and liver disease, and possibly osteoporosis and compromised immune function. In all of these areas, the importance of dose and molecular chain length also must be determined. A useful effect in any one of these situations would amply justify continued interest in inulin and oligofructose.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONThe dietary fiber hypothesis,...Colonic eventsRelation of colonic events...REFERENCES |
|---|
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