The term 'prebiotic' was first defined in 1995 by Gibson and Roberfroid as 'a non-digestible food ingredient that selectively stimulates growth and/or activity of one or a limited number of bacteria in the colon, thereby improving host health'. As research progressed, three criteria were accepted which a food ingredient should fulfil before it can be classified as prebiotic: firstly, it should be non-digestible and resistant to gastric acidity, hydrolysis by intestinal (brush border/pancreatic) digestive enzymes, and gastrointestinal absorption; secondly, it should be fermentable and; thirdly, it should in a selective way stimulate growth and/or metabolic activity of intestinal bacteria that are associated with health and wellbeing (1). Well established prebiotic compounds nowadays are inulin and oligofructose (or fructo-oligosaccharides), galacto-oligosacchardies and lactulose, however extensive research is ongoing to strengthen the scientific basis of promising new candidates.
Inulin-type fructans are naturally occurring oligosaccharides that represent
the carbohydrate reserve in plants. Plants containing inulin-type fructans primarily
belong to the Liliales, e.g.
leek, onion, garlic and asparagus; or the
Compositae, such as Jerusalem artichoke (Helianthus tuberosus
and chicory (Cichorium intybus
). Inulin is a polydisperse carbohydrate
material consisting of ß (2 ->1) fructosyl - fructose links (Fig. 1
A starting glucose moiety can be present. Inulin-type fructans can be represented
as both GFn
In chicory inulin, the number of fructose units linked to a terminal glucose
can vary from 2 to 70 units. By means of an endo-inulinase inulin is hydrolysed
into a DP between 2 and 8 (average DP=4) called oligofructose.
||Fig. 1. Chemical structure of inulin compounds.
Other interesting classes of dietary substances that arrive to a great extent
in the colon and are metabolised by the microbiota in the colon are the polyphenols.
Most polyphenols are in the form of esters, glycosides or polymers (proanthocyanidins)
and have to be hydrolysed by intestinal enzymes or by the colonic microflora
before absorption can occur (2-6). This complex group of plant derived-polyphenolic
compounds has been the focus of much research given their interesting anti-oxidant
properties which have been related to the protecting effect of diets rich in
fruits and vegetables against several chronic diseases such as cardiovascular
diseases and certain cancers (2, 7). Polyphenols can be classified in different
groups including phenolic acids (hydroxybenzoic acids and hydroxycinnamic acids),
flavonoids and the less common stilbenes and lignans. The flavonoids can be
further divided in flavones, isoflavones, anthocyanidins, flavanones, flavanols
and their polymers the proanthocyanidins (Fig. 2
). The main dietary sources
are fruits (e.g.
citrus fruit, apples, grapes and berries), wine, tea,
soy and cacao. Polyphenols are also found in vegetables (e.g.
artichokes) but are less abundant. Foods mostly contain complex mixtures of
polyphenols (2, 3, 8, 9). To understand their impact on human health, their
nature, origin, amount in the diet, bioavailability and microbial metabolisation
in the colon need to be investigated. In this respect, gaining understanding
of the metabolisation pathways of polyphenols by the microbiota and the kind
of bioactive metabolites that are formed during this process is of paramount
importance. Also in turn, the effects of such metabolites on the composition
of the microbiota might be subject of investigation. As such, in the future,
strategies that enhance bioactive formation by colonic microbiota manipulation
could be an important tool to enhance anti-oxidant or anti-inflammatory properties
INTESTINAL FUNCTION, METABOLISM
||Fig. 2. Subclasses of flavonoids.
Studies in ileostomised volunteers have demonstrated that orally ingested inulin
enters the colon almost quantitatively (>90%) where it is subsequently completely
metabolized by the endogenous colonic microbiota (10). In the colon, inulin-type
fructans are completely converted by the microbiota into bacterial biomass,
organic acids, like lactic acid and short-chain fatty acids (SCFA: acetic, propionic
and butyric acid) and gasses (CO2
). SCFA and lactate contribute to the host's
Inulin-type fructans, through their presence and subsequent fermentation in
the large bowel, influence the colonic metabolism in its lumen and the integrity
and functioning of the epithelial cell lining. Apart from their stool bulking
effect which has been demonstrated in randomised, double-blind and placebo-controlled
human studies in subjects with low stool frequency patterns or constipated patients
(11-13), more recently also a significant decrease in the intensity of digestive
disorders in patients with minor functional disorders was found in a randomised
and double-blind controlled, multicentre study set-up (14). An increase in stool
frequency with the administration of a synbiotic supplement (Bifidobacterium
and an oligofructose-enriched inulin) has been demonstrated in
elderly subjects also to be associated with an improved well-being and the quality
of life (15 - CROWNALIFE project, 'Crown of Life' Project on Functional Foods,
Gut Microflora and healthy Ageing, QLK1-2000-00067).
The intestinal microbiota can be considered as a metabolically adaptable and rapidly renewable organ of the body. Administration of oligofructose to post-weaning infants has been shown to increase the numbers of bifidobacteria (up to 9.5 log of colony-forming units per gram of faeces) (16). Also in adults and elderly subjects, administration of inulin and oligofructose alone or as synbiotic has been demonstrated to selectively increase numbers of bifidobacteria in the luminal as well as the mucosa-associated microbiota (11, 12, 15, 17-20), typically representing a prebiotic effect.
In certain conditions such as old age, the use of antibiotics or in case of
(critical) illnesses (acute or chronic such as inflammatory bowel diseases and
cancer) the intestinal barrier is functioning less and gastro-intestinal dysfunction
can occur. As a result, increased bacterial translocation may happen and leading
to systemic illness. The interdigestive intestinal motility (e.g.
myoelectric/motor complex) is one physiological mechanism that prevents bacterial
overgrowth and translocation in the gut and a relationship appears between the
intestinal motility and the composition of the intestinal microflora. Administration
of Lactobacillus rhamnosus
GG, Bifidobacterium lactis
oligofructose-enriched inulin to elderly rats regularised the occurrence of
intestinal contractions of high amplitude which are more effective in propelling
the residual food, debris, secretions and bacterial cells (21). Other animal
experiments to test the potential of modulating the microbiota to efficiently
discriminate and eliminate pathogenic organisms showed decreased translocation
of bacteria (total aerobic, anaerobic and the Enterobacteriaceae
the mesenteric lymph nodes and liver, after oral administration of probiotics
) and/or prebiotics (oligofructose-enriched
inulin) in DSS-colitis induced rats (22). These data are indicative of an improved
epithelial barrier function and in agreement with earlier studies. In mice infectioned
(intra-peritoneally) with virulent strains of systemic pathogens (Listeria
and Salmonella typhimurium
) mortality rates were much
lower upon inulin feeding (23). Other studies in rats, also infected with Salmonella
showed lower pathogen colonization in the intestines with oligofructose, however,
the authors observed an increase in translocation rate to the spleen and liver.
These observations can most likely be ascribed to the low calcium diet used
in this model which by itself damaged barrier function as the authors demonstrated
that increasing the calcium level of the diets was accompanied by a decrease
in the rate of translocation (24).
In humans, no effect on barrier function of (high dose) oligofructose was found
in healthy volunteers. Barrier function was measured by the levels of intestinal
epitheliolysis and the excretion of O-linked oligosaccharides in stools. The
latter refers to the production of glycoproteins which build up the mucus gel
layer that is covering the intestinal epithelium. The authors, however, did
observed a lower level of cytotoxicity of the faecal water with oligofructose
(25). In the SYNCAN Project, Synbiotics and Cancer Prevention (QLK-1999-00346),
the effect of oligofructose-enriched inulin (as synbiotic) on the epithelial
barrier function was studied. The trans-epithelial resistance (ex vivo
of cell lines subjected to the faecal water from polypectomized volunteers supplemented
with oligofructose-enriched was measured as an indicator of barrier functioning.
The faecal water is the faecal fraction in most intimate contact with the colonic
epithelium and mediates its functioning. A common observed effect of tumour
promoters is the reduction in barrier function of the epithelium inducing lower
protection of the mucosa to carcinogenic substances. Interestingly, the synbiotic
intervention increased the barrier function of the epithelium which was showed
by the significantly increased percentage in trans-epithelial resistance of
the Caco-2 cell monolayer when subjected to the faecal water of polyp patients
receiving the synbiotic (26).
PREBIOTICS AND BUTYROGENIC EFFECT
Studies (both in vitro
and in vivo
) have demonstrated that the
colonic fermentation of inulin-type fructans increases the production of butyrate,
which is the so-called 'butyrogenic effect'. However, as bifidobacteria are
primarily lactate and acetate producers, this effects remained until recently
unclear. Fermentation studies (in vitro
) using simple and complex bacterial
cultures or faecal slurries offer a valuable tool to study individual bacterial
metabolism and interspecies interactions. Kinetic analyses of co-cultures with
. and butyrate-producing colonic bacteria in the presence
of oligofructose revealed distinct types of cross-feeding reactions which were
strain-dependent. In such studies, butyrate-producing bacteria (e.g.
and R. intestinalis
) were unable to degrade oligofructose,
whereas in the presence of bifidobacteria and/or fermentation metabolites (acetate)
or breakdown products, degradation did occur with corresponding butyrate production
(27). Studies with stable isotopes, enabling to follow carbon flows, showed
that indeed Bifidobacterium sp
. in the presence of oligofructose produce
lactate (and/or breakdown products) which in turn are converted into butyrate
in the presence of butyrate-producing bacteria (e.g. R. intestinalis
and E. halli
) (28). Faecal batch cultures found that the addition of
oligofructose significantly increased butyrate production. About 80% of the
newly synthesised butyrate derived from oligofructose fermentation originated
from the inter-conversion of extracellular acetate and lactate. Also, Duncan
. found that the contribution of external acetate to butyrate formation
from oligofructose fermentation ranged from 82% (faecal slurry batch culture)
to 87% (continuous cultures). The increased flux of extracellular acetate to
butyrate upon oligofructose fermentation in mixed (faecal) slurries is in agreement
with butyryl CoA: acetyl CoA transferase being the dominant butyrate-producing
pathway. It appears that this pathway is selectively activated upon oligofructose
fermentation, with concomitant butyrate production. These cross-feeding mechanisms
could play an important role in the colonic ecosystem and contribute to the
combined bifidogenic and butyrogenic effect observed after addition of inulin-type
fructans to the diet (29).
INTESTINAL MICROBIOTA, INFECTION, INFLAMMATION AND IMMUNITY
Exploratory in vitro
work with fecal slurries, starting in the early
nineties, indicated that inulin and oligofructose are completely fermented by
the colonic microbiota and selectively stimulate bifidobacteria and lactobacilli
growth and activity at the expense of pathogenic bacteria (e.g.
Protective effects of bifidobacteria have been demonstrated in (gnotobiotic)
quails against the development of necrotizing enterocolitis (NEC)-like lesions
when inoculated with a pathogenic flora (containing Clostridium butyricum
and Clostridium perfingens
) from premature newborns. Lesions occurred
rapidly after establishment of the NEC-flora (e.g.
thickening of the
caecal wall with gas cysts, hemorrhagic ulcerations, necrotic areas), whereas
did less in the presence of Bifidobacterium infantis
(30). Supplementing the quails' diet with oligofructose induced an
increase in the level of bifidobacteria which prevented overgrowth of bacteria
implicated in NEC (e.g. Escherichia coli
, Clostridium perfringens
, and Clostridium ramosum
) and reduced NEC-like
lesions caused by polymicrobial infection (31). Other experiments in mice showed
that supplementation with inulin-type fructans reduced intestinal yeast densities
after oral challenge of mice with Candida albicans
, resulting in an enhanced
survival rate (23). A combination of oligofructose and Lactobacillus paracasei
also showed to suppress pathogens (Clostridium
, enterococci and enterobacteria)
in weaning pigs (32). Furthermore, in pigs with cholera toxin-induced secretory
diarrhea, oligofructose suppressed the presence of pathogens and increased lactobacilli
Clinical studies in humans have also shown that inulin-type fructans can protect
against pathogen colonization and infection. Critically ill patients have a
gut microbial ecology that is in dysbalance and is characterized by high numbers
of potential pathogens. Such patients, at risk for developing sepsis (at intensive
care unit), when receiving oligofructose (as a synbiotic), had lower numbers
of pathogens in their nasogastric aspirates. Treatment with antibiotics, on
the other hand, also changes the gut microflora and disrupts normal ecological
balance, which often leads to antibiotic-associated diarrhea. In the study of
Orrhage et al
. antibiotic treatment of patients induced a marked decrease
in the anaerobic microflora, mainly with a loss of bifidobacteria and an overgrowth
in enterococci. Oligofructose administration (as synbiotic) in those patients
restored their numbers of lactobacilli and bifidobacteria (34). Also, in patients
with Clostridium difficile
-associated diarrhea, which frequently occurs
after antibiotic-therapy, oligofructose suppressed colonization with C. difficile
and increased bifidobacteria levels. These changes were accompanied with a lower
relapse of diarrhea and reduced length of hospital stay (35).
Chronic inflammatory bowel diseases such as ulcerative colitis, Crohn's disease
and pouchitis are though to have their etiology to some extent linked to the
composition of the colonic microbial community and its activities. Although
members of the gut microbiota normally do not induce disease, in genetically
susceptible hosts, an altered immune response towards normal commensal organisms
is estimated to drive the inflammatory process towards a state of chronic inflammation
(36). The effect of inulin-type fructans in modulating the disease process has
been repeatedly demonstrated in experimental models in which inflammation was
induced by chemical agents such as DSS (37) or TNBS (38). In each of these,
administration of inulin-type fructans (alone or as symbiotic) to the diets
of animals reduced the inflammatory process (e.g.
), improved clinical and histological markers
with a reduction in corresponding lesions. The HLA-B27 transgenic (TG) rat is
a well-characterised model of chronic intestinal inflammation. The model spontaneously
develops colitis. Oral administration of oligofructose-enriched inulin to HLA-B27
TG rats decreased gross cecal and inflammatory histological scores in the caecum
and colon and altered mucosal cytokine profiles (decreased IL-1ß and increased
TGF-ß levels). Cytokine responses of mesenteric lymph node (MLN) cells
were also studied in vitro
by their response to cecal bacterial lysates
(CBL). Stimulation of MLN cells by CBL from oligofructose-enriched inulin-treated
TG rats induced a lower interferon-
In humans suffering from ulcerative colitis, it has been described that bifidobacteria
populations are about 30-fold lower compared to that in healthy individuals.
This let to the hypothesis that restoring bifidobacteria populations in these
patients by the use of pre- or synbiotics may influence the disease process.
Supplementation of the diet of patients with ulcerative colitis with oligofructose-enriched
inulin together with a probiotic (Bifidobacterium longum
) for 1 month
resulted in a 42-fold increase in bifidobacteria numbers in mucosal biopsies.
Clinical intervention study in ulcerative patients supplemented with the same
synbiotic as indicated above; showed improvement of the clinical appearance
of chronic inflammation, evidenced by a reduction in sigmoidoscopy scores, reduction
in acute inflammatory activity (TNF-
) and regeneration
of the epithelial tissue (40). In another placebo-controlled clinical trial
in patients with ulcerative colitis, oligofructose-enriched inulin lowered the
levels of calprotectin in the faeces thereby improving the patients' response
to therapy by mitigating intestinal inflammation (41). A reduction of the inflammation
and associated factors was observed also in patients with an ileal pouch-anal
anastomosis after therapy with inulin-type fructans (42). Moreover, in patients
with active ileo-colonic Crohn's disease, dietary intervention with a combination
of inulin and oligofructose has been shown to lead towards an improvement of
the disease activity (reduction in Harvey Bradshaw Index) and enhanced lamina
propria denritic cell IL-10 production and TLR2 and TLR4 expression. Strikingly
different changes in mucosa microbiota following inulin supplementation were
observed between patients who entered remission and those that did not. Patients
who entered remission had an increase in mucosal levels of bifidobacteria (43).
MICROBIOTA AND COLONIC CANCER
Diet has a strong influence on the etiology of colorectal cancers and intestinal
bacterial metabolism can generate substances derived from food with genotoxic,
carcinogenic, and tumour-promoting potential. Administration of weanling rats
with different types of inulin-type fructans induced a reduction in the number
of aberrant crypt foci (ACF) in the proximal, distal and total colon. ACF are
pre-neoplastic lesions found in the etiology of most colon cancers. Such reductions
in the distal parts of the colon (and the whole colon) were most pronounced
when rats were fed oligofructose-enriched inulin and resulted in the lowest
numbers of colonic ACF (44). Long-term studies with probiotics, prebiotics and
synbiotics in rats with AOM-induced colon cancer showed a reduction in the number
of colon carcinomas when supplemented with oligofructose-enriched inulin either
alone or given as a synbiotic (with Lactobacillus rhamnosus
GG and bifidobacterium
Bb12) (45). Treatment with the carcinogen AOM suppressed the rats'
natural killer (NK-) cytotoxicity in the Peyer's patches (PP). NK cells are
involved in both the recognition and subsequent elimination of tumour cells.
Suppression of this NK-cell activity may subsequently contribute to tumour growth.
Interestingly, the changes in tumour formation upon the intervention coincided
with a stimulation of immune functions within the gut-associated lymphoid tissue
(GALT) and PP which are the primary lymphoid tissues responsive upon oral intake
of prebiotics or synbiotics. The supplementation with oligofructose-enriched
inulin (alone or as a synbiotic) prevented such carcinogen-induced NK-cell suppression
in PP. After 33 weeks of treatment, immunological investigation of the rat's
PP revealed significant higher NK cell-like activity after intake of the pre-
or synbiotic. Other immunological markers in PP cells that differed upon both
interventions were the stimulation in IL-10 production. This increase in IL-10
cytokine production in PP was also found in a previous study of the same authors
after short-term exposure of AOM-rats to prebiotics, probiotics and synbiotics
A phase-II anticancer study, randomised, double-blind and placebo-controlled
in 80 patients with a history of colon cancer or polyps, and supplemented with
a synbiotic (oligofructose-enriched inulin and Bifidobacterium lactis
Bb12 and Lactobacillus rhamnosus
GG) for 12 weeks, showed increased levels
of bifidobacteria and lactobacilli. This was accompanied by a decrease in the
numbers of pathogens (coliforms and Clostridium perfringens
). The altered
composition of the colonic bacterial ecosystem beneficially affected the metabolic
activity in this organ. This was obvious from the decreased DNA damage in the
colonic mucosa (measured by the comet assay) and the tendency to lower the level
of colorectal proliferation (surrogate biomarker for colon cancer risk) in polyp
patients (no measures were taken in cancer patients). Other effects were the
decreased cytotoxicity of the faecal water. The fecal water of synbiotic-fed
polyp patients also showed a lower level of cell necrosis as demonstrated by
the lower cytotoxic potential in (HCT116 cell types). This indicates that the
synbiotic effectively prevented cell death of the colonic epithelium (26 - SYNCAN
project, Synbiotics and Cancer Prevention started, QLK-1999-00346).
INTESTINAL MICROBIOTA, ADIPOSE
TISSUE AND INFLAMMATION
Obesity and metabolic disorders (insulin resistance, hyperlipaemia) are tightly linked to a chronic low-grade state of inflammation (elevated levels of circulating inflammatory markers such as IL-6, and C-reactive protein). It is hypothesized that an altered gut microbiota in the obese state could contribute towards low grade inflammation resulting in the development of metabolic diseases associated with the condition (e.g.
diabetes, cardiovascular disease, etc.
) (46). However, the factors triggering such metabolic alterations remain to be determined.
In the obese lower levels of Bacteroidetes and higher levels of the phylum Firmicutes
in the colonic microbiota as compared to lean counterparts are found (47). These
observations have been associated with increased gut fermentation and calorific
bioavailability to the host. Moreover, feeding high fat diets have been demonstrated
to alter dramatically the microbiota composition in mice with reducing the quantities
of dominant Gram-positive groups, e.g. Bifidobacterium spp
- C. coccoides
groups, and the murine Gram-negative
MB (48). Recent studies in animal models have shown
that such changes within the microbial ecology or functional activities of the
gut microbiota can induce a metabolic shift towards a pro-inflammatory phenotype,
whole-body, liver and adipose tissue weight gain and impaired glucose metabolism.
Factors of microbial origins (e.g.
bacterial lipopolysaccharides) are
hypothesized to lie at the basis of such effects. In mice, high-fat feeding
let to (low level of) metabolic endotoxemia, low inflammatory tone, increasing
macrophage infiltration in adipose tissue and dysregulating lipid and glucose
metabolism. Multiple correlation analyses showed that the level of endotoxaemia
was negatively correlated with Bifidobacterium spp
., but no relationship
was seen between any other bacterial groups. On the other hand, restoration
of the levels of bifidobacteria in the intestine of mice upon oligofructose
supplementation lowered endotoxaemia and the level of microbial toxins and improved
mucosal barrier function. Interestingly, the lower body weight and visceral
adipose tissue mass in the oligofructose group (compared with the not supplemented
high-fat fed mice) showed a positive correlation with the endotoxin plasma levels
and negatively with the levels of bifidobacteria. Moreover, levels of mRNA of
plasminogen activator inhibitor type-1 (Pai-1, or Serpine-1) in adipose tissue
were increased in high-fat fed mice, whereas the levels were blunted with oligofructose
feeding. In addition, a normalisation of IL-1
and IL-6 cytokines was observed upon oligofructose feeding. These data indicate
that a lower fat mass and body weight 'only' are not a prerequisite for a lower
inflammatory tone and that this effect is accompanied by prebiotic changes in
the microbiota. Plasma cytokines were positively correlated with plasma endotoxin
levels and negatively with bifidobacteria levels (48). In diabetic mice, feeding
oligofructose reduced hepatic levels of phosphorylate IKK-ß and NF
suggestive of a reduction in the hepatic inflammatory status which might relate
to an improvement of the insulin sensitivity (49).
MICROBIOTA AND INTESTINAL METABOLISATION
Polyphenol aglycones and a few glucosides (e.g.
can be absorbed in the intestine, but the efficiency of polyphenol absorption
is generally low and differs widely depending on the type and structure of the
polyphenol. An extensive review comparing bioavailability and bioefficacy of
polyphenolic compounds showed that polyphenols which have high absorption (after
intake of 50 mg dose) are gallic acid (Cmax
4 ÁM), followed by isoflavones glycosides (daidzin, genistin) (Cmax
2 ÁM), flavanones and quercetin glucosides. Proanthocyanidins and anthocyanidins
are poorly absorbed (Cmax
= 0.02 ÁM) (8). Oral
administrations of chlorogenic and caffeic acid supplements, found that these
phenolic acids are absorbed for about 33 and 95 %, respectively. However, cholorogenic
acid accounts for 0.3% in urine and caffeic acid was found for 11% in urine.
Thus after absorption chlororgenic and caffeic acid are metabolised extensively
in other compounds (50).
Non absorbed polyphenols reach the colon. In the colon, the microbiota (e.g.
, Bifidobacterium sp
., Lactobacillus sp
., Eubacterium sp
.) hydrolyses the glycosides to
aglycones, which can further be metabolised to aromatic acids like phenylacetic,
phenylpropionic, phenylvaleric and benzoic acid. Those phenolic acids are well
absorbed through the colonic epithelium (2-6). With respect to the bioavailability
of dietary polyphenols and their colonic metabolites, more research is currently
needed in order to clarify the contribution of these different metabolites to
The importance of the colonic metabolisation has already been demonstrated for
some polyphenolic compounds in different studies. First, for the hydroxycinnamic
acids, which are naturally esterified in plant products, metabolisation is carried
out by the gut microflora (2, 3, 51). Bacterial species like Escherichia
, Bifidobacterium lactis
and Lactobacillus gasseri
cinnamoyl esterase activity and are responsible for the cleavage of the ester
bond between caffeic and quinic acid in chlorogenic acid (51). Secondly, regarding
the flavonoid group, the microbiota enzymes from Bacteroides distasonis
and B. ovatus
are important (e.g.
hydrolyse rutinoside to quercetin). Enterococcus casseliflavus
metabolise quercetin-3-O-glucoside to form formate, acetate, lactate,
the aglycone quercetin, butyrate, ethanol and 3,4-dihydroxyphenylacetic acid.
Strains belonging to the Clostridium
genera are also mentioned to cleave the C-ring of quercetin resulting in
3,4-dihydroxyphenylacetic acid and protocatechuic acid (2). Eubacterium ramulus
has also an impact on naringenin, apigenin and the isoflavone genistin (7, 9).
In another study, the role of gut microflora in the absorption and metabolism
of isoflavones and lignans was investigated using germ-free rats and rats associated
with human faecal bacteria. Soy and soy products contain the isoflavones genistein
and daidzein usually in the form of glycosides (genistin and daidzin). Germ-free
rats fed soy-isoflavone only excrete the aglycones daidzein and genistein. Hydrolysation
of the isoflavone glycosides occurs in the proximal intestinal tract. In contrast,
the metabolites equol, O-desmethylangolensin and the lignan enterolactone were
only detectable in the urine of human flora associated (HFA) rats. This demonstrates
the importance of the gut microbiota in the metabolisation of isoflavones and
lignans. The colonization of germ-free rats with faecal flora from human subjects,
capable to convert daidzein to equol, results in the excretion of the metabolites.
In the urine of HFA rats associated with a faecal flora from a low-equol producing
subject no detectable equol quantities were found. This indicates that some
subjects are unable to produce equol due to the lack of specific components
of gut microbiota (52).
Apart from inter-individual variation in daily intake of polyphenols, inter-individual differences in the composition of the human microbiota may lead to differences in bioavailability and bioefficacy of polyphenols and their metabolites. Research is needed to understand the role of the colonic microflora in the metabolisation of polyphenols and to evaluate the biological effects, including the anti-oxidative effects of these microbial metabolites.
In this respect, dietary strategies that modulate the composition of the microbiota enhancing metabolisation of polyphenols are hypothesized to improve bioavailability of polyphenols and could potentate their activity. In ovariectomized rats, feeding simultaneously soy isoflavones and fructo-oligosaccharides increased plasma levels of genistein, daidzein, and equol compared to isoflavone feeding alone. This effect also maximised the protective effects of isoflavones agains gonadal induced osteopenia (53). Inulin-type fructans have also been shown to increase plasma and urinary concentrations of soy-derived genistein and daidzein and their aglycone forms in humans. In post-menopausal women who were asked to consume a conjugated form of soybean isoflavones together with inulin it was found that 24 hr plasma levels (measured as the area under the curve) were resp. 38% for daidzein and 91% for genistein higher when compared to the isoflavone intake alone (54).
OUTLOOK AND PERSPECTIVES
The number of publications about food-based strategies to modulate the composition of the microbiota and their associated health effects has increased steadily over the last decade. This is expecting to continue since the importance of a well balanced colonic microbiota and its activities, as being a key factor in the modulation of human immunity, anti-oxidant defence, metabolism and endocrine activities, is more and more recognized. As new insights are being elucidated about the composition of the microbiota and its species diversity, the metabolic pathways of substrate degradation and the role in health and disease, interest will continue to rise. Together with this, it is of paramount importance to develop strategies to modulate this microbiota in a way to reduce the risk of developing disease through dietary means and the use of functional foods offers great value in this regard.
Conflict of interests: None declared.
- Gibson GR, Roberfroid MB. Dietary modulation of the human colonic microbiota - introducing the concept of prebiotics. J Nutr 1995; 125: 1401-1412.
- Manach C, Scalbert A, Morand C, Remesy C, Jimenez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004; 79: 727-747.
- D'Archivio M, Filesi C, Di Benedetto R, Gargiulo R, Giovannini C, Masella R. Polyphenols, dietary sources and bioavailability. Ann Ist Super Sanita 2007; 43: 348-361.
- Scalbert A, Morand C, Manach C, Remesy C. Absorption and metabolism of polyphenols in the gut and impact on health. Biomed Pharmacother 2002; 56: 276-282.
- Kroon AP, Clifford NM, Crozier A, et al. How should we assess the effects of exposure to dietary polyphenols in vitro? Am J Clin Nutr 2004; 80: 15-21.
- Manach C, Donovan LJ. Pharmacokinetics and metabolism of dietary flavonoids in humans. Free Rad Res 2004; 38: 771-785.
- Blaut M, Scoefer L, Braune A. Transformation of flavonoids by intestinal microorgansims. Int J Vitam Nutr Res 2003; 73: 79-87.
- Manach C, Williamson G, Morand C, Scalbert A, Remesy C. Bioavailability and bioefficacy of polyphenols in humans. I. Review of 97 bioavailability studies. Am J Clin Nutr 2005; 81: 230S-242S.
- Scalbert A, Williamson G. Dietary intake and bioavailability of polyphenols. J Nutr 2000; 130: 2073S- 2085S.
- Ellegard L, Andersson H, Bosaeus I. Inulin and oligofructose do not influence the absorption of cholesterol, or the excretion of cholesterol, Ca, Mg, Zn, Fe, or bile acids but increases energy excretion in ileostomy subjects. Eur J Clin Nutr 1997; 51: 1-5.
- Gibson GR, Beatty ER, Cummings J. Selective fermentation of bifidobacteria in the human colon by oligofructose and inulin. Gastroenterology 1995; 108: 975-982.
- Kleessen B, Sykura B, Zunft H-J, Blaut M. Effects of inulin and lactose on fecal microbiota, microbial activity, and bowel habit in elderly constipated persons. Am J Clin Nutr 1997; 65: 1397-1402.
- Den Hond E, Geypens B, Ghoos Y. Effect of high performance chicory inulin on constipation. Nutr Res 2000; 20: 731-736.
- Paineau D, Payen F, Panserieu S, et al. The effects of regular consumption of short-chain fructo-oligosaccharides on digestive comport of subjects with minor functional bowel disorders. Brit J Nutr 2008; 13: 311-318.
- Zunft H-J, Hanisch C, Mueller S, Koebnick C, Blaut M, Dore J. Synbiotic containing Bifidobacterium animalis and inulin increases stool frequency in elderly healthy people. Asia Pac J Clin Nutr 2004; 13: S112.
- Waligora-Dupriet A-J, Campeotto F, Nicolis I, et al. Effect of oligofructose supplementation on gut microflora and well-being in young children attending a day care centre. Int J Food Microbiol 2007; 113: 108-113.
- Rao A. The prebiotic properties of oligofructose at low intake levels. Nutr Res 2001; 21: 843-848.
- Tuohy KM. A human volunteer study on the prebiotic effects of HP-inulin-faecal bacteria enumerated using fluorescent in situ hybridisation (FISH). Anaerobe 2001; 7: 113-118.
- Langlands SJ, Hopkins MJ, Coleman N, Cummings JH. Prebiotic carbohydrates modify the mucosa associated microflora of the human large bowel. Gut 2004; 53: 1610-1616.
- Bartosch S, Woodmansey EJ, Paterson JCM, McMurdo ET, Macfarlane GT. Microbiological effects of consuming a synbiotic containing Bifidobacterium bifidum, Bifidobacterium lactis, and oligofructose in elderly persons, determined by real-time polymerase chain reaction and counting of viable bacteria. Clin Infect Dis 2005; 40: 28-37.
- Lesniewska V, Rowland I, Laerke HN, Grant G, Naughton PJ. Relationship between dietary-induced changes in intestinal commensal microflora and duodenojejunal myoelectric activity monitored by radiotelemetry in the rat in vivo. Exp Physiol 2006; 91: 229-237.
- Osman N, Adawi D, Molin G, Ahrne S, Berggren A, Jeppsson B. Bifidobacterium infantis strain with and without a combination of oligofructose-enriched inulin (OFI) attenuate inflammation in DSS-induced colitis in rats. BMC Gastroenterology 2006; 6: 31-35.
- Buddington KK, Donahoo JB, Buddington RK. Dietary oligofructose and inulin protect mice from enteric and systemic pathogens and tumor inducers. J Nutr 2002; 132: 472-477.
- Ten Bruggencate SJ, Bovee-Oudenhoven IM, Lettink-Wissink ML, Katan MB, Van der Meer R. Dietary fructo-oligosaccharides and inulin decrease resistance of rats to Salmonella: protective role of calcium. Gut 2004; 53: 530-535.
- Scholtens PAM, Alles MS, Willemsen LEM, et al. Dietary fructo-oligosaccharides in healthy adults do not negatively affect faecal cytotoxicity: a randomised, double-blind, placebo-controlled crossover trial. Brit J Nutr 2006; 95: 1143-1149.
- Rafter J, Bennett M, Caderni G, et al. Dietary synbiotics reduce cancer risk factors in polypectomized and colon cancer patients. Am J Clin Nutr 2007; 85: 488-496.
- Falony G, Vlachou A, Verbrugge K, De Vuyst L. Cross-feeding between Bifidobacterium longum BB536 and acetate-converting, butyrate-producing colon bacteria during growth on oligofructose. Appl Environ Microbiol 2006; 72: 7835-7841.
- Belenguer A, Duncan SH, Calder AG, et al. Two routes of metabolic cross-feeding between Bifidobacterium adolescentis and butyrate-producing anaerobes from the human gut. Appl Environ Microbiol 2006; 72: 3593-3599.
- Duncan SH, Holtrop G, Lobley GE, Calder AG, Stewart CS, Flint HJ. Contribution of acetate to butyrate formation by human faecal bacteria. Brit J Nutr 2004; 91: 915-923.
- Butel M, Roland N. Clostridia pathogenicity in experimental necrotising enterocolitis in gnotobiotic quails and protective role of bifidobacteria. J Med Microbiol 1998; 47: 391-399.
- Butel M, Catala I, Waligora-Dupriet A, et al. Protective effect of dietary oligofructose against cecitis induced by clostridia in gnotobiotic quails. Microbial Ecol Health Dis 2001; 13: 166-172.
- Bomba A, Nemcova R, Gancarcikova S, Herich R, Guba P, Mudronova D. Improvement of the prebiotic effect of micro-organisms by their combination with matlodextrins, fructo-oliogsaccharides and polyunsaturated fatty acids. Brit J Nutr 2002; 88: S95-S99.
- Oli MW, Petschow BW, Buddington RK. Evaluation of fructooligosaccharide supplementation of oral electrolyte solutions for treatment of diarrhea: recovery of the intestinal bacteria. Dig Dis Sci 1998; 43: 138-147.
- Orrhage K, Sjostedt S, Nord CE. Effects of supplements with lactic acid bacteria. J Antimicrob Chemother 2000; 46: 603-611.
- Lewis S, Burmeister S, Brazier J. Effect of the prebiotic oligofructose on relapse of Clostridium difficile-associated diarrhea: a randomized, controlled study. Clin Gastroenterol Hepatol 2005; 3: 442-448.
- Macfarlane S, Furrie E, Kennedy A, Cummings JH, Macfarlane GT. Mucosal bacteria in ulcerative colitis. Brit J Nutr 2005: 93: S67-S72.
- Videla S, Vilaseca J, Antolin M. Dietary inulin improves distal colitis induced by dextran sodium sulfate in the rat. Am J Gastroenterol 2001; 96: 1486-1493.
- Cherbut C, Michel C, Lecannu G. The prebiotic characteristics of fructooligosaccharides are necessary for reduction of TNBS-induced colitis in rats. J Nutr 2003; 133: 21-27.
- Hoentjen F, Welling GW, Harmsen HJM, et al. Reduction of colitis by prebiotics in HLA-B27 transgenic rats is associated with microflora changes and immunomodulation. Inflamm Bowel Dis 2005; 11: 977-985.
- Furrie E, Macfarlane S, Kennedy A. Synbiotic therapy (Bifidobacterium longum/Synergy1) initiates resolution of inflammation in patients with active ulcerative colitis: a randomized controlled pilot trial. Gut 2005; 54: 242-249.
- Casellas F, Borruel N, Torrejon A. Oral oligofructose-enriched inulin supplementation in acute colitis is well tolerated and associated with lower faecal calprotectin. Aliment Pharmacol Ther 2007; 25: 1061-1067.
- Welters CF, Heineman E, Thunnissen FB, van den Bogaard AE, Soeters PB, Baeten CG. Effect of dietary inulin supplementation on inflammation of pouch mucosa in patients with an ileal pouch-anal anastomosis. Dis Colon Rectum 2002; 45: 621-627.
- Lindsay JO, Whelan K, Stagg AJ, et al. Clinical, microbiological and immunological effects of fructo-oligosaccharides in aptients with Crohn's disease. Gut 2006; 55: 348-355.
- Verghese M, Walker LT, Shackelford L, Chawan CB. Inhibitory effects of nondigestible carbohydrates of different chain lengths on azoxymethane-induced aberrant crypt foci in Fisher 344 rats. Nutr Res 2005; 25: 859-868.
- Roller M, Femia AP, Caderni G, Rechkemmer G, Watzl B. Intestinal immunity of rats with colon cancer is modulated by oligofructose-enriched inulin combined with Lactobacillus rhamnosus and Bifidobacterium lactis. Brit J Nutr 2004; 92: 931-938.
- Cani PD, Amar J, Iglesias MA, et al. Metabolic endotoxemia initiates obesity and inulin resistance. Diabetes 2007; 56: 1716-1772.
- Ley ER, Turnbaugh JP, Klein S, Gordon IJ. Human gut microbes associated with obesity. Nature 2006; 444: 1022-1023.
- Cani PD, Neyrinck AM, Fava F, et al. Selective increases of bifidobacteria in gut microflora improve high-fat-induced diabetes in mice through a mechanism associated with endotoxaemia. Diabetologica 2007; 50: 2374-2383.
- Cani P, Daubioul C, Reusens B, Remacle C, Catillon G, Delzenne N. Involvement of endogeneous glucagon-like peptide-1(7-36) amide on glycemia-lowering effect of oligofructose in streptozotocin-treated rats. J Endocrinol 2005; 185: 457-465.
- Olthof MR, Hollman PCH, Katan MB. Chlorogenic acid and caffeic acid are absorbed in humans. J Nutr 2001; 131: 66-71.
- Rechner RA, Smith AM, Kuhnle G, et al Colonic metabolism of dietary polyphenols: influence of structure on microbial fermentation products. Free Radic Biol Med 2004; 36: 212-225.
- Bowly E, Adlercreutz H, Rowland I. Metabolism of isoflavones and lignans by the gut microflora: a study in germ-free and human flora associated rats. Food Chem Toxicol 2003; 41: 631-636.
- Mathey J, Puel C, Kati-Coulibaly S, et al. Fructo-oligosaccharides maximize bone-spearing effects of soy isoflavone-enriched diet in ovariectomized rat. Calcif Tissue Int 2004; 75: 169-179.
- Piazza C, Giovanna M, Melilli B, et al. Influence of inulin on plasma isoflavone concentrations in healthy postmenopausal women. Am J Clin Nutr 2007; 86: 775-780.