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The biological properties of dietary polyphenols are greatly dependent on their bioavailability that, in turn, is largely influenced by their degree of polymerization. The gut microbiota play a key role in modulating the production, bioavailability and, thus, the biological activities of phenolic metabolites, particularly after the intake of food containing high-molecular-weight polyphenols. In addition, evidence is emerging on the activity of dietary polyphenols on the modulation of the colonic microbial population composition or activity. However, although the great range of health-promoting activities of dietary polyphenols has been widely investigated, their effect on the modulation of the gut ecology and the two-way relationship "polyphenols ↔ microbiota" are still poorly understood. Only a few studies have examined the impact of dietary polyphenols on the human gut microbiota, and most were focused on single polyphenol molecules and selected bacterial populations. This review focuses on the reciprocal interactions between the gut microbiota and polyphenols, the mechanisms of action and the consequences of these interactions on human health.
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REVIEWS: CURRENT TOPICS
Benefits of polyphenols on gut microbiota and implications in human health
Fernando Cardona
a,b,
, Cristina Andrés-Lacueva
c,d
, Sara Tulipani
a
, Francisco J. Tinahones
b,e,
,
María Isabel Queipo-Ortuño
a,b
a
Laboratorio de Investigaciones Biomédicas del Hospital Virgen de la Victoria (FIMABIS), Málaga, Spain
b
CIBER de Fisiopatología de la Obesidad y Nutrición (CIBEROBN), Instituto de Salud Carlos III, Spain
c
Department of Nutrition and Food Science, XaRTA, INSA, Faculty of Pharmacy, University of Barcelona, Barcelona, Spain
d
INGENIO-CONSOLIDER Program, Fun-c-food CSD2007-06, Barcelona, Spain
e
Servicio Endocrinología y Nutrición del Hospital Virgen de la Victoria, Málaga, Spain
Received 18 January 2013; received in revised form 6 May 2013; accepted 24 May 2013
Abstract
The biological properties of dietary polyphenols are greatly dependent on their bioavailability that, in turn, is largely influenced by their degreeof
polymerization. The gut microbiota play a key role in modulating the production, bioavailability and, thus, the biological activities of phenolic metabolites,
particularly after the intake of food containing high-molecular-weight polyphenols. In addition, evidence is emerging on the activity of dietary polyphenols on
the modulation of the colonic microbial population composition or activity. However, although the great range of health-promoting activities of dietary
polyphenols has been widely investigated, their effect on the modulation of the gut ecology and the two-way relationship polyphenols microbiota are still
poorly understood.
Only a few studies have examined the impact of dietary polyphenols on the human gut microbiota, and most were focused on single polyphenol molecules
and selected bacterial populations. This review focuses on the reciprocal interactions between the gut microbiota and polyphenols, the mechanisms of action and
the consequences of these interactions on human health.
© 2013 Elsevier Inc. All rights reserved.
Keywords: Dietary polyphenols; Gut microbiota; Polyphenol bioavailability; Human health; Cancer; Immunity
1. Introduction
Dietary polyphenols are natural compounds occurring in plants,
including foods such as fruits, vegetables, cereals, tea, coffee and
wine [1]. Chemically, polyphenols are a large heterogeneous group of
compounds characterized by hydroxylated phenyl moieties. Based
on their chemical structure and complexity (i.e., the number of
phenolic rings and substituting groups), polyphenols are generally
classified into flavonoids and nonflavonoids [2]. Flavonoids form a
major (over 9000 structurally distinct flavonoids have b een
identified in nature) heterogeneous subgroup comprising a variety
of phenolic compounds with a common diphenylpropane skeleton
(C6-C3- C6). In turn, flavonoids are also classified into further
subclasses according to their structural differences (flavanones,
flavones, dihydroflavonols, flavonols, flavan-3-ols or flavano ls,
anthocyanidins, isoflavones and proanthocyanidins) [3,4]. In planta,
most p olyphenols occur i n t heir glycosylated forms, although
modifications such as esterification or polymerizati on are also
commonly found. Once ingested, polyphenols are recognized by
the human body as xenobiotics, and their bioavailability is therefore
relatively low in comparison to micro and macronutrients. Further-
more, depending on their degree of structural complexity and
polymerization, these compounds may be readily absorbed in the
small intestine (i.e., low-molecular-weight polyphenols such as
monomeric and dimeric structures) [5] or reach the colon almost
unchanged (oligomeric and polymeric polyphenols such as con-
densed or hydrolysable tannins, reaching molecular weight values
close to 40,000 Da) [610]. It has been estimated that only 510% of
the total polyphenol intake is absorbed in the small intestine. The
remaining polyphenols (9095% of total polyphenol intake) may
accumulate in the large intestinal lumen up to the millimolar range
where, together with conjugates excreted into the intestinal lumen
through the bile, they are subjected to the enzymatic activities of the
gut microbia l community [1126]. The colonic microbiota are
therefore responsible for the extensive breakdown of the original
polyphenolic st ructures into a series of low-molecular-weight
phenolic met abolites that, being absorbable, may actually be
Available online at www.sciencedirect.com
Journal of Nutritional Biochemistry 24 (2013) 1415 1422
Corresponding authors. F.C. Díaz is to be contacted at: Laboratorio de
Investigaciones Biomédicas del Complejo Hospitalario de Málaga (FIMABIS),
Campus de Teatinos s/n 29010 Málaga, Spain. Tel.: +34 951032647; fax: +34
951924651. F.J. Tinahones, Servicio Endocrinología y Nutrición, Complejo
Hospitalario de Málaga. Campus de Teatinos s/n 29010 Málaga, Spain. Tel.:
+34 951032734; fax: +34 951924651.
E-mail addresses: fernandocardonadiaz@gmail.com (F. Cardona),
fjtinahones@hotmail.com (F.J. Tinahones).
0955-2863/$ - see front matter © 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jnutbio.2013.05.001
responsible for the health effects derived from polyphenol-rich food
consumption, rather than the original compounds found in foods.
Currently, it is estimated that 5001000 different microbial
species inha bit the gastroi nte sti nal tract, rea chi n g the highest
concentrations in the colon (up to 10
12
cells per gram of faeces).
However, only a few bacterial species (e.g. Escherichia coli, Bifidobac-
terium sp., Lactobacillus sp., Bacteroides sp., Eubacterium sp.) catalyz-
ing the metabolism of phenolics have been identified so far, together
with the catabolic pathways implicated [26]. However, they do not
seem to be ubiquitous but reflect the interpersonal differences in the
gut microbial community.
Consequently, apart from the interindividual variation in daily
intake of polyphenols, interindividual differences in the composition
of the gut microbiota may lead to differences in bioavailability and
bioefficacy of polyphenols and their metabolites [27,28]. The scenario
appears even more complex when considering the two-way rela-
tionship polyphenols microbiota. Recent studies have in fact
suggested that both the phenolic substrates supplied to the gut
bacteria through different patterns of dietary intake and the aromatic
metabolites produced may in turn modulate and cause fluctuations in
the composition of the microflora populations through selective
prebiotic effects and antimicrobial activities against gut pathogenic
bacteria [2938]. The formation of bioactive polyphenol-derived
metabolites and the modulation of colonic microbiota may both
contribute to host health benefits, although the mechanisms have not
been delineated. The health properties attributed to beneficial
bacteria for human hosts include protection against gastrointestinal
disorders and pathogens, nutrient processing, reduction of serum
cholesterol, reinforcement of intestinal epithelial cell-tight junctions
and increased mucus secretion and modulation of the intestinal
immune response through cytokine stimulus [3941]. Likewise, in
the last decade, a growing body of in vivo interventional and
epidemiological studies has furnished new evidence on the wide
range of health promoting activities of dietary polyphenols, already
documented by in vitro data, including their antiinflammatory,
antioxidant, anticarcinogenic, antiadipogenic, antidiabetic and neu-
roprotective potentials, suggesting an association between the
consumption of polyphenol-rich foods and a reduced risk of several
chronic diseases [4248]. However, the effect of dietary polyphenols
on the modulation of the gut ecology, including the underlying
mechanisms and the actual benefits of such bioactive agents, is still
poorly understood.
The aim of this review is to provide an overview of recent reports
on the dual nature of polyphenolmicrobiota interactions and its
relevance to human health.
2. Polyphenols and their biotransformation in the gut
Fig. 1 schematically illustrates the metabolic fate of dietary
polyphenols in humans. Briefly, a small percentage of dietary
polyphenols (510% of the total intake, mainly those with monomeric
and dimeric structures) may be directly absorbed in the small
intestine, generally after deconjugation reactions such as deglycosy-
lation [7]. After absorption into the small intestine, these less complex
polyphenolic compounds may be subjected to extensive Phase I
(oxidation, reduction and hydrolysis) and particularly Phase II
(conjugation) biotransformations in the enterocytes and then the
hepatocytes, resulting in a series of water-soluble conjugate metab-
olites (methyl, glucuronide and sulfate derivatives) rapidly liberated
to the systemic circulation for further distribution to organs and
excretion in urine. In the large intestine, colonic bacteria are known to
act enzymatically on the polyphenolic backbone of the remaining
unabsorbed polyphenols (9095% of the total polyphenol intake),
sequentially producing metabolites with different ph ysiological
significance [49]. The metabolism of polyphenols by microbiota
involves the cleavage of glycosidic linkages and the breakdown of the
heterocyclic backbone [50]. As an example, the microbial catabolism
of proanthocyanidins (oligomers and polymers of flavan-3-ols) has
been extensively described in recent years. It results in the sequential
production of lactones and aromatic and phenolic acids with different
hydroxylation patterns and side-chain lengths, depending on the
precursor structures ( phenylvalerolactones, phenylvaleric acids,
phenylpropionic acids, phenylacetic acids, hippuric and benzoic
acids) [11,22]. The metabolism by gut microflora of these polyphenols
abundant in wine, tea, chocolate and many fruits may also influence
tissue exposure to high-molecular-weight polyphenols, including
proanthocyanidins or oxidized polymeric polyphenols, which are
poorly absorbed in the proximal part of the gastrointestinal tract [51].
In addition, the microbial transformation of nonflavonoid polymeric
molecules called ellagitannins (or hydrolysable tannins) has also been
investigated in the last decade [23,24]. After the consumption of
ellagitannin-rich food such as strawberries, raspberries, walnuts, oak-
aged wines and pomegranates, these tannin structures are subjected
to hydrolysis in the intestinal lumen, releasing free ellagic acid. Once
in the large intestine, ellagic acid is metabolized by human colonic
micro flora to produce a series of derivative compounds called
urolithins, characterized by a common 6H-dibenzo[b,d]pyran-6-one
nucleus and a decreasing number of phenolic hydroxyl groups
(urolithin DCAB). All these microbial-derived phenolic me-
tabolites may be absorbed or excreted by faeces. When absorbed, they
reach the liver through the portal vein where they may be further
subjected to extensive first-pass Phase II metabolism (including
glucuronidation, methylation, sulfation or a combination of these)
until they finally enter the systemic circulation and are distributed to
the organs or eliminated in urine. Microbial glucuronidase and
sulphatase activity may also deconjugate the Phase II metabolites
extruded via the bile throughout the enterohepatic circulation,
enabling their reuptake and effective bioavailability. Clostridium and
Eubacterium are the main genera involved in the metabolism of
many phenolics such as isoflavones (daidzein), flavonols (quercetin
and kaempferol), flavones (nar ingenin and ixoxanthumol) and
flavan-3-ols (catechin and epicatechin) [32].AsFirmicutes possess a
disproportionately smaller number of glycan-degrading enzymes
than Bacteroidetes [52], it might be hypothesized that intake of
different polyphenols could reshape the gut microbiota differently.
A major fraction of the polyphenols present in the plasma and
excreted in urine of rats fed with red wine polyphenols comprises
aromatic acid metabolites formed in the gut [53]. Incubating an
anthocyanin extract from Cabernet Sauvignon grapes with the
contents of the large intestine of pigs for 6 h results in a loss of the
parent compound but the generation of three identifiable metabolites
[54]. It is possible that these metabolites offer a protective effect
against colon cancer, such as decreased carcinogen-induced aberrant
crypt formation, colonic cell proliferation and oxidative DNA damage,
which have been attributed to anthocyanin consumption [55].
3. Effects of dietary polyphenols on modulation of
intestinal ecology
Previous human intervention trials have shown that apart from
interindividual variation in the daily intake of polyphenols, inter-
individual differences in the composition of the human microbiota
may lead to differences in bioavailability and bioefficacy of poly-
phenols and their metabolites [56,57]. In addition, polyphenols may
be converted by the colonic microbiota to bioactive compounds that
can affect the intestinal ecology and influence host health. There is
evidence from in vitro animal and human studies that certain doses of
selected polyphenols may modify the gut microbial composition, and
while certain bacterial groups can be inhibited, others can thrive in
the available niche of the ecosystem. Phenolic compounds alter gut
1416 F. Cardona et al. / Journal of Nutritional Biochemistry 24 (2013) 14151422
microbiota and, consequently, alter the Bacteroides/Firmicutes balance
[19,29,58]. For example, Tzounis et al.,inanin vitro study using a
batch-culture model reflective of the distal region of the human large
intestine, suggested that flavan-3-ol monomers such as ()epicatechin
and (+)catechin may be capable of influencing the large intestinal
bacterial population even in the presence of other nutrients, such as
carbohydrates and proteins. These authors found that (+)catechin
significantly inhibited growth of Clostridium histolyticum and enhanced
growth of E. coli and members of the Clostridium coccoidesEubacterium
rectale group, while growth of Bifidobacterium and Lactobacillus spp.
remained relatively unaffected [59].
Dietary administration of proanthocyanidin-rich extracts also
appears to have a similar effect. The faecal bacteria composition of
rats whose diet was supplemented for 16 weeks with a dealcoholized,
proanthocyanidin-rich red wine extract shifted from a predominance
of Bacteroides, Clostridium and Propionibacterium spp. to a predom-
inance of Bacteroides, Lactobacillus and Bifidobacterium spp. [60].
Yamakoshi et al. documented that a proanthocyanidin-rich extract
from grape seeds given to healthy adults for 2 weeks was able to
significantly increase the number of bifidobacteria [61]. Nevertheless,
recent studies indicate that monomeric flavan-3-ols and flavan-3-ol-
rich sources such as chocolate, green tea and blackcurrant or grape
seed extracts may modulate the intestinal microbiota in vi vo,
producing changes in beneficial bacteria such as Lactobacillus spp.
but inhibiting other groups such Clostridium spp. in both in vivo and
in vitro studies [30, 59,62, 63]. More recently, a cocoa dietary
intervention in a rat model showed a significant decrease in the
proportion of Bacteroides, Clostridium and Staphylococcus genera in
the faeces of cocoa-fed animals [64].
Other rat studies carried out by Smith et al. found that when rats
were gi ven a tanni n-rich diet, the Bacteroides group increased
significantly while the Clostridium leptum
cluster decreased signifi-
cantly [65]. Dolara et al. reported that, when rats were treated with
red-wine polyphenols, they had significantly lower levels of Clostri-
dium spp. and higher levels of Bacteroides, Bifidobacterium and Lac-
tobacillus spp. [60]. Similarly, the resveratrol commonly found in
grape promoted faecal cell counts of Bifidobacterium spp. and Lacto-
bacillus in a rat model [66].
A human intervention study indicated that consumption of red
wine polyphenols significantly increased the number of Enterococcus,
Prevotella, Bacteroides, Bifidobacterium, Bacte roides uni formis, Eg-
gerthella lenta,andBlautia coccoides-E. rectale group while the
quantity of Lactobacillus spp. was unaltered [31]. On the other hand,
when bacteria were cultured with various tea phenolics, the growth
of pathogenic bacteria such as Clostridium perfringens, Clostridium
difficile and Bacteroide s spp. was significantly repressed, while
commensal anaerobes like Bifidobacterium and Lactobacillus were
affected less [29]. Vendrame et al. found a significant increase in the
amount of Bifidobacterium after the consumption of a wild blueberry
drink, suggesting an important role of the polyphenol present in wild
blueberries on the intestinal microbiota composition modulation [67].
Cueva et al. analyzed the potential of flavan-3-ols from grape seed
to influence the growth of intestinal bacterial groups using in vitro
fermentation models. They found that the flavan-3-ol profile of a
particular food source could affect the microbi ota composition
(promoting the growth of Lactobacillus/Enterococcus and decreasing
the C. histolyticum group) and its catabolic activity, inducing changes
that could in turn affect the bioavailability and potential bioactivity of
these compounds [68].
Finally, important prebiotic effects and selective antimicrobial
activities against gut pathogenic bacteria have also been attributed to
the polyphenolic fraction contained in the skin covering the kernel of
several nuts, mostly composed of nonflavonoid tannin structures
(ellagitannins), flavan-3-ols and proanthocyanidins [3638].
4. Mechanisms of action of polyphenols on bacterial
cell membrane
The influence of polyphenols on bacterial growth and metabolism
depends on the polyphenol structure, the dosage assayed and the
microorganism strain [34]. For instance, Gram-negative bacteria are
more resistant to polyphenols than Gram-positive bacteria, possibly
due to the differences found in their wall composition [69]. Recent
findings suggest a variety of potential mechanisms of action of
polyphenols on bacterial cells. For example, polyphenols can bind to
bacterial cell membranes in a dose-dependent manner, thus
Fig. 1. Routes for dietary polyphenols and their metabolites in humans. Within the host, dietary polyphenols and their microbial metabolites successively undergo intestinal and liver
Phase I and II metabolism, biliary secretion, absorption in the systemic circulation, interaction with organs and excretion in the urine.
1417F. Cardona et al. / Journal of Nutritional Biochemistry 24 (2013) 14151422
disturbing membrane function and therefore inhibiting cell growth
[70]. Polyphenols, such as catechins, act on different bacterial species
(E . coli, Bordetella bronchiseptica, Serratia marcescens, Klebsiella
pneumonie, Salmonella choleraesis, Pseudomonas aeruginosa, Staphy-
lococcus aureus and Bacillus subtilis) by generating hydrogen peroxide
[71] and by altering the permeability of the microbial membrane [72].
Sirk et al. also reported that the mechanism of antimicrobia l,
anticancer and other beneficial health effects of catechins and
theaflavins may be governed by hydrogen bonding of their hydroxyl
groups to lipid bilayers of cell membranes. The molecular structure
and aggregated condition of the catechins significantly influences
their absorption, as well as their ability to form hydrogen bonds with
the lipid head groups. The molecular structure of the catechins and
theaflavins influences their configuration when binding to the bilayer
surface, as well as their ability to form hydrogen bonds with the lipid
head groups [73,74].
Another component of green tea, the ()-epicatechin gallate
(ECg), sensitize s methi cillin -resistant S. aureus to b eta- lactam
antibiotics, promotes staphylococcal cell aggregation and increases
cell-wall thickness. ECg-mediated alterations of the physical nature of
the bilayer can elicit structural changes to wall teichoic acid that
result in modulation of the cell-surface properties necessary to
maintain the beta-lactam-resistant phenotype [75].
Microbes stressed by exposure to polyphenols up-regulate pro-
teins related to defensive mechanisms, which protect cells while
simultaneously down-regulating various metabolic and biosynthetic
proteins involved, for example, in amino acid and protein synthesis as
well as phospholipid, carbon and energy metabolism [76]. Most
bacteria are able to regulate phenotypic characteristics, including
virulence factors, as a function of cell density under the control of
chemical signal molecules. Polyphenolic compounds can also inter-
fere with bacterial quorum sensing, which is achieved by producing,
releasing and detecting small signal molecules identified as auto-
inducers (acylated homoserine lactones in Gram-negative bacteria
and oligopeptides in Gram-positive bacteria) [77,78]. For example,
polyphenols have been reported to interfere with the production of
small signal molecules by bacterial cells of E. coli, Pseudomonas putida
and Burkholderia cepacia that trigger the exponential growth of a
bacterial population [79]. Studies performed with synthesized or
isolated Phase II-conjugated metabolites of flavan-3-ols have revealed
that they could have an effect beyond their antioxidant properties, by
interacting with signalling pathways implicated in important pro-
cesses involved in the development of diseases [10].
On the other hand, red wine and green tea polyphenols strongly
inhibit the VacA toxin, a major virulence factor of Helicobacter pylori
[80]. The inhibitory mechanisms of dietary polyphenols against H.
pylori may include suppression of urease activity, affecting bacterial
proliferation and damaging bacterial membranes, thus making cells
more sensitive to external compounds such as antibiotics and leading
to a disruption of proton motive force through the loss of H+
ATPase and membrane-associated functions [81].
Moreover, the B ring of the flavonoids may play a role in
intercalation or hydrogen bonding with the stacking of nucleic acid
bases, and this may explain the inhibitory action on DNA and RNA
synthesis [82]. Plaper et al. reported that quercetin binds to the GyrB
subunit of E. coli DNA gyrase and inhibits the enzyme's ATPase
activity [83]. In agreement with these earlier findings, more recently,
Gradisar et al. determined that the catechins inhibit bacterial DNA
gyrase by binding to the ATP (adenosine triphosphate) binding site of
the gyrase B subunit [84].
In both in vivo and in animal studies, the phenolic substances were
suggested to be responsible for the observed anticaries effect of cocoa
powder [85], possibly due to their inhibition of the synthesis of water-
insoluble glucans [86]. On the other hand, a rich source of flavonoids
such as onion extracts has been reported to act on Streptococcus
mutans and Streptococcus sobrinus as well as on Porphyromonas
gingivalis and Prevotella intermedia, which are considered to be the
main causal bacteria of adult periodontitis [87].
Another hypothesis leans toward the formation of polyphenol
metal ion complexes, which in turn would lead to iron deficiency in
the gut and could, therefore, affect sensitive bacterial populations,
mainly aerobic microorganisms [65]. Aerobic microorganisms need
iron for several functions, such as reduction of the ribonucleotide
precursor of DNA and to form heme groups. In contrast, it has been
demonstrated that dietary catechols may promote the growth of
enteropathogenic bacteria by providing iron under iron-restrictive
conditions and can enable gut bacterial growth [88]. Several
mechanisms of action of polyphenols on specific intestinal bacterial
functions are still unknown, and further research is needed for a
better understanding.
5. Polyphenols, microbiota and cancer
Several studies have linked the microbial metabolism of dietary
polyphenols to cancer prevention. These studies have found phylum-
level differences among the gut microbiota of patients with and
without colorectal cancer. Some phyla are increased, whereas others
are decreased, but exactly how these changes affect the cancer
process is not clear [89,90]. Studies done in vitro and in gnotobiotic
rats have shown that plant lignin secoisolariciresinol diglucoside can
be converted to enterodiol and enterolactone by a gut microbiota
consortia composed of Clostridium saccharogumia, Eggertella lenta,
Blautia producta and Lactonifactor longoviformis [91,92].Furthermore,
colonization with this lignin-metabolizing microbial community pro-
tected germ-free rats from 7,12-dimethylbenz(a)anthracene-induced
cancer. Moreover, colonization significantly decreased tumour number,
size and cell proliferation but increased tumour cell apoptosis [93].
Some polyphenol dietary components may also influence bacterial
metabolizing enzymes and thus influence the overall cancer risk. For
example, in a rat model, resveratrol supplementation (8-mg/kg body
weight/day, intragastrically) significantly reduced activities of faecal
and host colonic mucosal enzymes, such as β-glucoronidase, β-
glucosidase, β-galactosidase, mucinase and nitroreductase compared
to control animals (21%, 45%, 37%, 41% and 26%, respectively). The
reduced bacterial enzyme activity was associated with a significant
reduction in colonic tumour incidence in the resveratrol-fed rats
compared to control rats, but it is not clear if these changes were a
result of modifications of enzymatic activity within a subpopulation of
microorganisms or a change in the proportion of specific bacteria [94].
The stilbene resveratrol is important in relation with colon cancer. The
antiinflammatory activity of resveratrol includes inhibition of proinflam-
matory mediators, modification of eicosanoid synthesis and inhibition of
enzymes including COX-2, NF-κB, AP-1, TNF-α, IL6 and VEGF (vascular
endothelial growth factor) [95]. In cell culture, several phenolic
compounds inhibit COX-2 activity, possibly by binding to the enzyme [96].
Ellagic acid has been reported to show a multitude of biological
properties including antioxidant and cancer protective activities
[97,98]. Interestingly, both urolithins A and B, the most representative
microbial metabolites of dietary ellagitannins, have shown oestro-
genic activity in a dose-dependent manner, even at high concentra-
tions (40 microM), without antiproliferative or toxic effects towards
MCF-7 breast cancer cells [99]. Other authors have analyzed the
impact of selected intestinal polyphenol metabolites (with 3,4-
dihydroxyphenylacetic acid (ES) and 3-(3,4-dihydroxyphenyl)-pro-
pionic acid, metabolites of quercetin and chlorogenic acid/caffeic
acid) on modulation of enzymes involved in detoxification and
inflammation in LT97 human adenoma cells. They showed an up-
regulation of GSTT2 and a down-regulation of COX-2 that could
possibly contribute to the chemopreventive potential of polyphenols
after degradation in the gut [96]. Recently, Kang et al. reported that
1418 F. Cardona et al. / Journal of Nutritional Biochemistry 24 (2013) 14151422
coffee and caffeic acid specifically inhibited colon cancer metastasis
and neoplastic cell transformation in mice by inhibiting MEK1 and
TOPK (T-LAK celloriginated protein kinase) [100]. Several studies
using animal and cell culture models have shown that tea-derived
catechins, such as epigallocatechin-3-gallate, hold anticancer activity
and mediate various cellular events that could be protective against
cancer [101,102]. In addition, other nontea flavonoids such as
quercetin from apples and vegetables have been found to have
antic ancer effects, including inhibition of cell pr olifer ation and
induction of apoptosis [103]. Whether the concentration of these
compounds can be sufficiently achieved in human diets to affect these
pathways is not known. Based on these previous studies, multiple
mechanisms appear to be involved in the inhibition of carcinogenesis
by dietary polyphenols (Fig. 2).
6. Modulation of gut microbiota by polyphenols and the impact
on human gut health metabolism and immunity
In the following section, we summarize the effects of polyphenols
and metabolites from polyphenol microbial metabolism on specific
aspects of health and immunity. After a human intervention study,
Tzounis et al. reported that flavonols induced an increase in the
growth of Lactobacillus spp. and Bifidobacterium spp. and they may
have been partly responsible for the observed reductions in the
plasma C-reactive protein (CRP) concentrations, which are a blood
marker of inflammation and a hallmark of the acute phase response
[30]. Similarly, Fogliano et al.,inanin vitro model, found that the
bacterial fermentation of water-insoluble cocoa fractions was
associated with an increase in bifidobacteria and lactobacilli as well
as butyrate production. These microbial changes were associated with
significant reductions in plasma triacylglycerol and CRP, suggesting
the potential benefits associated with dietary inclusion of flavonol-
rich foods [104]. Recently, Queipo-Ortuño et al. [31] carried out a
human intervention study and found that the regular intake of red
wine polyphenols generated significant decreases in the plasma levels
of blood pressure, triglycerides and high-density lipoprotein choles-
terol, and these significant reductions may be partly due to the
polyphenol-induced increase in the growth of Bacteroides genera.
Moreover, they also reported a significant decrease in uric acid levels
after the consumption of red wine polyphenols that can be explained
by the significant increase in Proteobacteria observed in this stage,
which has previously been reported to degrade uric acid [105].
Finally, they noted a significant reduction in the concentration of CRP
after red wine treatment. This could be due to the increase seen in the
number of Bifidobacterium. CRP is a blood marker of inflammation,
and its concentration is a specific predictor of cardiovascular event
risk in healthy subjects. Its reduction in this study links polyphenol
intake to cardiovascular benefits in the host [106,107].
The weight-lowering property of fruits, green tea and vinegar
wine in obese people may be partly related to their polyphenol
content, which changes the gut microbiota either through the
glycan-degrading capability of Bacteroides, which is higher than
Firmicutes, or through the end products of colonic metabolism of
polyphenols [33].
Martin et al. performed a clinical trial in a population of human
subjects classified as having low or high anxiety traits using validated
psychological questionnaires. They found that the daily consumption
of dark chocolate (which is rich in flavonoids, mainly flavan-3-ols)
resulted in a significant modification in the metabolism in healthy and
free living human subjects, with potential long-term term health
consequences, as per variation of both host and gut microbial
metabolism. Human subjects with higher anxiety traits, however,
showed a distinct metabolic profile, indicative of a different energy
homeostasis (lactate, citrate, succinate, trans-aconitate, urea and
proline), hormonal metabolism (adrenaline, DOPA [dihidroxifenila-
lanina] and 3-methoxy-tyrosine) and gut microbial activity (methyl-
amines, p-cresol sulfate and hippurate) [108].
Monagas et al. observed that dihydroxylated phenolic acids (3,4-
dihydroxyphenylpropionic acid, 3-hydroxyphenylpropionic acid and
3,4-dihydroxyphenylacetic acid) derived from microbial metabolism
of proanthocyanidins presented marked in vitro antiinflammatory
properties, reducing the secretion of TNF- α, IL-1b and IL-6 in
Fig. 2. Possible mechanisms proposed for the prevention of cancer by dietary polyphenols.
1419F. Cardona et al. / Journal of Nutritional Biochemistry 24 (2013) 14151422
lipopolysaccharide-stimulated peripheral blood mononuclear cells
from healthy subjects. It has been suggested that these microbial
metabolites could be among the new generation of therapeutic agents
for the management of immunoinfl ammatory diseases such as
atherosclerosis [109], as well as for dampening the inflammatory
response to bacterial antigens, which may have implications for
chronic inflammatory or autoimmune diseases such as inflammatory
bowel disease [110].
Larrosa et al., after screening different microbial catabolites of
polyphenols for their antiinflammatory potential in vitro, found that
hydrocaffeic, dihydroxyphenylacetic and hydroferulic acid reduced
prostaglandin E2 production by at least 50% in CCD-1 8 colon
fibroblast cells stimulated with IL-1β. These results suggest that
foods containing significant hydrocaffeic acid precursors (procyani-
dins, hydroxycinnamic acid derivatives, etc.) such as artichoke, cocoa,
apples and strawberries could exert antiinflammatory activity and
reduce intestinal inflammation in humans [111].
In addition, it has been shown that microbial metabolites of plant
polyphenols may also affect disease risk in the metabolic syndrome.
Verzelloni et al. demonstrated that two microbial metabolites of
polyphenols, urolithins and pyrogallol derived from ellagitannin are
highly antiglycative compared to parent polyphenolic compounds in
an in vitro model of protein glycation. Moreover, it is known that
protein glycation plays an important pathological role in diabetes and
diabetes-associated disorders, including blindness [112].
Tucsek et al. [113] induced an inflammatory response by treating
macrophages with bacterial endotoxin and found that end products of
polyphenol degradation, such as ferulaldehyde, exerted a beneficial
antiinflammatory response by diminishing MAP (mitogen-activated
protein) kinase activation, thereby inhibiting NF-κB activation, mito-
chondrial depolarization and reactive oxygen species production.
Similar results were found by Chirumbolo using many purified
aglycone flavonoids [114]. It is arguable that the antimicrobial activity
of polyphenols might be principally due to their well-recognized
antiinflammatory potential.
Very recently, Beloborodova et al. [115] analyzed the role of
phenolic acids of microbial origin as biomarkers in the progress of
sepsis. They found that p-hydroxyphenylacetic acid showed the
capacity to inhibit ROS (reactive oxygen species) production in
neutrophils. By affecting neutrophils, they retard the immune
response, whereas, while acting on mitochondria, they prevent or
red uce the development of multiple organ failure. Thus, during the
development of bacteremias and purulent foc i of infec tion associ-
ated with P. aeruginosa and Acinetobacter bauma nii, their metabolite
p-hydroxyphenylacetic acid can directly enter the systemic blood
flow and inhibit the pha gocytic ac tivity of neutrophils.
Finally, all these results support the hypothesis that not only the
food polyphenols but also their microbial metabolites must be
taken into account when assessing the impact of polyphenols on
host health.
7. Conclusion
The bioavailability and effects of polyphenols greatly depend on
their transformation by components of the gut microbiota. Different
studies have been carried out to understand the gut microbiota
transformation of particular polyphenol types and identify the
microorganisms responsible. The modulation of the gut microbial
population by phenolics was also reviewed in order to understand the
two-way phenolic-microbiota interaction. It is clear that dietary
polyphenols and their metabolites contribute to the maintenance of
gut health by the modulation of the gut microbial balance through the
stimulation of the growth of beneficial bacteria and the inhibition of
pathogen bacteria, exerting prebiotic-like effects. However, data on
the impact of polyphenols on the gut microbiota and their
mechanisms of action in humans are scarce. In addition, a better
understanding of the dietary phenolic and gut microbiota relationship
by the c ombination of metagenomic and metabolom ic studies
provides more insight into the health effects of polyphenols.
Acknowledgments
The research group belongs to the Centro de Investigación en
Red (CIBEROBN, CB06/03/0018) of the Instituto de Salud Carlos III,
Madrid, Spain. The grant Miguel Servet (CP07/00095) from the
Instituto de Salud Carlos III (Fernando Cardona).
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