Am J Physiol Gastrointest Liver Physiol 302: G1–G9, 2012.
First published October 20, 2011; doi:10.1152/ajpgi.00048.2011.
Review
Functional analysis of colonic bacterial metabolism: relevant to health?
Henrike M. Hamer, Vicky De Preter, Karen Windey, and Kristin Verbeke
Translational Research Center for Gastrointestinal Disorders and Leuven Food Science and Nutrition Research Center,
University Hospital Gasthuisberg, Katholieke Universiteit Leuven, Leuven, Belgium
Submitted 7 February 2011; accepted in final form 16 October 2011
fermentation; metabolites; metabolomics; microbiota; short-chain fatty acids
THE HUMAN INTESTINAL MICROBIOTA complements our physiology
with functions that we have not had to develop on our own. In
fact, the intestinal microbiota have a metabolic capacity that is
comparable to that of the liver (83). The human colon contains
an extremely complex and dynamic microbial ecosystem with
high densities of living bacteria in concentrations of 1011-1012
cells/g of luminal contents belonging to more than 1,000
different species. In healthy adults, 80% of the identified fecal
microbiota can be classified into three dominant phyla: Firmicutes, Bacteroidetes, and Actinobacteria, but there is substantial variation in the species composition between individuals
(30, 101).
The intestinal microbiota plays an important role in human
physiology. For example, the intestinal microbiota is responsible for the further metabolism and energy harvest from
nondigested nutrients, is involved in the synthesis of vitamins
such as B and K and metabolism of polyphenols, provides
colonization resistance toward potential pathogens, is involved
in the metabolism of bile acids, and stimulates the immune
function of the host (74, 83).
Molecular approaches, mainly based on the 16S ribosomal
RNA gene, have revolutionized the field of gut microbial
Address for reprint requests and other correspondence: K. Verbeke, TARGID, Univ. Hospital Leuven, Herestraat 49, 3000 Leuven, Belgium (e-mail:
[email protected]).
http://www.ajpgi.org
ecology. Nowadays, the uncultured and complex microbial
communities can be characterized with greater sensitivity by
using high-throughput technologies, such as pyrosequencing
(5) and phylogenetic microarrays (79), compared with former
molecular fingerprinting methods, such as PCR-denaturing
gradient gel electrophoresis (DGGE). Complementary quantitative technologies, such as fluorescence in situ hybridization
(FISH) and real-time quantitative PCR, can be used to confirm
shifts in particular groups or species (113).
In recent years an increasing amount of literature has demonstrated that several diseases are related to alterations in the
intestinal microbiota (known as dysbiosis), such as irritable
bowel syndrome (54), inflammatory bowel disease (93), diabetes (55), atopic diseases (76), cancer (85), and obesity (7).
For example, a reduction in the abundance and diversity of
Firmicutes is frequently associated with inflammatory bowel
disease and irritable bowel syndrome (105, 113). These studies
have mainly shown that differences in the composition of the
intestinal microbiota are associated with disease.
Functional Capacity of the Microbiota
The human microbiota is characterized by a significant
degree of functional redundancy, meaning that different bacteria can perform similar functions and metabolize the same
substrate (60, 64). Therefore, not only the composition but also
0193-1857/12 Copyright © 2012 the American Physiological Society
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Hamer HM, De Preter V, Windey K, Verbeke K. Functional analysis of colonic
bacterial metabolism: relevant to health?. Am J Physiol Gastrointest Liver Physiol 302:
G1–G9, 2012. First published October 20, 2011; doi:10.1152/ajpgi.00048.2011.—With the
use of molecular techniques, numerous studies have evaluated the composition of
the intestinal microbiota in health and disease. However, it is of major interest to
supplement this with a functional analysis of the microbiota. In this review, the
different approaches that have been used to characterize microbial metabolites,
yielding information on the functional end products of microbial metabolism, have
been summarized. To analyze colonic microbial metabolites, the most conventional
way is by application of a hypothesis-driven targeted approach, through quantification of selected metabolites from carbohydrate (e.g., short-chain fatty acids) and
protein fermentation (e.g., p-cresol, phenol, ammonia, or H2S), secondary bile
acids, or colonic enzymes. The application of stable isotope-labeled substrates can
provide an elegant solution to study these metabolic pathways in vivo. On the other
hand, a top-down approach can be followed by applying metabolite fingerprinting
techniques based on 1H-NMR or mass spectrometric analysis. Quantification of
known metabolites and characterization of metabolite patterns in urine, breath,
plasma, and fecal samples can reveal new pathways and give insight into physiological regulatory processes of the colonic microbiota. In addition, specific metabolic profiles can function as a diagnostic tool for the identification of several
gastrointestinal diseases, such as ulcerative colitis and Crohn’s disease. Nevertheless, future research will have to evaluate the relevance of associations between
metabolites and different disease states.
Review
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FUNCTIONAL ANALYSIS OF COLONIC BACTERIAL METABOLISM
Types of Fermentation
The colonic microbiota ferment endogenous host-derived
substrates such as mucus, pancreatic enzymes, and exfoliated
epithelial cells, as well as dietary components that escape
digestion in the upper gastrointestinal tract. Two main types of
colonic microbial fermentation can be distinguished, including
saccharolytic fermentation of carbohydrate and proteolytic
fermentation of protein (Fig. 2). In the proximal part of the
colon, mainly saccharolytic fermentation takes place, since
most microorganisms preferentially ferment carbohydrates and
switch to protein fermentation when carbohydrate sources are
depleted (75). The main products of carbohydrate metabolism
are short-chain fatty acids (SCFA), mainly acetate, propionate,
and butyrate, which have been shown to contribute to colonic
health. SCFA provide energy to the colonic epithelial cells,
decrease luminal pH, and improve mineral absorption. Furthermore, butyrate has been shown to possess an anti-inflammatory
Fig. 1. Simplified scheme of the metabolites produced by the interplay between
the microbial and human metabolome. After passing through the small intestine, unabsorbed dietary substrates (mainly unabsorbed carbohydrates and
proteins) along with other endogenous substrates (such as bile enzymes, mucus
and exfoliated cells) reach the large intestine, where they are fermented by the
intestinal microbiota. These microbial metabolites are excreted in feces or
absorbed and transported to the liver. Subsequently, these absorbed metabolites are subject to further human metabolism and can be excreted in urine and
breath (50).
and anticarcinogenic potential (46, 63). Besides their contribution to gut health and maintenance, SCFA may provide further
benefits for the systemic metabolism. For example, it has been
shown that acetate and propionate affect hepatic lipid metabolism. Acetate is the primary substrate for cholesterol synthesis, whereas propionate can inhibit cholesterol synthesis (27,
110). Since the different SCFA may show distinct effects, not
only the amount of SCFA produced but also their ratio is of
importance with regard to these health effects. Furthermore,
SCFA stimulate increased plasma levels of satiety hormones
such as peptide YY (PYY), leptin, and glucagon-like peptide-1
(GLP-1) and may attenuate insulin resistance (1, 34). These
effects may occur due to the fact that SCFA are ligands for G
protein-coupled receptors (GPR) 41 and 43, expressed on
adipocytes, enteroendocrine L-cells, and immune cells (88). In
addition, recent studies have linked SCFA activation of GPR
43 to the suppression of colon cancer (94, 96). Proteolytic
fermentation also leads not only to the production of SCFA
(lower amounts than produced from carbohydrates) and
branched-chain fatty acids but also to potentially toxic metabolites such as phenolic compounds, sulfur-containing compounds, amines, and ammonia (Fig. 2) (48). The toxicity of
these protein fermentation metabolites has mainly been established in in vitro studies (6, 11, 58) and animal studies (4, 97).
However, human epidemiological studies do not consistently
support an association between protein intake and colorectal
cancer (2) and inflammatory bowel disease (47). Unfortunately, actual protein fermentation metabolites were not determined in these studies. The different metabolites of protein
fermentation and their potential involvement in colorectal
cancer have previously been reviewed (48).
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the functional capacity of the intestinal microbiota is highly
important regarding the clinical end points.
Next-generation pyrosequencing can be applied to further
evaluate the functional capacity of the colonic microbiota by
creating a catalog of the genetic potential of the microbiota.
However, it has to be realized that the detection of genes in a
metagenomic library does not necessarily mean that these are
functionally important (113). To gain better insight into the
activity and functionality of the intestinal microbiota, other
meta-“omics” approaches can be applied that use RNA, proteins, and metabolites as targets. In this review, we summarize
the different approaches that have been used to quantify and
characterize the metabolites produced by the microbiota, which
yield information on the actual end products of metabolism.
Colonic microbial fermentation results in the production of
large amounts of different end products. The type and amount
of these fermentation-derived metabolites largely depend on
the composition of the microbiota, transit time, and the substrates available for fermentation (95). Some of these end
products have been shown to be protective to the colonic
epithelium, and others have proved to be proinflammatory or
procarcinogenic metabolites (3, 48). By using knowledge of
these specific metabolites, a hypothesis-driven targeted approach can be applied to evaluate changes in colonic metabolism following dietary interventions or during different disease
states, for example, through quantification of selected metabolites from carbohydrate and protein fermentation, secondary
bile acids, or colonic enzymes. On the other hand, a top-down
approach can be followed by applying metabolite fingerprinting techniques based on 1H-nuclear magnetic resonance
(NMR) or mass spectrometric analysis. By following this
approach, novel metabolites and mechanisms can be identified
that are involved in health and disease. This is, however, not an
easy task, since the signals first have to be identified and their
metabolic roles elucidated.
For analyses of metabolites as end products of intestinal
metabolism in humans, we mainly rely on fecal samples or on
breath (for example, hydrogen, methane, and carbon dioxide),
urine, and plasma samples due to the relative inaccessibility of
the colon to sample at different locations (Fig. 1) (50). In these
human studies, an elegant solution to study metabolic pathways in vivo is the application of stable isotope tracers.
Review
FUNCTIONAL ANALYSIS OF COLONIC BACTERIAL METABOLISM
Fig. 2. Different metabolites produced from colonic fermentation of carbohydrates and proteins. BCFA, branched-chain fatty acids; SCFA, short-chain
fatty acids.
Targeted Approach
barley has evaluated the kinetics of SCFA appearance in the
systemic circulation in healthy volunteers. In this study, the
pattern of SCFA appearing in the systemic circulation was
different after consumption of a meal with dietary fiber (nonstarch polysaccharides) combined with resistant starch compared to a meal with dietary fiber alone (102). Further studies
are necessary to address the significance of these different
SCFA profiles with regard to health benefits. In another study,
stable isotope tracers were applied to determine the production
rate of acetate during colonic fermentation of lactulose in
humans. Healthy volunteers received a primed continuous
infusion of [1-13C]acetate followed by the ingestion of lactulose. The colonic acetate production was calculated from the
reduction in the plasma [13C]acetate enrichment as a result of
the colonic fermentation of lactulose (77). With the use of a
dynamic in vitro model of the large intestine, the fermentation
of 13C-labeled starch was evaluated by determination of the
label incorporation into the microbial biomass and metabolites
using stable isotope probing and NMR analysis (53). From the
labeling pattern of microbial metabolites, it was concluded that
cross-feeding between Ruminoccus bromii and Eubacterium
rectale occurred, wherein R. bromii produced acetate, which
was subsequently converted to butyrate by E. rectale.
Quantification of protein fermentation. The degree of proteolytic fermentation can be determined by quantification of
typical end products of protein fermentation (Fig. 2) (48).
Some of these metabolites are reused as nitrogen source for
bacterial growth, whereas others will be taken up by colonocytes and transported into the bloodstream. For instance, phenols and indoles are breakdown products of the aromatic acids
tyrosine and tryptophan, respectively. Generally, once these
compounds are produced, they enter the hepatic circulation to
be detoxified in the liver and eventually excreted in urine.
Since p-cresol and phenol are unique bacterial metabolites
from protein fermentation that are not produced by human
enzymes, these metabolites have been frequently used to assess
the degree of proteolytic fermentation (25, 37). Urinary levels
of p-cresol and phenol have shown to be increased during high
Fig. 3. Proposed mechanism of action of prebiotics.
IBD, inflammatory bowel disease; IBS, irritable bowel
syndrome.
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Quantification of carbohydrate fermentation. The beneficial
effects of saccharolytic fermentation have mainly been ascribed to the production of SCFA (68, 91). Several studies have
evaluated SCFA concentrations and profiles in patients with
different diseases. For example, a lower butyrate-to-acetate
ratio has been found in colonic luminal content of patients with
adenomatous polyps or colon cancer compared with healthy
controls (107), and increased amounts of SCFA were found in
fecal samples from obese compared with lean individuals (89).
Dietary interventions with prebiotics, such as inulin and oligofructose, defined as “nondigestible food ingredients that stimulate the growth and/or activity of bacteria in the digestive
system which are beneficial to the health of the host,” aim to
increase and prolong the saccharolytic fermentation toward the
distal colon and thereby aim to reduce proteolytic fermentation
(38) (Fig. 3). Increased SCFA production after addition of
different prebiotics to the diet has previously been demonstrated by measuring fecal or plasma concentrations of SCFA
(52, 73). However, the in situ production of SCFA is difficult
to determine, since more than 95% of the produced SCFA are
absorbed and metabolized by the host depending on the gastrointestinal transit time (59). To gain more insight into the
actual SCFA production over time, the use of stable isotope
tracers can be considered. A recent study with 13C-labeled
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FUNCTIONAL ANALYSIS OF COLONIC BACTERIAL METABOLISM
after conversion to [15N]urea in the liver. When microbial
activity was stimulated after intake of a prebiotic, the urinary
15
N excretion decreased, whereas the fecal 15N excretion
increased (20, 36). In general, fermentable carbohydrates stimulate bacterial proliferation, which leads to incorporation of
nitrogen (from ammonia and other sources) into bacterial cells
and consequent excretion in feces (25, 26, 108). Thus a shift of
nitrogen excretion from urine to feces can be explained by
increased bacterial protein synthesis and a subsequent decrease
in colonic absorption of nitrogen in the form of ammonia. The
removal of ammonia from the colonic lumen might be considered a health benefit, since it may prevent ammonia from
damaging colonocytes (87) and increased systemic levels of
ammonia cause neurotoxic effects (106). Accumulation of
ammonia in the bloodstream is associated with hepatic encephalopathy. Lactulose is frequently utilized in the treatment to
reduce ammonia levels in these patients (90, 106).
Secondary bile acids. Bile acids are natural amphipathic
detergents that assist the emulsification and absorption of lipids
and fat-soluble vitamins. The human liver synthesizes the
primary bile acids, cholic acid and chenodeoxycholic acid.
Primary bile acids are secreted in bile from the gallbladder into
the small intestine during digestion. They are then actively
absorbed in the ileum and returned to the liver via the portal
vein (Fig. 5). About 5% of bile salts escape this enterohepatic
circulation and enter the colon, where they are subject to
bacterial biotransformation reactions. When the primary bile
acids reach the colon, they are deconjugated and successively
undergo other enzymatic reactions, the most predominant being the dehydroxylation by bacterial 7␣-dehydroxylase to form
the secondary bile acids, primarily deoxycholic (DCA) and
lithocholic acids (LCA) (8). Secondary bile acids have been
hypothesized to be cocarcinogenic and tumor promoters (18,
112). It has been reported that patients with adenomatous
polyps have a higher concentration of fecal bile acids and total
secondary bile acids compared with control subjects (49).
Furthermore, the ratios of the primary bile acids and their
secondary bile acids are significantly lower in cancer patients
compared with controls (17, 49). However, a large metaanalysis of 20 clinical trials and a total number of 1,226
individuals showed no difference between the fecal concentrations of secondary bile acids (DCA and LCA) in colorectal
cancer patients compared with controls (98). It has been shown
that dietary interventions are able to modulate fecal bile acid
concentrations. For example, Boutron-Ruault et al. (15) have
shown that ingestion of short-chain fructooligosaccharides decreased fecal concentrations of LCA. This decrease may be
Fig. 4. Lactose[15N,15N=]ureide is resistant to
digestion but is fermented by colonic bacterial
enzymes into [15N,15N=]urea and subsequently
[15N]ammonia (NH3). The formed [15N]ammonia can either be taken by the microbiota
and excreted via the feces or be absorbed
through the mucosa and renally excreted after
conversion to [15N,14N=]urea in the liver. The
proportionate recovery of 15N in feces or urine
indicates the degree of colonic protein fermentation.
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protein intake (37) and decreased after oral supplementation
with oligofructose-enriched inulin (OF-IN) (25).
In hemodialysis patients, p-cresol generation and p-cresyl
sulfate serum concentrations were lowered after oral OF-IN
administration (69). As renal function declines in these patients, substances that are either excreted or metabolized by the
kidney accumulate in the circulation, resulting in increased
levels of numerous molecules in blood (33). A number of these
retained solutes originate from colonic bacterial protein metabolism (33). In addition, small intestinal assimilation of
proteins is impaired in renal failure (10), resulting in an
increased availability of proteins for fermentation in the colon
(31). Accumulation of the protein fermentation metabolites in
serum has been suggested to alter endothelial function (32, 70)
and has been associated with increased mortality in hemodialysis patients (9). As a consequence, a dietary strategy with
OF-IN that contributes to a lower generation of protein fermentation metabolites might constitute a significant improvement in the treatment of these patients. In addition to p-cresol
and phenol, other protein fermentation metabolites, such as
sulfur-containing metabolites, were decreased in fecal samples
after incubation with OF-IN in vitro (22). Increased concentrations of sulfides have been associated with the pathogenesis
of ulcerative colitis (UC). Hydrogen sulfide (H2S) has been
found to provoke genomic DNA damage in colonic cancer
cells (HT-29 cells) in concentrations similar to those found in
the human colon (6). In addition to inducing DNA damage,
sulfide impairs the oxidation of butyrate, the major energy
substrate in colonocytes, by inhibition of cytochrome c oxidase
(81). These observations are the basis for Roediger’s “energy
deficiency” hypothesis as a cause of UC (67, 82). Different
studies have reported increased luminal concentrations of sulfide as well as high numbers of sulfide-reducing bacteria in
patients with UC (84). This patient group may therefore also
benefit from dietary interventions with prebiotics.
Colonic ammonia metabolism. To evaluate the ammonia
metabolism in the colon, the stable isotope-labeled biomarker
lactose[15N,15N=]ureide has been validated (20, 36) (Fig. 4).
Human enzymes are not able to hydrolyze the bond between
the sugar moiety and urea, whereas this bond can be split by
bacterial enzymes when it reaches the colon. Released
[15N,15N=]urea is rapidly hydrolyzed to [15N]ammonia by ubiquitous bacterial urease. As such, lactose[15N,15N=]ureide is
used as a vehicle to introduce a known amount of 15N, in the
form of ammonia, to the colon. The formed [15N]ammonia can
be either taken up by the colonic microbiota, followed by fecal
excretion, or absorbed through the mucosa and renally excreted
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FUNCTIONAL ANALYSIS OF COLONIC BACTERIAL METABOLISM
G5
related to the increased production of SCFA, since SCFA
decrease the colonic pH. A low pH inhibits dehydroxylation of
bile acids and, consequently, conversion to secondary bile
acids (19). This hypothesis is supported by the negative correlation between butyrate and acetate production and bile acid
metabolism (100).
Bacterial enzyme activity. Several bacterial enzyme activities have been studied in relation to their effect on host health.
Bacterial ␤-glucosidase and ␤-glucuronidase hydrolyze the
glycosidic bond of glycosides and glucuronide conjugates,
respectively, to release aglycones. Glycosides in the gut originate from the plant material in the diet. Glucuronide conjugates are endogenously produced compounds that are metabolized in the liver and conjugated to glucuronic acid before
being excreted by the liver via the bile into the small intestine
(86). Since some aglycones have been reported to be potentially toxic or carcinogenic, elevated activity of these bacterial
enzymes is hypothesized to be a risk factor for colon cancer
(39, 43). On the other hand, anticarcinogenic effects of flavonoid aglycones also have been reported (29). Other examples of bacterial enzymes are azoreductase, nitrate reductase,
nitroreductase, and sulfatases (39). Azoreductase leads to the
production of amines, whereas nitrate reductase converts nitrate into nitrite and nitroreductase is involved in the formation
of heterocyclic and aromatic nitro compounds (66). Sulfatases
are involved in the degradation of mucosal glycans, such as
colonic mucins (14). Activities of these bacterial enzymes are
related to the composition of the microbiota. For example,
lactobacilli and bifidobacteria produce low levels of ␤-glucuronidase but also low levels of azoreductase and nitroreductase, whereas strict anaerobes (Bacteroides sp., Eubacterium
sp., and Clostridium sp.) produce high levels of these enzymes
(14, 71). A number of animal and human intervention studies
have investigated the ability of pre- and probiotics to modulate
these bacterial enzyme activities in the colon. However, these
studies have obtained conflicting results (23, 40 – 42, 45, 62,
65, 66, 92), and their significance for host health is often
difficult to determine (23, 65).
Top-Down Approach/Metabolomics
New techniques such as metabolomics allow evaluation of
the colonic metabolism by a top-down approach, bypassing the
need for an a priori hypothesis. Metabolomics has been defined
as “the quantitative measurement of the multiparametric metabolic responses of a living system to pathophysiological
stimuli or genetic modification” (72).
Metabolomic analysis requires analytical technologies that
are capable of detecting and quantifying the large number of
metabolites in biological samples, such as feces (24, 35, 37,
103), intact tissue (12, 16), breath (80), blood (111), and urine
(109). Analyses of these multiple biological samples complement each other, since they yield different information with
regard to the origin of the metabolites. Certain breath and
urinary metabolites result from cometabolism by the host and
the intestinal microbiota, whereas fecal metabolites are primarily derived from the intestinal microbiota (Fig. 1).
Ideally, the sample preparation for metabolomic analysis
should be minimal to enable the quantification of as many
metabolites as possible. However, no single analytical technique can measure all the metabolites present in a particular
sample due to the diverse chemical properties of metabolites
and the limitations of each analytical technique (57). Commonly used analytical platforms are mass spectrometry (MS)
(28) and NMR (13). Multivariate statistical techniques are
required to determine which metabolites differ between samples from different groups. Commonly used multivariate discriminant tests in metabolomic studies include principal component analysis (PCA) and partial least-squares discriminant
analysis (PLS-DA). PCA may be considered an unsupervised
classification method, since the variation in the data is analyzed
without a priori designation of samples into their classes. In
contrast, PLS-DA is considered a supervised classification
method, because the samples are designated into their classes
for comparison. These different platforms to detect hundreds,
or even thousands, of metabolites in one single analysis can be
used to evaluate the differences in metabolite profiles between
different diseases (51, 109) or different stages of disease (56)
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Fig. 5. Enterohepatic circulation of bile acids. After each meal, gallbladder contraction empties bile acids into the intestinal tract. When passing through the
intestinal tract, some bile acids are reabsorbed in the upper intestine by passive diffusion, but most bile acids (⫾95%) are reabsorbed in the ileum. Bile acids
are transdiffused across the enterocyte to the basolateral membrane and excreted into portal blood circulation back to the liver. In the colon, secondary bile acids
are formed by bacterial 7␣-dehydroxylation of their primary bile acid precursor. Secondary bile acids may have a cytotoxic effect on the colonic epithelium. Bile
acids lost in the feces (0.2– 0.6 g/day) are replenished by de novo synthesis in the liver to maintain a constant bile acid pool. BA, bile acids; CA, cholic acid;
CDCA, chenodeoxycholic acid; DCA, deoxycholic acid; LCA, lithocholic acid.
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FUNCTIONAL ANALYSIS OF COLONIC BACTERIAL METABOLISM
Microbial Metabolites Relevant to Health?
The top-down evaluation of microbial metabolites may lead
to new insights into which components could be associated
with different diseases and (dietary) interventions. Another
future challenge remains to associate which microbial species
or consortia are responsible for the production of distinct types
of metabolites. Several recent studies have used a “transgenomic” approach to link gut microbiome and metabolic
phenotype variation (61, 104). These approaches can offer new
possibilities for future interventions. However, more knowledge on the clinical effects of a decrease or increase in certain
metabolites and microbial species is warranted, because detected associations may not always be relevant. It remains
unknown whether changes in metabolites are a cause or a
consequence of the disease. Large prospective cohort studies
are warranted to evaluate whether there exists a causal relationship between the production of different metabolites and
the different diseases, their clinical features, or disease progression.
Metabolic profiles can also be used for diagnosis of different
diseases or classification of patients. This is a promising
noninvasive, rapid, and relatively inexpensive diagnostic tool.
Before introduction into the clinical practice, the specificity
and sensitivity needs to be further evaluated (35, 78).
Conclusions
Changes in colonic metabolism can be evaluated following
a targeted approach or a top-down approach. The targeted
approach may include quantification of known metabolites
from carbohydrate fermentation (SCFA) or protein fermentation (such as p-cresol, phenol, ammonia, or H2S), levels of
secondary bile acids, or bacterial enzyme activities. All these
“biomarkers” have been identified in epidemiological or intervention studies and have been related to different pathologies,
such as colorectal cancer, inflammatory bowel disease, or
metabolic syndrome. However, at present, these biomarkers
have been insufficiently validated in large prospective trials.
With the emergence of metabolomic analyses, new insight into
metabolic pathways in relation to health and disease will
evolve. Furthermore, these specific metabolic profiles can
function as a diagnostic tool for the identification of several
diseases, such as ulcerative colitis and Crohn’s disease. Until
now, the described biomarkers of bacterially mediated colonic
metabolism can only be regarded as intermediate end points.
An important challenge for current and future research remains
to relate changes in bacterial metabolism to human health.
GRANTS
V. De Preter is a postdoctoral fellow of the Fund for Scientific ResearchFlanders (F.W.O. Vlaanderen, Belgium).
DISCLOSURES
All authors declare that they have nothing to disclose and that there is no
potential conflict of interest.
AUTHOR CONTRIBUTIONS
Author contributions: H.M.H. conception and design of research; H.M.H.
and K.V. prepared figures; H.M.H., V.D.P., and K.V. drafted manuscript;
H.M.H., V.D.P., K.W., and K.V. edited and revised manuscript; H.M.H.,
V.D.P., K.W., and K.V. approved final version of manuscript; K.V. interpreted
results of experiments.
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increased SCFA levels as expected. However, a more discriminatory factor for the clustering of the fecal slurries incubated
with different doses of OF-IN was an increase in the concentration and number of esters (22). Increased acid production
might be the origin of a higher presence of esters. The relevance of esters to health is yet unknown. Future studies are
warranted and may also have to consider esters as a marker of
saccharolytic activity. Incubation of fecal slurries with OF-IN
was furthermore associated with a reduction in sulfur-containing compounds and phenols in a dose- and time-dependent way
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