gut microbiota and endotoxemia

Meghan McGillin January 15, 2016

Dysbiosis of the Gut and Obesity

The obesity pandemic

In the past thirty years the prevalence of obesity has more than doubled (W.H.O., 2015).

In 2014, over 600 million adults had a body mass index (BMI) over 31, classifying 11% of the

global adult population as obese. The World Health Organization defines obesity as excessive

accumulation of fat tissue and is a major risk factor for noncommunicable diseases like

cardiovascular disease (CVD), diabetes, musculoskeletal disorders, and some cancers (W.H.O.,

2015). Obesity is the result of an imbalanced energy homeostasis, which is the consequence of

greater intake of energy than expenditure (W.H.O., 2015). However, the doubling of global obesity rates over the past three decades suggests that the traditional equation for obesity is

incomplete. As more parts of the world transition from developing to industrial societies, a shift

in quality of life follows. These changes are most prominent in diet, where a diet high in whole foods is replaced with a more western-style diet, which is considered a diet high in refined carbohydrates, processed meats and low in dietary fiber (Carnahan, S. 2014). It is speculated that the soaring rates of obesity and metabolic syndrome are partially the consequence of the

shift from a whole to a processed food diet. Obesity is associated with a cluster of several metabolic disorders, such as insulin

resistance, hyperglycemia, hyperlipidemia, and hepatic steatosis (Chassaing,B. 2014 CT

Nathan 2008). This cluster of metabolic disorders is related to increased risk of diabetes,

cardiovascular diseases, and liver dysfunction (Cani, Patrice D 2009). The underlying cause of

obesity is complex and confounding, and involves various environmental, genetic, and lifestyle

factors. Recently, the human gut microbiota has been implicated as a major force in host

metabolism and the development of obesity.

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The human gut microbiota

The human gut microbiota refers to the trillions of microorganisms that reside in the

human gut (Qin, J., 2009). It is estimated to be home to 1000 different species (Qin, J., 2009),

and contains greater than 100 times the genes found in the human genome (Carnahan, S.,

2014). The human gut microbiota can be viewed as “a responsive intestinal endocrine organ”

(Rosenbaum, Michael, 2015), capable of secreting or modifying the production of molecules

involved in energy balance and energy stores (Rosenbaum, Michael, 2015).

The gut microbiota is intertwined with many aspects of host metabolism of nondigestible

carbohydrates, absorption of micronutrients, synthesis of essential vitamins, and mediation of

immune responses (Requena,Teresa, 2013). Recent advances in analytical methods like DNA

sequencing, metabolomics, and glycomics have lead to a series of discoveries pairing the gut

microbiota with obesity. Beginning with the discovery that the presence of a gut microbiota

increases energy harvest has fueled scientific inquiry in the gut microbiota involvement in host

metabolism (Bäckhed, F. 2004). It has lead to other fundamental findings within the field of research, such as the gut microbiota role in the regulation of fat storage. This phenomenon was

demonstrated when conventionalization of germ-free (GF) mice led to increases triglyceride

storage in adipocytes through a mechanism involving hepatic lipogenesis and fasting-induced

adipocyte factor (Fiaf) (Bäckhed, F. 2004). It has also been shown that certain interactions between the gut microbiota and the host immune system can exacerbate the development of

metabolic disorders (Cani, Patrice D 2009). Hansen touched on this concept when they reported that a dietary change in pregnant mice significantly altered the susceptibility of type I

diabetes in their offspring, despite the offspring being being fed a standard diet. It is speculated

that this finding stems from a shift towards an anti-inflammatory state following diet-induced

alterations in the gut microbiota (Hansen 2014). Altogether, these findings suggest that the gut microbiota can affect an individual’s energy harvest capacity and fat storage, and influence their

susceptibility to diseases, like diabetes and obesity (Requena,Teresa, 2013).

The vast diversity and highly individualized nature of one’s gut microbiota complicates

the already convoluted task of defining the gut microbiota exact involvement in host metabolism.

That being said, a more complete picture of the mechanisms behind the development of

metabolic disorders, like obesity, is beginning to surface. This new picture, which replaces the

old energy equation described earlier, illustrates the how such metabolic disorders might affect

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the status of one's gut microbiota and how one’s gut microbiota may affect the status of one’s

health.

Diet induced alterations in the gut microbiota represents both a contributing factor and a

ultimately a treatment for the obesity epidemic. Over the course in which one becomes obese,

abnormalities in host metabolism develop concurrently. The process is not fully understood, but

it is speculated that the dysfunctional metabolism can be attributed to disturbances in the gut

microbiome. These alterations in the gut bacteria composition can result from diet and may

further perpetuate the ill effects of diet-induced obesity (DIO) (Carnahan, S., 2014). The aim of

this review is to explore how alterations in the gut microbiota is linked to obesity and to explore

the relationship between them and diet.

Bacterial richness of the gut

The composition of the gut microbiota has been associated with obesity, as well as other

diseases and disorders such as allergies, diabetes, and irritable bowel disease (Le Chatelier, E.,

2013). The number of gut microbial genes, also referred to as the “bacterial richness” of the gut,

has the potential to identify individuals at increased risk for obesity, and other related diseases

(Le Chatelier, E., 2013).

A study to investigate the relationship between bacterial richness of the gut microbiome

and metabolic markers related to adiposity and inflammation found that variations in gut

bacterial richness can distinguish populations who may be at increased risk of developing

obesity-related diseases (Le Chatelier, E., 2013). In this study 209 subjects fell into two groups,

the subjects with less than 480,000 genes were placed in the low gene count (LGC) group and

those with more were placed in the high gene count (HGC) group (Le Chatelier, E., 2013). The

LGC group was characterized by increased adiposity, insulin resistance and inflammation (Le

Chatelier, E., 2013). The obese population was heavily represented by the LGC group. The

same was seen for other groups like individuals afflicted with inflammatory bowel disorder (IBD),

and elderly patients suffering from inflammation, all whom are most commonly associated with

low bacterial richness (Le Chatelier, E., 2013).

There were direct variations in gut microbiotas that were associated by marked differences in

bacterial gene richness. Starting at the phylum level, Proteobacteria and Bacteroidetes were

seen in greater abundance among the LGC group, the same was not observed in the HGC

group. Increased abundance of Verrucomicrobia, Actinobacteria, and Euryarchaeota were

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associated with high gene count individuals (Le Chatelier, E., 2013). Le Chatelier found 46

different genera between the two groups. The most dominant genera of the LGC subset was

Bacteroides, Parabacteroides, Ruminococcus, Campylobacter, Dialister, Porphyromonas,

Staphylococcus and Anaerostipes. In total, 36 genera were significantly associated with HGC, including Faecalibacterium, Bifidobacterium, Lactobacillus, Butyrivibrio, Alistipes, Akkermansia, Coprococcus and Methanobrevibacter (Le Chatelier, E., 2013). 73 species were identified in total from the 97 genomes analyzed. Interestingly, more than 93% of the total microbial

population belonged to just nine different species. There were significant differences in the

diversity and abundance of the nine dominant species observed between LGC and HGC

individuals. The known species that differed in abundance among the two groups are

Clostridium bolteae, Clostridium clostridioforme, Coprococcus eutactus, Clostridium ramosum, Clostridium symbiosum, Faecalibacterium prausnitzii, Methanobrevibacter smithii,

Ruminococcus gnavus, and Roseburia inulinivorans (Le Chatelier, E., 2013). This particular study found anti-inflammatory species, like Faecalibacterium prausnitzii to be associated with high bacterial richness;; whereas Bacteroides and Ruminococcus gnavus, both pro-inflammatory species associated with IBD, are more prevalent in LGC individuals.

Metabolic activity of HGC and LGC

There were significant differences in the metabolic activity of the LGC and HGC groups.

The metabolic activity of HGC individuals were associated with increased production of SCFA

(lactate, propionate, and butyrate), and demonstrated a higher potential for hydrogen

production. LGC individuals appeared to have an increased capacity to withstand oxidative

stress based off of the increased production of peroxides, catalase, and TCA modules. The

LGC gut microbiota had the potential to produce possible harmful metabolites, like

pro-carcinogens and ammonium and modules for degradation of aromatic amino acids and

β-glucuronide (Le Chatelier, E., 2013). Le Chatelier speculate the significant increase in potentially noxious modules may stem from the increased abundance of Bacteroides associated with the LGC.

Compared to the metabolic activity observed with HGC microbiota, the LGC had reduced

levels of butyrate-producing bacteria and a decrease in hydrogen and methane production. The

LGC group was associated with increased mucus degradation potential, as told by the reduced

ratio of Akkermansia to Ruminococcus torque and Ruminococcus gnavus (Le Chatelier, E., 2013). The weakened mucosal barrier protection, in addition to the increased presence of

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opportunistic pathogens, like Campylobacter and Shigella, and the improved oxidative stress threshold links that low bacterial richness to a pro-inflammatory gut microbiota (Le Chatelier, E.,

2013).

Phenotypes of HGC and LGC

Bacterial richness was significantly associated with biochemical and anthropometric

aspects of the host’s phenotype. As mention earlier, LGC represented a majority of the obese

subjects involved in this study. Low bacterial richness was correlated with increased adiposity,

body weight, and greater fat mass percentage. The LCG group was associated with insulin

resistance, hyperinsulinemia and inflammation (Le Chatelier, E., 2013). Le Chatelier suggested that the imbalance of pro- and anti-inflammatory bacteria that is responsible for the inflammation

and insulin resistance observed with LGC individuals. Low bacterial richness was also

associated increased levels of leptin, triglyceride, and free fatty acids. It was proposed that the

increase in free triglycerides and free fatty acids was triggered by increased expression in

fasting induced adipose factor (FIAF also known as ANGPLT4) resulting from an altered gut

microbiota. Members of the LGC group had lower serum adiponectin and HDL-cholesterol

compared to HGC individuals (Le Chatelier, E., 2013). The significant differences observed

between the two different groups suggest that the metabolic disturbances associated with low

levels of gut bacterial richness increase the risk for T2D and cardiovascular disorders (Le

Chatelier, E., 2013).

Le Chatelier found that those with low bacterial richness gained significantly more weight than their HGC counterparts over the course of nine years. Based off the differences

discussed earlier, Le Chatelier speculated that it may be the butyrate-producing bacteria found in high abundance in HGC individuals that discouraged the weight gain seen in LGC individuals.

The findings of this study demonstrated perfect stratification related to bacterial richness. The

significance of this was their ability to identify HGC and LGC based off of a few select bacteria,

like Faecalibacterium prausnitzii for HGC individuals and Bacteroides for LGC individuals (Le Chatelier, E., 2013).

Modulation of energy intake and metabolism by SCFA

A major function of the gut microbiome is the production of short chain fatty acids

(SCFA), which are the product of microbial fermentation of nondigestible polysaccharides in the

colon and small intestine (Lin,H.V. 2012). The predominant SCFA in the gut lumen of humans

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and rodents are acetate, butyrate, and propionate (Lin,H.V. 2012). SCFAs act as energy

sources for various cells in the body (Lin,H.V. 2012). Butyrate is utilized by the colonic

epithelium whereas propionate is prefered by liver cells. Acetate tends to circulate the body,

reaching it’s final destination in peripheral tissue (Lin,H.V. 2012). Acetate appeared to serve as

a substrate in cholesterol synthesis (Hartstra,A.V. 2015).

Despite providing an estimated 6-10% of the daily needed energy, SCFAs are also

directly involved in energy homeostasis by acting as signalling molecules in various host

metabolic pathways (Requena,Teresa, 2013). It’s becoming increasingly evident that SCFAs

protect against obesity by modifying host energy balance and appetite regulation.

In mice, there was an improvement in insulin sensitivity and energy expenditure after

being administered butyrate (Hartstra,A.V. 2015). Butyrate also functions as a regulator of the

production of glucose in the liver. Animal studies found propionate reduces lipogenesis in the

liver (Oh, Susan 2014). Propionate and butyrate effect gluconeogenesis in the liver. Intestinal

gluconeogenesis (IGN) releases glucose, which sends signals to the brain which result in

improved glucose metabolism and food intake (Hartstra,A.V. 2015). One study found that

butyrate was involved in initiating IGN gene expression in rats (Hartstra,A.V. 2015).

Another way in which SCFA interact with host metabolism is through regulation of gut

peptide secretion. SCFA act as signalling molecules for receptors involved in hormone

production and secretion, like G-coupled protein receptors (GPR) (Lin,H.V. 2012) (figure 1.).

Acetate has a high affinity for GPR43, whereas GPR41 is activated butyrate. Propionate is

indifferent and binds to both endogenous receptors (Lin,H.V. 2012). GPR41 activation induces

growth and stimulates the sympathetic nervous system (Rosenbaum, Michael, 2015). Activation

of GPR41 by butyrate induces the release of peptide YY (PYY), a satiety hormone (Delzenne,

N. M. 2011)]. GPR43 activation has been shown to decrease release of inflammatory cytokines

which may also increase hypothalamic sensitivity to leptin (Rosenbaum, Michael, 2015). Leptin

is a peptide released in the adipose tissue that regulates food intake (Oh, Susan 2014).

Butyrate and propionate have demonstrated a more direct effects on feeding behavior

compared to acetate (Rosenbaum, Michael, 2015).

Butyrate and propionate, specifically, have accumulated interest in regards to their

possible anti-obesity effects. Both have shown to stimulate gut hormone secretion and to reduce

appetite in humans resulting in decreased food intake (Shen, Jian, 2013). The observed

decrease in appetite and consumption is speculated to be the result of increased secretion by

endocrine cells of two gut peptide hormones, glucagon-like peptide-1 (GLP-1) and PYY. The

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expression of GLP-1 and PYY showed to decrease significantly in plasma levels of DIO mice

(Lin,H.V. 2012). It was also shown that these signalling molecules interfered with ghrelin

secretion, which provides further explanation for the observed increase in satiety and decrease

in energy intake (Shen, Jian, 2013). Consumption of dietary fiber has been correlated with

increased production of GLP-1 and PYY, and a decrease in ghrelin levels (Requena,Teresa,

2013). These findings offer an explanation to the observed increase in satiety, and decreased

energy intake associated with diets high in fiber.

Obesity-induced changes in composition and function

Gut microbial profiles of obese rodents

One of the first discoveries highlighting the collaboration between the gut microbiota and

human energy homeostasis was when Bäckhed found that germ-free (GF) mice were protected

from diet induced weight gain. When GF mice were colonized with gut bacteria, weight gain and

increased fat mass followed (Bäckhed, 2004). Perhaps the most important finding from this

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discovery was the significance of the microbial type on the mice’s weight gain outcome

(Rosenbaum, Michael, 2015 CT Turnbaugh 2006).

Obesity showed a positive relationship to increases in the Firmicutes to Bacteriodetes

ratio populating the gut of rodents (Rosenbaum, Michael, 2015). This was also observed along

side an overall decrease in gut diversity resulting from both weight and diet composition

(Rosenbaum, Michael, 2015). When fecal transplants from obese subjects were introduced to

GF mice, they developed the obese phenotype following no significant change in daily chow

consumption (Rosenbaum, Michael, 2015) (figure 2.).

A study conducted on obese mice fed a HF diet found that those HF-DIO mice placed on

a calorie-restricted HF diet had greater microbial diversity in their gut than compared to when

they were at their maximal weight (Rosenbaum, Michael, 2015). The significance of this finding

was that it demonstrated that changes in energy intake, independent of diet, can alter gut

bacterial diversity of obese mice.

Shortcomings in translating mice to human gut microbiota studies

There are several anatomic, physiological, and behavioral factors that should be

accounted for when applying mouse based models to humans. The diet composition typical of

mice is higher in carbohydrate content, and lower in fat content compared to humans, which can

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demonstrate much more variety. Additionally, mice show significant differences in

thermogenesis by brown and beige adipose tissue (<50%) compared to humans (<5%)

(Rosenbaum, Michael, 2015).

Obesity and diversity in the human gut

Phylum-wide change

The link between the gut microbiota composition and obesity has yet to be defined. The

dominant phyla in the human gut are Bacteroidetes, Firmicutes, Proteobacteria, and

Actinobacteria (Rosenbaum, Michael, 2015). Collectively, these four phyla make up 97% of the

total microbial population (Rosenbaum, Michael, 2015). It is important to consider the high

degree of taxonomic variability in the human gut when using averages to describe phylum

representation. That being said, Firmicutes, make up around 60 to 65% of the gut microbiota,

and are associated with the fermentation of fiber to butyrate (Rosenbaum, Michael, 2015).

Bacteroidetes, which make up around 20 to 25% of the total gut microbiota, are associated with

polysaccharide degradation, and involve some butyrate producing species by fermentation of

fiber (Rosenbaum, Michael, 2015). Proteobacteria compose of approximately 5 to 10% of the

gut microbiota and its function in host metabolism is not known (Rosenbaum, Michael, 2015).

Actinobacteria represent approximately 3% of the gut microbiota and are associated with

vitamin biosynthesis (Rosenbaum, Michael, 2015). It’s being explored the significance of the ratio of Firmicutes to Bacteroidetes in the gut.

Firmicutes and Bacteroidetes are the dominant phyla responsible for the production of SCFAs

(Rosenbaum, Michael, 2015). A negative correlation between Bacteroidetes and obesity was

one of the first reported gut microbial alterations associated with host metabolism. In rodent

studies, and in some human, an increase in relative abundance of Firmicutes to Bacteroidetes

has been associated with obesity (Rosenbaum, Michael, 2015). However, whether this finding

translates to the human gut microbiota is unclear. There have been studies exploring the human

gut microbiota relationship between obesity and F/B ratio, some have produced results that

question, and in some cases, contradict the previous positive association between obesity and

the relative increase in Firmicutes at the expense of the Bacteroidetes (Rosenbaum, Michael,

2015). Different studies in both humans and rodents found that no difference in relative

abundance of Firmicutes and/or Bacteroidetes in obese vs lean individuals (Duncan, Sylvia, H.,

2008), others reported no effect of weight loss on the F/B ratio (Duncan, Sylvia, H., 2008), and

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some reported even greater relative abundance of Bacteroidetes in obese individuals

(Collado,M.C. 2008). The only fair conclusion one can make in regards to the relative

abundance of Firmicutes to Bacteroidetes is that it is uncertain whether that is an effective

predictor for obesity (Shen, Jian, 2013). The controversy arising from attempts to determine the

microbial profiles of obese human guts compared to their lean counterparts suggests

differences of the gut microbiota at the phylum level between obese and lean individuals might

not be so easily typecast.

Finer grain changes

One study set out on better defining the influence of obesity on the gut microbiota

composition, focusing members of the Lactobacillus genus (Million, M., 2012). Million et. al. compared the Lactobacillus population at the species level between obese and lean humans. They found an increase in Lactobacillus reuteri abundance and reduced levels of L.casei/paracasei and L. plantarum in obese patients. They also found that Methanobrevibacter smithii , a member of Archaea domain, was significantly less abundant in obese patient, in general (Million, M., 2012). Methanobrevibacter was also one of the dominant genera seen with individuals with high gene count, discussed earlier. This finding appears to contradict previous

studies, which reported increased levels of Methanobrevibacter smithii in obese patients (Zhang,H. 2009), and speculated that their increased capacity to ferment dietary

polysaccharides was their link to the observed increase in adipose tissue(Requena,Teresa,

2013). Methanobrevibacter smithii role in host metabolism remains unclear, but it does appear to be a key member of the gut.

Functional changes in the gut microbiota of obese individuals Gut dysbiosis has been characterized by a shift from SCFA producing microbes to

opportunistic pathogens (Qin, Junjie 2012). There were noted increases in potential

opportunistic pathogens associated with obesity. One study found that overweight and obese

pregnant women and their infants who later in childhood became overweight had a higher

abundance of Staphylococcus aureus (Santacruz, Arlette 2010). One study found that members belonging to the gram negative Enterobacter genus comprised a staggering 35% of a morbidly

obese patient (Fei, Na 2013).

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In addition to the increase in the prevalence of pathogenic microorganism in the obese

gut microbiota, there was an observed decrease in SCFA producing bacteria. SCFA producing

bacteria, like Bifidobacteria were in lower abundance in obese subjects (den Besten,G. 2013).

Role of gut microbiota in metabolic syndrome

The gut microbiota and metabolic enterotoxaemia, inflammation and gut permeability

It has been established that conventionalization of GF mice with a microbiota from a

diseased donor partially transfers aspects of the affected phenotype. This phenomenon

highlights the impact an impaired microbiota has in the establishment and manifestation of

noncommunicable disease (Chassaing,B. 2014).

Low-grade inflammation

Changes in the gut microbiota can promote low-grade inflammation (Chassaing,B.

2014). Obesity is considered as a state chronic low grade inflammation and is a major

contributor to the metabolic disturbances and disorders associated with increased BMI (Shen,

Jian, 2013). Low grade inflammation can be assessed by measuring levels of lipocalin-2 (Lcn2)

in fecal samples (Chassaing,B. 2014 CT Chassaing 2012). The production of pro-inflammatory cytokines responsible for the inflamed response is associated with increased adipose tissue

(Cani, P.D., , 2007). These cytokines, such as IL-1, IL-6 and TNF-α, can disrupt insulin

sensitivity and promote hyperinsulinemia and lipid storage in adipose and liver tissue (Cani,

P.D., , 2007). Inflammation results from the activation of toll-like receptors (TLR) by lipotoxic

substances, like saturated fatty acids, produced by the gut microbiota (Carnahan, S., 2014).

The relationship between the gut microbiota and low-grade inflammation was first

reported when studying colitis, which is inflammation in the colon. In this study, they found mice

lacking gene expression for TLR5, a receptor that detects bacterial flagellin, were more

susceptible to developing colitis. However, less than 30% of the TLR5 deficient mice actually

developed colitis, the majority of them failed to show any histopathological characteristics

associated with colitis. What they found in those mice was mild increases in expression of

proinflammatory genes, leading to what was eventually called, low-grade inflammation

(Chassaing,B. 2014 CT Vijay-Kumar 2007). Interestingly, the phenotype of the non colitis TLR5

deficient mice paralleled that of metabolic syndrome seen in humans. Specifically, increased

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body mass (mostly fat mass), followed by increased macrophage infiltration in adipose tissue,

which lead to a deposit of proinflammatory cytokines to the adipose tissue. The non colitis TLR5

deficient mice also developed hypertension and were more susceptible to insulin resistance and

impaired glucose tolerance. The mechanism behind the development of metabolic syndrome in

TLR5 deficient mice was hypothesized to be driven by the increased levels of proinflammatory

cytokines brought on by the inflamed state which was caused initially by alterations in the gut

microbiota.

Low bacterial diversity in the gut microbiota has been associated with low-grade

inflammation. When considering overall bacterial diversity in the gut, individuals with LGC or

less bacterial richness were characterized by pro-inflammatory bacteria, Bacteroides and Ruminococcus gnavus (Le Chatelier, E., 2013). Whereas individuals with HGC or greater

bacterial richness in the gut was associated with an increased abundance of anti-inflammatory

species, such as Faecalibacterium prausnitzii (Le Chatelier, E., 2013). The gut microbiota from children living in rural Burkina Faso who consumed a diet high in fiber and plant-based

polysaccharides also were enriched with SCFA producing bacteria, like Faecalibacterium prausnitzii , and also exhibited a lower prevalence of inflammation (De Filippo, C. 2010).

Metabolic Endotoxemia

Increased plasma levels of lipopolysaccharide (LPS), a cell wall component derived from

gram negative bacteria, is closely tied to the development of obesity (Carnahan, S., 2014)

(Shen, Jian, 2013) (Cani, P.D., , 2007). Elevated levels of circulating LPS is known as

metabolic endotoxaemia and is associated with many negative health effects, such as insulin

resistance, glucose intolerance, fasting insulinaemia, increased adipose tissue and body weight

gain (Cani, P.D., , 2007)(Carnahan, S., 2014). LPS is a product of the gut microbiota and its

plasma concentration is a major driver in inflammation and an indicator for reduced gut barrier

integrity (Pendyala, Swaroop 2012). The inflamed state observed with metabolic endotoxaemia

is characterized by increased pro-inflammatory cytokines (Carnahan, S., 2014). The

mechanism involves LPS binding to CD14, a coreceptor found on the membrane of host cells.

The endotoxin is then transferred to TLR4, resulting in the activation NF-kβ pathway (Thomson

Reuters). The NF-kβ complex is involved in the production of cytokines, which, when released

into circulation, assists in the infiltration of macrophages in adipose tissue. This influx of

macrophages further induces the NF-kβ expression of cytokines, which leads to an acceleration

in the inflammatory response (Carnahan, S., 2014).

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Reduced levels of Bifidobacteria has been associated with metabolic endotoxemia in

multiple studies (Cani, P.D., , 2007). The relative abundance of circulating LPS is associated

with alterations in the gut microbiota brought on by other factors like diet and BMI.

Gut barrier integrity

Increased levels of circulating LPS appear to be the common denominator between

obesity, inflammation and the gut microbiome. The mechanism in which metabolic endotoxemia

develops is speculated to be the result of impaired gut barrier integrity (Cani, P. D. 2009). In

healthy individuals, the intestinal epithelium acts as a continuous barrier to prevent LPS from

entering circulation;; yet this protective function can be compromised by changes in the gut

microbiota. Such changes include the reduced levels of bifidobacteria, a commensal gut

bacteria that is negatively correlated with plasma LPS and associated with enhanced gut

integrity (Cani, P. D. 2009).

The onset of metabolic endotoxemia seen with obese and diabetic mice was in

accordance with reduced abundance of tight-junction proteins (Cani, P. D. 2009). The proposed

mechanism behind this observation involves increased production of plasma cytokines, like

TNF-α, IL-1α, IL-1β and IL-6. It was proposed that this influx in pro-inflammatory cytokines

further propagated the inflammatory response, which in turn, induced modifications of both

tight-junctional proteins (Cani, P. D. 2009). In this study they found that modifications in the gut

microbiota influenced GLP-2 production, which was strongly associated with altered gut barrier

functions in a mechanism based off of GLP-2 gut peptide (Cani, P. D. 2009). This suggests that obesity promotes metabolic endotoxemia and low-grade inflammation through disruption of

tight-junction proteins involved in gut barrier function.

The gut microbiota and insulin sensitivity and glucose homeostasis

Insulin plays a major role in glucose homeostasis, for it is the only hormone in humans

capable of lowering blood glucose levels (Oh, Susan 2014). Diabetes mellitus is a metabolic

disease described by abnormally high levels of glucose in the blood (Oh, Susan 2014). The

development of type two diabetes (T2D) is closely associated to obesity and inflammation. T2D

is characterized by insulin resistance and impaired β-cell function, resulting in diminished

sensitivity of insulin in peripheral tissue, and inadequate insulin release (Oh, Susan 2014).

There were marked alterations in the gut microbiome of individuals inflicted with T2DM

compared to healthy individuals. One study found reduced levels of butyrate-producing bacteria

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Roseburia and Faecalibacterium prausnitzii in T2DM patients compared to healthy subjects (Qin, J., 2009). There was an increase in abundance of opportunistic pathogens, like

Clostridium hathewayi and Clostridium ramosum in type two diabetics (Shen, Jian, 2013 CT Qin , 2012).

Obesity is associated with insulin resistance, and has been shown to disrupt epithelial

barrier function resulting in the activation of toll-like-receptor 4 (TLR4) by a LPS-dependent

mechanism (Chassaing,B. 2014). The activation of TLR4 by LPS induces the release of

cytokines that promote insulin resistance (Chassaing,B. 2014 CT Cani 2007). The signaling of

TLR4 prompts the upregulation of inflammatory pathways, like nuclear factor-κB (NF-κB), that

are involved in the development of insulin resistance (Caricilli, Andrea M 2013).

Diet-induced changes in gut microbiota composition, function, and

metabolism

Diet and microbial diversity in the human gut

The influence of diet on the composition and activity of the gut microbiota has gained a

lot of interest as the incidences of obesity and other metabolic diseases continue to increase.

The extent of diet’s influence on composition is not fully known. Comparison of human gut

metagenomes have found that the gut microbiota can be distinguished by distinct bacterial

clusters, or enterotypes, which demonstrate high association to certain diets. These enterotypes

can be distinguished by the dominant phyla in the individual's gut microbiota (Wu, G.D., , 2011),

and recent studies suggest enterotypes could be potential indicators for disease.

Plant-based and animal-based diets

Several studies have examined the alterations in the gut microbiota brought on by

animal-based and plant-based diets. An animal-based diet is characterized as being high in fat

and protein, whereas a plant-based diet is one high in carbohydrates and fiber. Clusters of

specific gut bacteria demonstrate a strong associations with these two contrasting diets. Wu

found that these enterotypes could be distinguished by the relative abundance of Bacteroides

and Prevotella (Wu, G.D., , 2011). Bacteroidetes and Actinobacteria were associated with fat,

and carbohydrates and fiber were associated with Firmicutes and Prevotella (Wu, G.D., , 2011).

There were six different genera that differed between the two enterotypes examined in the

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study. The Prevotella enterotype included Paraprevotella, a genus within the Bacteroidetes, and Catenibacterium, a genus within Firmicutes. The Bacteroides enterotype included Alistipes and Parabacteroides, all of the Bacteroidetes phylum(Wu, G.D., , 2011). Greater consumption of

animal-based products, as seen in the western diet, was associated with the Bacteroides enterotype. This enterotype is highly associated with animal protein, various amino acids, and

saturated fat. The opposite was seen for the Prevotella enterotype, which demonstrated a high affinity for carbohydrates and simple sugars. This enterotype demonstrated a strong association

to carbohydrate-based diets, typical of plant-based diets (Wu, G.D., , 2011).

One study analyzed the gut microbiota of children living in Europe (EU) compared to

children living in a rural African village in Burkina Faso (BF), and discovered distinct differences

in enterotypes of the two populations (De Filippo, C. 2010). The european subjects consumed a

typical western diet, one high in animal protein, sugar, fat, and low in fiber. The BF subjects ate

a diet resembling that of Neolithic subsistence farmers, which unlike the European diet, was low

in fat and animal protein, and high in fiber and other plant polysaccharides (De Filippo, C. 2010).

Actinobacteria and Bacteroidetes were the dominant phyla in the BF children’s microbiota,

whereas Firmicutes and Proteobacteria were greater represented in the EU children’s

microbiota. The ratio between Firmicutes and Bacteroidetes showed that the Firmicutes were

twice as abundant in the microbiota of EU children compared to their BF peers.

Contrary to the enterotype observed in European children, the enterotype observed in

African children was characterized by the Prevotella genus (De Filippo, C. 2010). Two genera within Bacteroidetes;; Prevotella and Xylanibacter, and Treponema of the Spirochaetes were absent from the microbiota of the EU children, but present in the microbiota of the BF children.

This particular bacterial community seen exclusively in BF children are SCFA-producers. It was

speculated that the BF diet, which was rich in plant polysaccharides and low in sugar and fat,

could selectively promote the growth of SCFA-producing bacteria. The greater abundance in

SCFA-producers observed in the BF children could possibly protect them from the colonization

of potentially pathogenic microbes, like Enterobacteriaceae, Shigella, and Escherichia. Interestingly, there potential pathogens which were much more abundant in the gut microbiota

of EU children than in BF children (De Filippo, C., 2010).

Interestingly, there was a major divergence in gut microbiota composition of the two

populations that occurred when the diet shifted from breast milk to solid foods. This observation

continued to diverge ultimately leading to two different gut microbiota profiles. The BF children’s

gut microbiota was comprised of more gram negative bacteria (Bacteroidetes), whereas the EU

15


children were much more abundant in gram positive bacteria (Firmicutes). This observation

nicely depicts the impact of the dietary and environmental influences that alter the

Firmicutes/Bacteroidetes (F/B) ratio (De Filippo, C. 2010). The overall bacterial diversity was

less in the microbiomes of EU children compared to BF children. De Filippo believed that the

reduced microbial diversity demonstrated the impact of a HF/HS diet on the “adaptive potential”

of the gut microbiota of children in developed societies (De Filippo, C. 2010).

There was another study that examined the short-term changes in the human gut

microbiota brought on by the animal-based and plant-based diets. It was discovered that the

animal-based diet had a greater effect on the overall gut microbial diversity than the plant-based

diet. Alterations in the subject’s gut microbiota became detectable in just 24 hours on the

selected diets, and became more prominent over time (David, A. L., 2014). The animal-based diet lead to significant changes in the relative abundance of 22 clusters after five days, whereas

only 3 clusters significantly altered in their abundance after five days on the plant-based diet

(David, A. L., 2014). They found that the animal-based diet promoted the growth of bile-tolerant

bacteria, like Alistipes, Bilophila and Bacteroides and discouraged the growth of Firmicutes involved in plant polysaccharide fermentation, like Roseburia, Eubacterium rectale, and Ruminococcus bromii .

There was an observed differences in gene expression followed the taxonomic shifts

seen with the animal-based diet compared to the plant-based diet. The animal-based diet was

associated with the increased expression of β-lactamase and genes involved in vitamin

biosynthesis and polycyclic aromatic hydrocarbons degradation, which are carcinogenic

compounds produced during the charring of meat (David, A. L., 2014). Another study examined

the short-term change induced by the two diets and found that the differences in the gut

microbiota activity reflected the diet type (David, A. L., 2014). Analysis of fecal SCFA indicated

alterations in microbial activity resulting from the different diets. Compared to the plant-based

diet, the animal-based diet produced significantly lower levels of fermented carbohydrate-based

metabolites, compensating the loss of SCFA with increased amino acids fermentation (David, A.

L., 2014).

When comparing the metagenomes of the western diet to rural African diets, it was

discovered that the westernized diet was enriched in genes associated in enzymes involved in

the degradation of amino acids and simple sugars (Hannelore, D., 2014 CT Yatsunenko

2012). Mice fed a typical diet high in sugar and fat (HF/HS) were enriched in genes associated

with glutamate metabolic pathways. It was suggested that the HF/HS microbiota has increased

16


fitness for the transport and conversion of simple sugars and glycoproteins. This is based off of

the occurrence of four enzymes;; 2-dehydro-3-deoxygluconokinase, 6-phosphofructokinase,

N-acetylglucosamine-6-phosphate deacetylase and two sugar-binding proteins, that were all

present in the gut microbiome of mice fed a western diet (Hannelore, D., 2014 CT Turnbaugh ,

2008].

The high fat diet

Several studies have explored alterations in the gut microbiota brought on by a high fat

diet. Hannelore found that in addition to increased body weight gain, fasting hyperglycemia, and reduced cecal mass, mice fed a HF diet for 12-weeks also underwent significant changes in

their gut microbial composition (Hannelore, D., 2014). The Firmicutes were the dominant

phylum, making up approximately 71-98% of the total sequences. That being said, there was a

significant decrease in Ruminococcaceae of the Firmicute phylum. This decrease in Ruminococcaceae is expected as they are a major utilizer of plant polysaccharides;; a food component lacking in the experiment being described. There was an increase in Rikenellaceae, of the Bacteroidetes phylum in HF mice. Overall, there was no reported decrease in

Bacteroidetes after HF feeding. Mice fed a HF diet, in general, showed no significant change in abundance of lactobacilli compared to the control group (Hannelore, D., 2014). The HF-diet altered 19 distinct species. There was an increase in 2 Alistipes species (Hannelore, D., 2014). The majority of the analyzed species that decreased after HF feeding belonged to the

Clostridiales order (Hannelore, D., 2014). HF feeding appeared to promote two Clostridium species, and one species belonging to the Clostridium cluster XIVa (Hannelore, D., 2014).

When analyzing cecal samples of the mice fed a HF diet, it was reported that the HF diet

altered the biochemical environment and microbial activity of the gut. This study found that the

HF-diet induced changes in the biochemical fingerprints of the bacteria belonging to the

Bacteroidetes and Lachnospiraceae (Hannelore, D., 2014). The HF diet impact on the gut metaproteome activity was also analyzed. Interestingly, they reported that the majority of

proteins in the metaproteome was involved in carbohydrate metabolism. As a consequence of

this, it appeared that the gut microbiota is not equipped for adequate energy harvest for diets

where dietary fat exceeds 60% of calories needed for dietary energy intake. The gut bacteria

responded to the HF diet by reducing the expression of protein associated with carbohydrate

metabolism, and shifting the focus to amino acid metabolism (Hannelore, D., 2014). These

findings are consistent with the increased protein to carbohydrate ratio observed in HF diets

17


compared to CARB diets (1:1 vs 1:3). This is also in accordance with the increased production

of branched-chain FA from leucine, isoleucine, and valine. The focus on amino acid

fermentation is consistent with previously made associations between HF feeding in humans

and protein fermentation in vitro (Hannelore, D., 2014 CT MacFarlane , 1992;; Russell , 2011).

Similar to the LGC microbiome, HF feeding led to alterations that increased oxidative stress

tolerance by increasing the activity of three enzymes;; glutaredoxin, alkyl hydroperoxide and

thioredoxin reductase (Hannelore, D., 2014 CT Xiao , 2010).

The effects of a diet high in saturated fat the development of metabolic endotoxemia was determined by comparing the LPS levels of western diet to a prudent diet (Pendyala, Swaroop

2012). The prudent diet was low in fat and high in fiber, and of equal calories to the western

diet. There were significant differences in plasma LPS levels of the eight healthy subjects over

the duration of four weeks (Pendyala, Swaroop 2012). The western-diet lead to significant

increases (71%) in plasma endotoxin levels, whereas the prudent diet resulted in a 38%

decrease in endotoxin activity. The prudent-diet also showed significant decreases in the level

of pro-inflammatory chemokines and cytokines, like TNF-α, IL-8 and in the protein MCP-1

(Pendyala, Swaroop 2012). There are striking similarities between the inflammatory response induced by the HF-diet

compared to LPS. This observation is in accordance with the belief that elevated plasma LPS

may be the molecular link between HF feeding, microbiota, and inflammation (Cani, P.D., , 2007). It’s been established that feeding mice a high fat diet can trigger an inflammatory

response (Cani, P.D., , 2007). One study found that HF feeding of mice increased levels of

pro-inflammatory cytokines, specifically, plasma IL-1α, IL-1β, and IL-6 concentrations (Cani,

P.D., , 2007). They also found increases in adipose tissue mRNA concentrations of

proinflammatory cytokines, TNF-α, and PAI-1 after HF feeding, further supporting the

association between a high fat diet and low-grade inflammation (Cani, P.D., , 2007).

The gut microbiota impact on gut integrity has been tested in previous studies. The HF

diet demonstrated diminished gut barrier function in obese and diabetic mice (Cani, P. D.

2009). It was also associated with increased plasma LPS levels and low-grade inflammation

(Cani, P. D. 2009). The increase in intestinal permeability followed alterations in the gut

microbial composition brought on by the HF-diet. Specifically the decrease in bifidobacteria, a

group of bacteria associated with reduced LPS levels and improved mucosal barrier function

(Cani, P. D. 2009). The compromised gut barrier function may be due to decreased activity of

ZO-1 and occludin, two tight-junction proteins found on the plasma membranes of host cells

18


(Cani, P. D. 2009). Immunofluorescence assay performed on the intestines of

chow-fed-wild-type mice found a greater abundance localized network of ZO-1 and occludin

proteins (Cani, P. D. 2009 CT Brun P, Castagliuolo I, Leo VD, , 2007). This starkly contrasted

the reduced and discontinuous ZO-1 and occludin protein network observed in the obese

control group tissue (Cani, P. D. 2009). Tight-junction proteins mRNA was negatively correlated

with gut permeability biomarkers used in the immunofluorescence assays (Cani, P. D. 2009).

This suggest that the HF-diet promotes metabolic endotoxemia by way of increasing gut

permeability, resulting in elevated plasma LPS and low-grade inflammation.

Dietary fat type

There were significant alterations in the composition of the gut microbiota based off of

the dietary lipid type. Mice fed fish oil had increased Lactobacillus character. This particular genera is an established probiotic, marketed towards reducing inflammation and treating IBD

(Caesar, Robert 2015). There was also an observed increase in Akkermansia muciniphila, a bacterial species associated with improved gut barrier function and glucose metabolism

(Caesar, Robert 2015). Akkermansia muciniphila was negatively correlated with fat mass gain and WAT macrophage infiltration in obese mice (Caesar, Robert 2015). A similar decrease in

Akkermansia genus was observed individuals with LGC, as discussed in a previous section. The mice fed a diet high in saturated fat displayed an increase in Bilophila, as seen with the animal-based diet. Previous studies have associated Bilophila wadsworth with increased colitis (Caesar, Robert 2015). These changes demonstrate the significance of dietary fat type on the

community structure of the intestinal microbiota. This study highlighted the impact of dietary fat

on both composition and diversity of the gut microbiota (Caesar, Robert 2015).

When discussing the manifestation of metabolic diseases, the type of dietary fat appears

to significantly influence the susceptibility and development of the disease. Although the

association between fat and metabolic endotoxemia is well established, it does not consider the

effects of the type of fat being consumed. One study comparing the influence of different dietary

lipids found that mice fed a lard-based diet had higher serum levels of LPS than compared to

mice fed fish-oil, highlighting the impact of saturated fats on metabolic endotoxemia (Caesar,

Robert 2015).

Similar distinct effects of fat type appear when applying the same criticalness to

development of inflammation. One study demonstrated that lard-fed mice had increased

expression of TNF-α in both adipocytes and macrophages compared to mice fed a fish-oil-diet

19


(Caesar, Robert 2015). Mice fed lard showed increased TLR4 activity (Caesar, Robert 2015).

It’s speculated that TLR4 signalling, which is associated with inflammation and insulin

resistance, is activated by saturated FA found in greater quantity in lard compared to fish oil

(Caesar, Robert 2015). The lard-based-diet showed significantly higher CCL2 levels when

compared to fish-oil diet. CCL2 is a chemokine that undergoes secretion when activated by TLR

ligands. The overexpression of CCL2 resulted in white adipose tissue (WAT) inflammation and

insulin resistance (Caesar, Robert 2015). In this particular study, Caeser demonstrated CCL2 vitalness in WAT macrophage infiltration, thus fortifying the link between the gut microbiota to

WAT inflammation (Caesar, Robert 2015). All of these finding highlight the impact of fat type on the gut microbiota and the development of low-grade inflammation.

HF-feeding reported significant increase in fasting insulinaemia and glucose intolerance

in mice (Cani, P.D., 2007). Diabetic individuals fed a HF-diet were enriched with Alistipes (Hannelore, D., 2014 CT Qin , 2012). This is consistent with previous studies that Alistipes with HF diets. The HF diet appeared to interfere with glucose-induced insulin secretion, which was

severely reduced in mice fed a HF diet. Cani et al. noted a positive correlation with insulin resistance and plasma endotoxin levels (Cani, P.D., , 2007). Similarly, there were significantly

higher plasma endotoxins levels and inflammatory markers, like TNF-α, in obese and diabetic

mice (Cani, P.D., , 2007 CT Brun ). The cytokine, TNF-α, disrupts insulin sensitivity by

phosphorylating serine residue substrate (IRS-1) from the insulin receptor, leading to its

inactivation (Cani, Patrice D 2009). The HF-diet was associated with greater levels of ceramide

(N-acylsphingosine) in cecal samples of mice. Ceramide is a fatty acid component of cell

membrane and have been associated in diet-induced insulin resistance (Hannelore, D., 2014

CT Longato , 2011).

Potential treatments

Prebiotics

Prebiotics are nondigestible carbohydrates that are fermented by the gut microbiota

(Carnahan, S., 2014). The criteria for prebiotics requires them to tolerate gastric acidity,

withstand host enzymes, and avoid absorption in the upper gastrointestinal tract while

selectively promoting the growth of beneficial gut bacteria (Carnahan, S., 2014). For example,

insulin and fructooligosaccharides (FOS) are two well studied inulin-type-fructan (ITF) prebiotics,

20


that are effective in stimulating growth of health-promoting genera like Bifidobacterium and Lactobacillus (Cani, Patrice D 2009). The majority of studies conducted on prebiotics focus on enriching the bifidobacterium genus which are considered beneficial to human health (Hildebrandt, Marie A 2009). Prebiotics counter-acted diet induced metabolic endotoxemia by

promoting the Bifidobacteria (Cani, Patrice D 2009). Prebiotic treatment normalized low-grade

inflammation by decreasing metabolic endotoxemia through reduced plasma LPS. This

decreased the amount of proinflammatory cytokines in circulation and in the tissue. There was

an improvement in insulin sensitivity and steatosis from the prebiotic treatment (Cani, Patrice D

2009).

Recently, a lot of interest in treatment and prevention of disease through targeted

modification in the gut microbiota by prebiotic supplements.The treatment potential of

dietary-fiber prebiotics include improved gut barrier function and host immunity, reduced

pathogenic bacteria, and enhanced SCFA production (Slavin, J., 2013). Growing evidence is

depicting prebiotics as potential treatment for metabolic syndrome and metabolic endotoxemia.

Prebiotics and gut microbial composition

Prebiotics can alter the gut microbiota by selectively promoting the growth of a specific

bacteria, altering the composition and diversity of the gut. There were significant differences in

the bacterial profile of the gut of obese mice fed a dietary fiber prebiotic compared the obese

microbiome standard fed a standard chow diet (Geurts, L, , 2013). The effects of the HF diet

appear to be blunted by probiotics. The HF-diet showed to significantly decrease the

bifidobacteria content in the gut of humans and mice(Cani, P.D., , 2007). Supplementation of

the prebiotic oligofructose [OFS] to a HF-diet restored the quantities of bifidobacteria in mice (Turnbaugh, P. J. 2009)]. Inulin-type-fructans (ITF) and arabinoxylan oligosaccharides (AXOS) also increased bifidobacteria levels (Schwiertz, A., , 2010). The increase in bifidobacteria observed with ITF treatment appeared to be at the expense of the Bacteroides (Geurts, L, , 2013).

ITF showed to increase Firmicutes and Actinobacteria (Dewulf, E.M. 2012). The increase

in Firmicutes attributed to increases in Bacilli and Clostridium clusters IV and XVI. Bacilli are negatively associated with LPS levels (Dewulf, E.M. 2012). ITF supplementation lead to

reductions in two bacteria involved in fat metabolism, Propionibacterium and Bacteroides vulgatus (Geurts, L, , 2013) (Wu, G.D., , 2011). ITF increased Faecalibacterium prausnitzii , a bacteria that is negatively correlated with metabolic endotoxemia (Schwiertz, A., , 2010).

21


Prebiotics effects on metabolic endotoxemia

Prebiotics counter-acted diet induced metabolic endotoxemia by promoting bacteria that

is negatively associated with circulating LPS (Cani, Patrice D 2009). ITF-treatment increased

Faecalibacterium prausnitzii , a bacteria that is counteracts metabolic endotoxemia. Supplementation of OFS to HF diet normalized plasma LPS

levels (Cani, P.D., , 2007). The prebiotic treatment

counteracted metabolic endotoxemia by reducing the level of

circulating endotoxins. This reduced the amount of

proinflammatory cytokines in circulation and was followed by

improved insulin sensitivity and reduced inflammation (Cani,

Patrice D 2009).

Prebiotics effects on gut permeability

Prebiotic feeding improved the tight-junction integrity of the

epithelial cells lining the intestines (Cani, P. D. 2009)]. Obese

mice treated with prebiotics had greater abundance of ZO-1

and occludin proteins localized along the host cell membrane

compared to obese mice (Cani, P. D. 2009). Prebiotic-fed

mice had lower level of TNF-α, IL-1β, IL-1α and IL-6, all

cytokines known to promote tight-junction disruption (Cani, P.

D. 2009).

One possible mechanism behind prebiotics protective

effect on gut barrier permeability is by the promotion of

Bifidobacterium species (Cani, P. D. 2009). Unlike other bacteria, Bifidobacterium doesn’t degrade intestinal mucus

glycoproteins, and improve gut barrier integrity (Cani, P. D. 2009). Bifidobacterium promote healthy microvilli and are associated with thick mucosal layer in the intestine

(Cani, P. D. 2009).

Obese mice administered a dietary-fiber prebiotic had increased proglucagon mRNA in

their intestine (Cani, P. D. 2009). The prebiotic treatment induced alterations in the gut

microbiota composition, a shift that promoted GLP-1 and GLP-2 synthesis. Gut peptide

hormones, GLP-1 and GLP-2 are associated with increased satiety and improved mucosal

22


barrier function, respectively (Cani, P. D. 2009). The improved gut barrier observed with

prebiotic treat was correlated to the lower portal plasma LPS levels and inflammation (Cani, P.

D. 2009) (Figure 3.).

Prebiotics effects on inflammation

Prebiotic supplementation has been shown to effectively abate inflammation through

activation of G-coupled protein receptor GPR43. Prebiotics increase levels of short chain

carboxylic acids in the colon and circulating in the plasma, which activates GPR43. Activation of

GPR43 results in a decrease in lipolysis and free fatty acids in plasma. The reduction of free

fatty acids decrease their interaction with membrane TLR, which subdue the inflammatory

response.

Supplementation of AXOS is believed to discouraged metabolic endotoxemia through

increased production of anti-inflammatory cytokine in serum ( Neyrinck, AM., , 2012). It is

speculated that the prebiotic increases expression of IL-10 and reduces macrophage infiltration

in adipose tissue ( Neyrinck, AM., , 2012).

Another study found that treatment of OFS to mice restored the increased levels of

pro-inflammatory cytokines IL-1α and IL-6 that resulted from a HF feeding (Cani, P.D., , 2007).

The increased mRNA concentrations of TNF-α, and PAI-1(Serpine1) in adipose tissue reported with HF feeding was normalized after OFS supplementation (Cani, P.D., , 2007).

Prebiotics and insulin sensitivity

Supplementation of OFS to the HF-diet appeared to reverse the glucose intolerance

brought on by the HF-diet in mice. Additionally, glucose-induced insulin secretion and normal

fasting plasma insulin levels also improved after administration of the OFS prebiotic to the

HF-diet (Cani, P.D., , 2007). AXOS supplementation demonstrated improved

fasting-hyperinsulinemia and HOMA-IR brought on by a HF diet ( Neyrinck, AM., , 2012).

Prebiotics effects on satiety and energy intake

Studies comparing lean and obese mice establish the gut microbiota influence on host

energy balance. Promoting weight loss through directed changes in the gut microbiota brought

on through prebiotic treatment is appearing as an attractive method for the treatment of obesity.

One study found that fructooligosaccharides (FOS) supplementation reduced energy intake and body weight in overweight and obese subjects by suppressing ghrelin levels and

23


enhancing PYY levels (Slavin, J., 2013). Similar results were found with supplementation of

AXOS to a high fat diet, which increased activity in satieogenic peptides GLP-1 and PYY and

blunted body weight gain, fat mass development as a result ( Neyrinck, AM., , 2012).

The 2.5-fold increase in body weight gain and 3- to 4-fold in fat mass after HF-feeding in

mice was significantly reduced following OFS supplementation (Cani, P.D., , 2007). Additionally,

HF-OFS mice resulted in decreased energy intake. OFS prebiotic also appeared to stimulate

GLP-1 production (Cani, P.D., , 2007).

Conclusion

Obesity is a disease associated with a cluster of several metabolic disorders like insulin

resistance, hyperglycemia, and hyperlipidemia (Chassaing,B. 2014). This cluster of metabolic

disorders is known as metabolic syndrome and is related to increased risk of diabetes,

cardiovascular diseases, and liver dysfunction (Cani, Patrice D 2009). The gut microbiota has

an established role in the development of obesity. Obesity is associated with changes in the gut

microbiota and is shown to reduced bacterial diversity and altered metabolic pathways (Slavin,

J., 2013). Gut dysbiosis has been characterized by a shift from SCFA producing microbes, like

bifidobacteria, to opportunistic pathogens (Qin, Junjie 2012). Reduced levels of bifidobacteria

has been associated with metabolic endotoxemia in multiple studies (Cani, P.D. 2007).

Elevated levels of circulating LPS is known as metabolic endotoxaemia and is

associated with many negative health effects, such as insulin resistance, fasting insulinaemia,

increased adipose tissue and body weight gain (Cani, P.D., , 2007, Carnahan, S., 2014). LPS is

a product of the gut microbiota and its plasma concentration is a major driver in inflammation

and an indicator for reduced gut barrier integrity (Pendyala, S. 2012). The inflamed state

observed with metabolic endotoxaemia is characterized by increased pro-inflammatory

cytokines (Pendyala, Swaroop 2012). It was proposed that the HF-diet promotes metabolic

endotoxemia by way of increasing gut permeability, resulting in elevated plasma LPS and

low-grade inflammation.

Prebiotic intervention appeared to improve gut barrier function, reduce metabolic

endotoxemia and promote beneficial gut bacteria. It appears that alterations in the gut

microbiota with prebiotics may participate in the control of the development of metabolic

diseases associated with obesity (Cani, Patrice D 2009). For this reason, it would be useful to

24


further explore the potential of alterations in the gut microbiota that discourage the harmful

effect of the high-fat or obesity-induced metabolic diseases.

25


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gut microbiota and endotoxemia

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