Review Article Role of Gut Microbiota in the Aetiology of Obesity: … · 2019. 7. 30. · Stomach, small and large intestine gut permeability and apelin and APJ mRNA expression Hormonal

Review ArticleRole of Gut Microbiota in the Aetiology of Obesity:Proposed Mechanisms and Review of the Literature

Muhammad Jaffar Khan,1,2 Konstantinos Gerasimidis,2

Christine Ann Edwards,2 and M. Guftar Shaikh3

1 Institute of Basic Medical Sciences, Khyber Medical University, Phase V Hayatabad, Peshawar, Khyber Pakhtunkhwa, Pakistan2Human Nutrition, School of Medicine, Dentistry and Nursing, College of Medical Veterinary and Life Sciences, University of Glasgow,Level 3, New Lister Building, Glasgow Royal Infirmary, 10-16 Alexandra Parade, Glasgow G31 2ER, UK3Department of Endocrinology, Royal Hospital for Children, 1345 Govan Rd, Govan, Glasgow G51 4TF, UK

Correspondence should be addressed to Muhammad Jaffar Khan; [email protected]

Received 1 February 2016; Revised 21 May 2016; Accepted 21 August 2016

Academic Editor: R. Prager

Copyright © 2016 Muhammad Jaffar Khan et al.This is an open access article distributed under the Creative CommonsAttributionLicense, which permits unrestricted use, distribution, and reproduction in anymedium, provided the originalwork is properly cited.

The aetiology of obesity has been attributed to several factors (environmental, dietary, lifestyle, host, and genetic factors); howevernone of these fully explain the increase in the prevalence of obesity worldwide. Gut microbiota located at the interface of host andenvironment in the gut are a new area of research being explored to explain the excess accumulation of energy in obese individualsand may be a potential target for therapeutic manipulation to reduce host energy storage. Several mechanisms have been suggestedto explain the role of gut microbiota in the aetiology of obesity such as short chain fatty acid production, stimulation of hormones,chronic low-grade inflammation, lipoprotein and bile acid metabolism, and increased endocannabinoid receptor system tone.However, evidence from animal and human studies clearly indicates controversies in determining the cause or effect relationshipbetween the gut microbiota and obesity. Metagenomics based studies indicate that functionality rather than the composition ofgut microbiota may be important. Further mechanistic studies controlling for environmental and epigenetic factors are thereforerequired to help unravel obesity pathogenesis.

1. Introduction

Initial Evidence of the Role of Gut Microbiota in Obesity.The worldwide increase in obesity has prompted researchersto investigate its aetiology which is multifactorial, involvingenvironmental, dietary, lifestyle, genetic, and pathologicalfactors. Although the gut microbiota were already estab-lished as a metabolic organ that could ferment nondi-gestible dietary components (particularly nondigested carbo-hydrates) to generate short chain fatty acids (SCFA), their roleas a significant environmental factor affecting host adipositythrough an integrated host signalling pathway was exploredin 2004 by Bäckhed and colleagues [1]. This breakthroughevidence suggested that the gut microbiota induced adiposityby stimulating hepatic de novo lipogenesis and triglyceridestorage through carbohydrate response element binding pro-tein (ChREBP) and sterol response element binding protein

1 (SREBP1) and by suppressing fasting induced adipocytefactor (fiaf ) which is an inhibitor of adipocyte lipoproteinlipase [1].The same group proposed that this intestinal “high-efficiency bioreactor” in certain individuals might promoteenergy storage (obesity), whereas a low-efficiency reactorwould promote leanness due to lesser energy harvest fromcarbohydrate fermentation [2]. Differences in the gut micro-biota between obese and lean people were therefore worthyof further exploration.

Subsequent studies conducted by the same group sug-gested that although gut microbiota communities wereshared between mothers and offspring regardless of ob geno-type in genetically obese leptin deficient C57BL/6J ob/obmiceand lean mice (ob/+ and +/+ wild-type siblings) fed similarpolysaccharide rich diets, the ob/obmice had reduced relativeabundance of Bacteroidetes (by 50%) and a proportionalincrease in Firmicutes regardless of kinship [3]. A higher

Hindawi Publishing CorporationJournal of ObesityVolume 2016, Article ID 7353642, 27 pageshttp://dx.doi.org/10.1155/2016/7353642

2 Journal of Obesity

Firmicutes to Bacteroidetes ratio was therefore suggestedto be associated with increased energy harvest from foodfacilitated by the gut microbiota. However, no evidence waspresented to show increased expression of genes related tobacterial metabolic activity and how this could be affected bydiet and lifestyle norwhether these changes could also be seenin humans.

Turnbaugh et al. (2006) used whole genome shotgunmetagenomic and microbiota transplantation studies toinvestigate the mechanisms [4]. They observed a high Firmi-cutes rich microbiome in ob/ob mice clustered together (innonmetric multidimensional scale plot), richer in enzymesfor degradation of polysaccharides, higher faecal acetateand butyrate, and less stool energy loss than in lean mice.Transplantation of gut microbiota from ob/ob mice or leanmice to germ-free mice resulted in obese (high Firmicutes)or lean (high Bacteroidetes) gutmicrobiome in the recipients.Obese microbiome recipients had higher percentage body fatdespite similar food intake.

In a human study [5], obese adults were randomisedonto fat or carbohydrate restricted diets and followed upfor one year. Despite marked interpersonal variations ingut microbiota diversity, obese people had a lower relativeabundance of Bacteroidetes and a higher relative abundanceof Firmicutes before the restricted calorie intake. However,over the period of follow-up, the relative abundance ofBacteroidetes significantly increased while that of Firmicutessignificantly reduced. Increased Bacteroidetes was signifi-cantly positively correlated with percentage weight loss andnot the caloric content of diet [5]. This suggested that the gutmicrobiota restructured, changing their metabolic prioritiesto support coexistence in a changed environment. However,this study did not explore the same relationship in a parallellean group to see whether the lean phenotype had the sameresponse to dietary intervention.

Further evidence suggested the presence of the gutmicrobiota was necessary for development of obesity asgerm-free mice were resistant to obesity even when theyconsumed more calories from normal chow or a high fatWestern-type diet comparedwithCONVmice [13]. However,this idea was challenged in a later study by Fleissner et al.(2010) [14] who found that germ-free mice on a high fatdiet gained significantly more weight and body fat and hadless energy expenditure than lean CONV mice. Addition-ally, intestinal fiaf increased in HF and WD fed GF micecompared to CONVmice but not in the systemic circulation[14].

Several possible mechanisms were proposed to explainthe impact of structural and functional differences in gutmicrobiota in lean and obese individuals that may contributeto host adiposity and whether an obese phenotype is trans-missible by transplantation of gut microbiota. However, mostof these studies were conducted in experimental animalswhich exhibited different anatomical, physiological, and bac-terial colonisation patterns from humans. Several humanand animal based studies have now revealed controversialevidence attributing differences in gut microbiota to thedifferences in diet [15–17] while others suggested no suchassociation [18].

2. Proposed Mechanisms for the Role of GutMicrobiota in Obesity

The gut microbiota can be regarded as a “microbial organ”contributing to a variety of host metabolic processes fromdigestion to modulation of gene expression. The differencesin gut microbiota between lean and obese animals or humansubjects suggest a link between gut microbiota and energyhomeostasis although there is still some debate as to whetherthese differences are causally related to an obese or leanphenotype. Various mechanisms have been suggested to linkgut microbiota with obesity-genesis and other metabolicdisorders (Table 1). However, it is still unclear how thesemechanisms interact to influence the overall metabolic statusof an individual.

2.1. Energy Harvest from Diet (Short Chain Fatty Acids).Dietary polysaccharides and proteins that escape digestionin the small intestine are fermented in the colon by thegut microbiota into SCFA mainly acetate propionate andbutyrate. The amount of energy harvested is hypothesised tobe influenced by the composition of the gut microbiota [2]. Ithas been estimated that up to 10%of daily energy requirementand up to 70% of energy for cellular respiration for thecolonic epithelium may be derived from SCFA. Chronicexcess energy harvest may cause long term increased fataccumulation in the body [72].

To a greater extent, there is a general agreement frommany studies that the obese phenotype is associated withexcess SCFA in caecal and faecal samples in animal andhuman studies compared with the nonobese (Table 2).However, there is considerable disagreement and controversyover the population of the gut microbiota that may beassociated with increased caecal or faecal SCFA measured(Table 3). Whether increased SCFA production results inincreased energy harvest from the diet in obese phenotypesdepends on several factors such as substrate availability, guttransit, mucosal absorption, gut health, production by thegutmicrobiota, and symbiotic relationships between differentgroups of gut microbiota [66]. Based on the equation derivedby Livesey (1990), approximately 50% (2 kcal/g) of the energyderived from glucose is available after fermentation. Thenet amount of energy derived will therefore vary dependingupon the amount of indigestible carbohydrate available forfermentation [73].

The obese phenotype in animals is associated with highertotal caecal SCFA, acetate, and butyrate and higher expressionof bacterial genes responsible for polysaccharide metabolism[4]. Increased efficiency in production of SCFA in obesitymight also result from crosstalk between different species andgenera to maintain their growth and population. Absorptionof these SCFA, coupledwith other lifestyle and environmentalfactors may result in excess energy storage and obesity. It isnot clear whether this is an effect of substrate (i.e., carbohy-drates) or the population of specific gutmicrobiota associatedwith increased SCFA production, absorption, and storage inadipose tissues and liver. The results are largely confoundedby the study settings, lifestyle, and environmental factors ofthe study subjects.

Journal of Obesity 3

Table 1: Suggested mechanisms for the role of gut microbiota in the aetiology of obesity.

Proposed mechanism Mediators Source ofmediators Target tissues/organs Local/systemic effects

Metabolic

Increased production ofshort chain fatty acids [1]

Bacterial glycosylhydrolases

Colon, distalileum, and rectum Colonic enterocytes

↑ energy harvestEnergy for colonocytesAlteration in cholesterol

metabolismMuscle fatty acid oxidation

[1] ↓ AMP kinase Small intestine Muscle, liver↓muscle fatty acid

oxidation

Bile acid circulation [19] Secondary bile acidproduction Colon ColonReverse cholesterol

transportExpression of liver

ChREBP/SREBP-1 [1]↑ glucoseabsorption Liver Liver ↑ hepatic lipogenesis

Inflammatory

Chronic low-gradeinflammation [9]

LPS, NF-kappaB,and TNF-𝛼mRNA Colon, ileum

Endothelium,hypothalamus?

Metabolic endotoxemiaand hyperphagia

↑ endocannabinoid (eCB)system tone [10, 20] Bacterial LPS Ileum, colon

Stomach, small andlarge intestine

↑ gut permeability and ↓apelin and APJ mRNA

expression

Hormonal

Suppression of Fiaf [1] Colonic L-cells Colon Adipose tissue ↑ lipolysis, ↓muscle fattyacids oxidation

↑ PYY [21] Satiety centre Ileum, colon Hypothalamus↓ appetite, ↓ gastricmotility, and ↓ gut

emptyingExpression of G protein

coupled receptors 41 and 43(GPR41 and GPR43) [22]

SCFA (acting as aligand)

Colon, distalileum, and rectum Liver, brain

↑ peptide YY (PYY), ↑ denovo hepatic lipogenesis

AMP: adenosinemonophosphate, ChREBP: carbohydrate response element binding protein, SREBP-1: sterol response element binding protein-1, PYY: peptideYY, LPS: lipopolysaccharide, NF-kappaB: nuclear factor-kappaB, TNF-𝛼: tumour necrosis factor alpha, mRNA: messenger RNA, GPR41 and GPR43: G proteincoupled receptors 41 and 43, SCFA: short chain fatty acid, and eCB: endocannabinoid.

2.2. Gut Microbiota and Fasting Induced Adipocyte Factor.Fasting induced adipocyte factor or angiopoitein-like protein4 (Fiaf /ANGPTL4) is a target gene for peroxisome receptoractivated proteins (PPARs) and is produced by large intestinalepithelial cells and the liver. Fiaf /ANGPTL 4 inhibits lipopro-tein lipase (LPL) which causes accumulation of fat in periph-eral tissues. Inhibition of fiaf by the gut microbiota with aresultant increase in LPL may be one mechanism for gutbacterial induced host adiposity [1].This is further supportedby studies on GF mice, genetically deficient in fiaf genes(fiaf −/−). Lack of the fiaf gene causes disinhibition of LPLwhich leads to deposition of up to 60% higher epididymalfat compared to germ-free wild-type littermates expressingfiaf genes (fiaf +/+). fiaf /ANGPTL4 is therefore involved inthe regulation of fat storage mediated by the gut microbiota.Controlled manipulation of the gut microbiota may alterthe expression of this hormone [74]. Normal weight SPFC57B/6J mice were fed either with high fat (20%) diet or highfat diet supplemented with probiotic Lactobacillus paracaseiF19 for 10 weeks. Compared to the nonsupplemented group,plasma fiaf /ANGPTL4 was upregulated in the Lactobacillusparacasei F19 supplemented group with significantly elevatedplasmaVLDL but no change in other lipoproteins. In anotherstudy, Lactobacillus paracasei F19 and Bifidobacterium lactisBB12 were found to upregulate ANGPTL4 in the coloncarcinoma HCT116 cell line in a dose and time dependentmanner while Bacteroides thetaiotaomicron had no effect[74]. In the same study, the authors fed germ-free NMRI

mice with normal chow and exposed them to F19. Theyfound an increasing trend of ANGPTL4 in the serum after2 weeks of colonisation, while the effect was not observedwith heat killed F19 [74]. This study suggested that manip-ulation of expression of fiaf /ANGPTL4 is dependent on thegut microbiota and future interventional studies on weightmanagement can be based on modification of ANGPTL4 bymanipulating the gut microbiota.

Whether the increase in levels of fiaf in systemic cir-culation and the subsequent suppression of LPL and fatstorage is associated with a change in gut microbiota hasbeen questioned in some studies as there was no differencein fiaf in serum of GF and conventionally raised mice [14].GF and CV mice were fed a low fat diet (LF), high fat diet(HF), and commercial high fat Western diet (WD). GF micegained more weight and body fat than CV mice on HF andvice versa on WD. Although intestinal fiaf /ANGPTL4 washigh in GF mice on HF and WD, circulating levels of fiafdid not change significantly compared to CV mice. The gutmicrobiota changed differently with HF andWD in CVmice.These observations suggested that diet affects the type of gutmicrobiota in the gut and that fiaf does not play a major rolein peripheral fat storage as mentioned by other studies.

2.3. Gut Microbiota and Fatty Acid Oxidation. The gutmicrobiota are thought to reduce muscle and liver fatty acidoxidation by suppressing adenosine monophosphate kinase(AMPk), an enzyme in liver and muscle cells that acts as


Table 2: Studies looking at differences in SCFA in faecal or caecal samples in obese versus lean phenotypes in animal and human studies.

Reference Technique used SCFA differences Gut microbiota differences

Turnbaugh et al. 2006[4]

GC-MS,pyrosequencing

↑ caecal acetate and ↑ butyrate in obeseob/ob mice compared to lean

↑ Firmicutes and lower Bacteroidetes in obesethan lean mice. No differences in genera leveldiversity

Zhang et al. 2009[23]

GC, qPCR, andpyrosequencing

↑ acetate in obese compared to lean andgastric bypass group

↑M. smithii and Prevotellaceae in obesecompared to lean and gastric bypass

Schwiertz et al. 2010[18]

GC and qPCR withSYBR Green

↑ total SCFA and propionate (conc. & %) inobese compared to lean

↑ Bacteroides and ↓ Firmicutes, ↓ Ruminococcusflavefaciens, ↓ Bifidobacterium, and↓Methanobrevibacter in obese compared tolean

Payne et al. 2011[24]

qPCR, TGGE, andHPLC

↑ butyrate, propionate, and isobutyrate inobese compared to lean↑ lactate and valerate in lean compared toobeseNo difference in acetate and total SCFA

No difference in Firmicutes and Bacteroidetes,Firmicutes/Bacteroides ratio, Bifidobacteria,Enterobacteriaceae, and sulphate reducingbacteria between lean and obese children↑ Roseburia/E. rectale in obeseHighly variable banding pattern on TGGE forboth obese and healthy

Yang et al. 2013[25] GC

↑ ratio of molar propionate: total SCFA and↓ acetate : SCFA ratio in obese versus lean Not measured

Teixeira et al. 2013[26] GC

↑ acetate, propionate, and butyrate in obeseversus lean womenSCFA correlated with body fat, bloodpressure, waist circumference, insulin, andHOMA index

Not studied

Belobrajdic et al. 2012[27] GC

Increase in total SCFA pool and stool energyirrespective of obese or lean phenotype(obesity prone or obesity resistant) inresponse to 0, 4, 12, and 16% resistant starchdiet for 4 weeks

Not studied

Rahat-Rozenbloom etal. 2014[28]

GC

↑ total SCFA, acetate, and butyrate in obesecompared to leanNo differences in isobutyrate, isovalerate,and valerate

↑ Firmicutes : Bacteroidetes ratio in obese.Firmicutes correlated with SCFA in obese

Fernandes et al. 2014[29] GC, qPCR

Significantly ↑ propionate and valerateMarginally ↑ acetate and butyrate

Escherichia Coli higher in lean than obeseNo difference in Bacteroides/Prevotella,Clostridium coccoides and C. leptum group,Bifidobacteria, and total bacteria, F/B ratio

Li et al. 2013[30] GC Higher SCFA in obese than lean ↑ Firmicutes and lower Bacteroidetes in obese

GC: gas chromatography, GC-MS: gas chromatography-mass spectrometry, SPME-GCMS: solid phase microextraction-gas chromatography mass spectrom-etry, v1-v2: variable regions 1 and 2, HPLC: high performance liquid chromatography, TGGE: temperature gradient gel electrophoresis, CHO: carbohydrate,EU: European Union, qPCR: quantitative polymerase chain reaction, and F/B ratio: Firmicutes to Bacteroidetes ratio.

a fuel gauge monitoring cellular energy status. Inhibition ofAMPk results in reducedmuscle and liver fatty acid oxidationultimately leading to excess fatty acids storage in these tissues[1].

Phosphorylated AMPk inhibits the formation of malonylCoA via acetyl CoA carboxylase. Inhibition of malonyl CoAcauses disinhibition of carnitine palmitoyltransferase-1 (Cpt-1) which in turn catalyses the rate limiting step in the entryof long chain fatty acyl-CoA into mitochondria for fatty acidoxidation [75]. Increased fatty acid oxidation is associatedwith enhanced cellular energy status coupled with glycogenlevel reduction and increased insulin sensitivity [75].

Germ-free mice have a consistently raised level of phos-phorylated acetyl CoA carboxylase (Acc) and carnitinepalmitoyltransferase-1 (Cpt-1) activity in gastrocnemiusmus-cles and raised AMPk in liver and skeletal tissue compared

to CONV mice [13, 76]. This effect has also been observedwith high calorie diet suggesting that enhanced or suppressedmuscle fatty acid oxidation is dependent on the presence orabsence of gut microbiota. The gut microbiota may thereforeinfluence storage of peripheral adipose tissue and hence hostadiposity by inhibiting fatty acid oxidation.

2.4. Gut Microbiota and Bile Acids Circulation. Primary bileacids (cholic and chenodeoxycholic acids) are ligands for thefarnesoid x receptor (FXR) which plays a key role in the con-trol of hepatic de novo lipogenesis, very low density lipopro-tein (VLDL) triglyceride export, and plasma triglycerideturnover leading to improved lipid and glucose metabolism[6]. By binding to FXR in ileal cells, bile acids are able tostimulate the expression of genes (Asbt, IBABP, and Ost𝛼/𝛽) which help in absorption, intracellular transport, and


Table3:Ev

idence

from

anim

alstu

dies

abou

tthe

roleof

gutm

icrobiotainob

esity.

Reference

Stud

ymod

elAim

ofthes

tudy

Stud

ydesig

nandou

tcom

esmeasures

Results

Con

clusio

n

Stud

iessuggestin

gasso

ciatio

nofgutm

icrobiota

with

obesity

Fleissnere

tal.

2010

[14]

Malea

dultC3

HGF

andCV

mice

Influ

ence

ofdifferent

dietso

ntheb

ody

compo

sitionof

GFand

CVmice

Adlibitu

mintake

oflowfat(LF

),high

fat(HF),and

commercial

Western

diet(W

D)for

GFand

CVmice.Re

al-timeP

CR,FISH,

andfia

f/angplt4

ingutand

bloo

d

GFmiceg

ainedmorew

eightand

body

fatand

hadlessenergy

expend

iture

than

CVmiceo

nHF.HigherF

irmicutes

(especially

Erysipelotric

haceae)a

ndlower

Bacteroidesin

CVmiceo

nHFandWD.Intestin

alFiaf

increasedin

GFmiceb

utno

change

inplasma

fiaflevelsa

scom

paredto

CVmice

GFmicea

reno

tprotected

from

diet

indu

cedob

esity.D

ietaffectsg

utmicrobiotac

ompo

sitionandfia

fdo

esno

tplay

aroleinfatstorage

mediatedby

gutm

icrobiota

Šefč́ıkováe

tal.

2010

[31]

8–10

pups

pern

est,

Sprague-Daw

leyrats,

from

day21

today40

Effecto

fnormaland

overnu

trition

onthe

developm

ento

fgut

microbiota,intestinal

alkalin

epho

sphatase,

andoccurrence

ofob

esity

Standard

labo

ratory

dietfor

controlgroup

andadditio

nal

milk

basedliq

uiddietforstudy

grou

p.Ba

cterialenu

merationvia

FISH

,alkalinep

hosphatase

activ

ityvia

immun

ocytochemistry

Obese

ratsgained

moree

nergy(25%

)and

high

erbo

dyfat(27%)thanlean

rats.

Alkaline

phosph

ataseincreased

inob

eser

ats.

Lactob

acilliincreased

whileBa

cteroides

decreasedin

obeser

atssignificantly

Thisstu

dymay

providea

baselin

eforfurther

insig

htinto

thew

ayso

finvolvem

entinprogrammingof

asustainedintake

anddigestion

Dingetal.2010

[32]

GF/CO

NVmicea

ndNF-𝜅Bkn

ockinmice

(GF/CO

NV)

Hypothesis:intestin

alinflammationis

prom

oted

bythe

interactionof

gut

bacteriaandhigh

fat

diet,con

tributingto

the

progressionof

insulin

resistancea

ndob

esity

Highandlowfatd

ietsfor2

,6,or

16weeks.G

Fmicefed

with

diet

after

expo

sureto

faecalslu

rrieso

fCO

NVmice.Bloo

dglucosea

ndEL

ISAforinsulin.T

NF-𝛼mRN

Aexpressio

nby

qPCR

.Expression

ofNFk

Bmiceb

yflu

orescent

light

microscop

y

CONVmiceg

ainedmorew

eightthanGF.

Increasedexpressio

nof

TNF-𝛼mRN

Aand

NF-𝜅Bin

CONVHFdietmice.TN

F-𝛼changes

precedew

eightchanges.E

nhancedNF-𝜅Bin

GFNF-𝜅Bmiceo

nfeedingCO

NVNFk

Bfaecal

slurry

HFdietandenteric

bacteriainteract

toprom

oteinfl

ammationand

insulin

resistancep

riortothe

developm

ento

fweightg

ain,

adiposity,and

insulin

resistance

Turnbaug

hetal.

2008

[17]

8-9-week-old

GF/CO

NVmice

Tostu

dythe

interrelationship

betweendiet,energy

balance,andgut

microbiotau

singmou

semod

elof

obesity

Con

ventionalisationof

GFmice

with

HFWestern

dietfollo

wed

byintro

ductionof

Western

orCH

Odietin

CONVmice.

CARB

-reduced

orFA

T-redu

ced

dietsinanothersub

set.qP

CR,

DEX

Ascan,and

weight

measurementsdo

ne

Western

diet-associatedcaecalcommun

ityhad

asignificantly

high

errelativea

bund

ance

ofthe

Firm

icutes

(specificallyMollicutes)a

ndlower

Bacteroidetes.Miceo

ntheW

estern

dietgained

morew

eightthanmicem

aintainedon

the

CHOdietandhadsig

nificantly

more

epididym

alfat.Miceo

nCA

RB-R

andFA

T-R

dietconsum

edfewer

calorie

s,gained

less

weight,andhadlessfat

Thereisrestructurin

gof

gut

microbiotaw

ithWestern

diet,

specifically

redu

ctionof

Bacteroides

andsurgeinMollicutes

classof

Firm

icutes

with

increasedcapacity

toharvestenergyfro

mdiet

Danieletal.

2014

[33]

MaleC

57BL

/6NCr

lmice(𝑛=6,per

grou

p)

Toinvestigatec

hanges

infunctio

nandactiv

ityof

theg

utecosystem

inrespon

seto

dietary

change

LC-M

S/MSform

etaproteom

e,FT

-ICR

-MSform

etabolom

e,Mise

qillum

inap

yrosequencing.

Interventio

nwith

high

fat(HF)

andcontrol(carboh

ydrate)d

iet

for12weeks

HFdietdidno

taffectcaecaltaxa

richn

ess.

Bacterialcom

mun

ities

cluste

redaccordingto

diet.Significantly↓Ru

minococcaceae

(Firm

icutes)a

nd↑Rikenellaceae

(phylum

Bacteroidetes),Lactobacilli,and

Erysipelo

trichiales

inHFfedversus

carboh

ydratefeddiet.19OTU

saffected

byHF

diet.C

arbo

hydratea

ndHFgrou

phaddistinct

proteomea

ndmetabolom

e

Highfatd

ietaffectsg

utmicrobial

ecolog

ybo

thin

term

sof

compo

sitionandfunctio

n


Table3:Con

tinued.

Reference

Stud

ymod

elAim

ofthes

tudy

Stud

ydesig

nandou

tcom

esmeasures

Results

Con

clusio

n

Cani

etal.2007

[9]

C57bl6/Jmice

and

CD14−/−

mutantstrain

Toevaluatetheinfl

uence

ofgutm

icrobiotao

nthe

developm

ento

fmetabolicendo

toxemia

Metabolic,infl

ammatory,and

microbiologicaldifferences

(byFISH

)betweenhigh

fatfed

obeseo

rrod

entleanchow

-fed

mice

Highfatfeeding

andob

esity

decimateintestin

almicrobiota–

Bacteroides-m

ouse

intestinal

bacteria,B

ifidobacterium,and

Eubacterium

rectale-Clostridium

coccoidesg

roup

sall

significantly↓comparedto

incontrolanimals

Highfatd

ietind

uces

changesingut

microbiotathatleads

toele

vated

plasmaL

PSleadingto

metabolic

endo

toxemia,byalterin

gtheg

utbarrierfun

ction

Cani

etal.2008

[34]

C57bl6/Job/ob

mice

Manipulatinggut

microorganism

sthrou

ghantib

iotic

sto

demon

stratewhether

changesingut

microbiotac

ontro

lthe

occurrence

ofmetabolic

synd

romes

Caecalmicrobiotao

fmiceu

nder

high

fatlow

fibre

dietand

antib

iotic

s.qP

CRandDGGE

Antibiotic

redu

cedLP

Scaecalcontentand

metabolicendo

toxemiain

both

ob/obandhigh

fatg

roup

s.Highfatd

iet↑

intestinal

perm

eabilityandLP

Sup

take

leadingto

metabolicendo

toxemia.A

bsence

ofCD

14mim

ickedthem

etabolicandinflammatory

effectsof

antib

iotic

s

Highfatd

ietm

odifies

gut

microbiotaw

hich

indu

ceinflammationandmetabolic

endo

toxemia.A

ntibioticsc

anreversethese

changes

Murph

yetal.

2010

[16]

HFfedwild

-type

micea

ndleptin

deficient

ob/obmice

(𝑛=8perg

roup

)

Toinvestigatethe

effect

ofhigh

fatd

ietand

genetic

allydeterm

ined

obesity

forc

hang

esin

gutm

icrobiotaa

ndenergy

harvestin

gcapabilityover

time

GC,

metagenom

icpyrosequ

encing

high

fato

rnormalchow

dietfed

toob/obmicea

ndwild

-type

mice

for8

weeks

↑in

Firm

icutes

andBa

cteroidetesinHFfed

andob

esem

iceb

utno

tinlean.C

hanges

inmicrobiotan

otassociated

with

markersof

energy

harvest.Initialincrease

incaecalSC

FA(acetate)a

nd↓in

stoolenergy

with

HFdietdid

notrem

ainsig

nificanto

vertim

e

Changesinbacterialphylaarea

functio

nof

high

fatd

ietand

aren

otrelatedto

them

arkersof

energy

harvest

deWitetal.

2012

[35]

MaleC

57BL

/6Jm

ice

Tostu

dythee

ffectof

dietaryfattype

(polyunsaturated

and

saturatedfatty

acids

ratio

)onthe

developm

ento

fobesity

Phylogeneticmicroarray

(MITCh

ip)a

nalysis

,bom

bcalorim

etry,m

easuremento

ftriglycerid

es,and

plasmainsulin

HFdietwith

high

saturatedfatty

acids(palm

oil)indu

ced↑weightg

ainandliver

TGcomparedto

HFdietwith

oliveo

iland

safflow

eroil.HFdietwith

palm

oil↓

microbial

diversity

and↑Firm

icutes

(Bacilli,Clostridium

cluste

rsXI,X

VII,and

XVIII)Ba

cteroidetes

ratio

.Upregulationof

69lip

idmetabolism

genesindistalsm

allintestin

eand↑fatinsto

ol

Type

ofdietaryfatinfl

uences

the

weightg

ainandhepatic

lipid

metabolism

Faith

etal.2011

[36]

MaleC

57BL

/6Jm

ice

(𝑛=10perg

roup

)

Changesin10

mod

elgut

commun

ities

species’

abun

dancea

ndmicrobialgenesw

ithchangesinpeculiard

iet

Shotgu

nsequ

encing

offaecal

DNA

dietsu

sedfore

achcommun

ity:

casein

(forp

rotein),corn

oil(for

fat),

starch(fo

rpolysaccharides),

andsucrose(forsim

ples

ugars)

61%varia

nceinabun

danceo

fthe

commun

itymem

berswas

explainedby

dietparticularly

casein.A

bsolutea

bund

ance

ofE.

recta

le,Desulfovibriopiger,andM.formatexigens↓

by25–50%

whileBa

cteroidescaccae↑

with

increase

incasein,alth

ough

thetotal

commun

itybiom

ass↑

Hostd

ietexplainsc

onfig

urationof

gutm

icrobiotab

othforrefined

dietsa

ndcomplex

polysaccharid

es

Hild

ebrand

tet

al.200

9[37]

RELM

-𝛽kn

ockout

femalem

icea

ndwild

-type

mice

Toassesstheinfl

uenceo

fho

stph

enotype,

geno

type,immun

efunctio

n,anddieton

gut

microbiota

16SrD

NA454FL

Xpyrosequ

encing

,metagenom

icsequ

encing

Switching

tohigh

fatd

ietcaused↓

Bacteroidetesa

nd↑Firm

icutes

and

Proteobacteriain

both

wild

-type

andRE

LM-𝛽

knockout

miceirrespectiveo

fthe

geno

type.

Geneticmakeupon

lymod

estly

influ

encedthe

gutm

icrobiom

ecom

position.

Changesingene

contentw

ithHFdiet

Dietd

etermines

theg

utmicrobiota

compo

sition


Table3:Con

tinued.

Reference

Stud

ymod

elAim

ofthes

tudy

Stud

ydesig

nandou

tcom

esmeasures

Results

Con

clusio

n

Huang

etal.

2013

[38]

Adultm

aleC

57BL

/6

Toassesstherela

tionship

ofdietcontentand

source

ongutm

icrobiota

andadiposity

16SrRNAanalysis,

term

inal

restric

tionfragmentlength

polymorph

ismandV3-V4

sequ

ence

taganalysisvian

ext

generatio

nsequ

encing

.Mesenteric

fatand

gonadalfat

tissuea

nalysis

.Milk

,lard,or

safflow

erbased

dietsfor

4weeks

↑weightg

ainandcaloric

intake

with

HF

comparedto

lowfatd

iet.Milk

basedandPU

FAbaseddietsa

nimalsh

ad↑adiposetissue

inflammationthan

lard

basedor

lowfatd

iet.

Milk

basedandPU

FAdiethadsig

nificantly↑

Proteobacteriaand↓Teneric

utes.P

UFA

based

fedanim

alsh

ad↑expressio

nof

adiposetissue

inflammationgenes(MCP

1,CD

192,and

resistin

)

Dietary

fatcom

ponentsreshape

gut

microbiotaa

ndaltera

dipo

sityand

inflammatorystatus

oftheh

ost

Jakobsdo

ttire

tal.2013[39]

MaleW

ister

rats

Toinvestigatethe

effect

ofdietaryfib

reon

metabolicris

kmarkers

inlowandhigh

fatd

iets

at2,4,and6weeks

Gas

liquidchromatograph

y,liver

fatcon

tent,cho

lesteroland

triglycerid

esanalysis,

and

term

inalfragmentlength

polymorph

ism.D

iets

supp

lementedwith

guar

gum

oram

ixture

↓in

weightgain,

liver

fat,cholesterol,and

triglycerid

eswith

fibre.C

hangeinform

ationof

SCFA

.↓in

serum

SCFA

with

HFdietfollo

wed

byrecovery

after

4weeks.Succinica

cid↑with

HFconsum

ption.

Dietary

fibre↓thiseffectand

also↓inflammation.

Bacteroidesw

ere↑

with

guar

gum

andAk

kerm

ansia

was↑with

fibre-fr

eediet

HFdiet↑metabolicris

kfactors

which

arep

artly

reversed

byhigh

fibre

diet

Stud

iessuggestin

geffectofdieton

changesingutm

icrobiota

andresulta

ntobesity

delaSerree

tal.

2010

[15]

MaleS

prague-D

awley

rats

Toevaluatewhether

changesingutb

acteria

andgutepithelial

functio

nared

ieto

rob

esea

ssociated

Intestinalperm

eability,intestinal

Alk-Pase,plasmaL

PS,tissue

myeloperoxidase

(MPO

)activity,

immun

ochemicallocalizationof

TLR4

/MD2complex,and

Occludin.

Sequ

ence

analysisof

them

icrobial16SrRNAgene

Appearance

oftwodistinctg

roup

s;diet

indu

cedob

esity

pron

e(DIO

-P)and

diet

indu

cedob

esity

resis

tant

(DIO

-R)g

roup

s.DIO

-Pratshad↑features

ofadiposity,↑

MPO

activ

ity,↑

TLR4

MD2im

mun

oreactivity

and↑

plasmaL

PSlevels,↑gutp

ermeability,

immun

oreactivity

ofOccludin,

and↓alkalin

eph

osph

ataselevels

than

LFandDIO

-Rgrou

p.HFdietwas

associated

with↑Clostridiales

regardlessof

prop

ensityforo

besity.Amarked

differenceinEn

terobacterialesinDIO

-Panim

alsc

omparedwith

either

DIO

-Ror

LFfed

anim

als

Changesingutb

acteria

are

independ

ento

fobese

status.G

utinflammationmarkedby

increased

LPSmay

beatrig

gerin

gmechanism

forh

yperph

agiaandob

esity

Bäckhedetal.

2004

[1]

Adultgerm-fr

ee(G

F)C5

7BL/6mice

Toevaluatethee

ffectof

gutm

icrobiotao

nthe

hostenergy

metabolism

usinganim

almod

el

Con

ventionalisationof

GFmice

with

murineg

utmicrobiotao

rB.

thetaiotaomicron,intestin

alfia

f,liver

metabolism

,totalbo

dyfat,

LPLactiv

ityin

adiposetissue,

andfaecalmicrobiota

compo

sitionby

qPCR

Con

ventionalized

GFmices

howed

57%↑in

body

fat,increasedenergy

expend

iture,↓

intestinalfia

f,increasedLP

Lactiv

ity,and↑

expressio

nof

ChRE

BPandSR

EBP-1inliver.

Firm

icutes

toBa

cteroidesratio

similarinGF

andCO

NV

Gut

microbiotaa

lterh

ostenergy

storage

byaffectin

gfia

fand

LPL

activ

ity


Table3:Con

tinued.

Reference

Stud

ymod

elAim

ofthes

tudy

Stud

ydesig

nandou

tcom

esmeasures

Results

Con

clusio

n

Bäckhedetal.

2007

[13]

AdultG

FC5

7BL/6

mice(𝑛=5)a

ndCO

NVmice(𝑛=5)

Toassesswhether

GF

micea

reprotected

againsto

besityon

high

fatW

estern

diet

Dietary

interventio

nwith

lowfat

follo

wed

byhigh

fatW

estern

diet

for8

weeks

CONVmiceg

ained↑weighto

nHFdietwhile

conventio

nalised

GFmiced

idno

t.Stoo

lenergy

was

similartotheL

FfedGFmice.Persistent↑

TGin

HFfedGFmice.GFmiceh

ad↑Ac

c-p,

AMPK

-P,and

Cpt-1

activ

ity.G

Fmiceh

ad↓

hepatic

glycogen

andglycogen-synthase

activ

ity.↑

fiafinHFfedGFmice

GFmicea

reprotectedagainstd

iet

indu

cedob

esity

bytwo

mechanism

s:(1)increased

phosph

orylated

AMPK

and(2)

increasedfia

f

Vijay-Ku

mar

etal.2010[40]

TLR5

knockout

mice

(T5K

O),wild

-type

mice(WT)

Toshow

thatmice

deficient

inTL

R-5

exhibith

yperph

agia,

which

isap

rincipal

factor

inthe

developm

ento

fobesity

andmetabolicsynd

rome

Broadspectrum

antib

iotic

s.Py

rosequ

encing

of16SrRNA

genesinthec

aecum.

Transplantationof

TLR5

-KO

micem

icrobiotaintoWT

germ

-free

hosts

Antibiotic

treatment↓

theb

acteria

lloadby

90%,correctionof

metabolicsynd

romes

imilar

tothew

ild-ty

pemice.Re

lativea

bund

ance

ofbacterialphylawas

similarinbo

th,w

ith54%

Firm

icutes,39.8

%Ba

cteroides.116

phyla

observed

tobe

enric

hedor↓in

TLR5

-KO

relativetoWTmice.Microbiotao

fWTmice

transplanted

totheT

LR5-KO

micer

esultedin

allfeatureso

fmetabolicsynd

romeinthe

TLR5

-KOgrou

p

Lossof

TLR-5results

inmetabolic

synd

romea

ndalteratio

nin

gut

microbiota

Leyetal.2005

[3]

Leptin

deficient

C57B

L/6J

ob/obmice,

lean

ob/+,and

+/+

mice(𝑛=19)

Tostu

dydifferences

inbacteriald

iversity

betweenob

eseg

enetic

mod

elof

obesity

andits

relationshipwith

kinship

16SrRNAgene

amplificatio

nof

caecalbacteriafollo

wed

byanalysisusingPH

REDand

PHRA

Psoftw

are.Allmicefed

thes

amep

olysaccharider

ich

chow

ob/obmicec

onsumed

42%morec

howand

gained

significantly↑weight.Mothersand

offsprin

gshared

bacterialcom

mun

ity.O

bese

ob/obmiceh

ad50%redu

ctionin

Bacteroidetes

andap

ropo

rtional↑

inFirm

icutesas

compared

tolean

regardlessof

thek

inship

andgend

er

Obesityisassociated

with

altered

bacterialecology.Th

isho

wever

needstobe

correlated

with

the

metabolicattributes

ofgut

microbialdiversity

Turnbaug

hetal.

2006

[4]

Leptin

deficient

C57B

L/6J

ob/obmice

(𝑛=13)a

ndlean

ob/+

and+/+mice

(𝑛=10)

Whether

gutm

icrobial

gene

contentcorrelates

with

characteris

ticdistal

gutm

icrobiom

eofleptin

deficient

ob/obmicea

ndtheirleancoun

terparts

1SrRNAwho

legeno

mes

hotgun

metagenom

ics,GC-

MSforS

CFA

analysis,

bombcalorim

etry,gut

microbiotatransplantatio

n,and

DEX

A

Firm

icutes-enrichedob

esem

icrobiom

eclu

steredtogether

whilelean

phenotypew

ith↓

Firm

icutes

toBa

cteroidetesratio

cluste

red

together.O

bese

microbiom

erichin

enzymes

forb

reakdo

wnof

dietarypo

lysaccharid

esparticularlyglycosideh

ydrolases.ob/obhad↑

acetatea

ndbu

tyrateandsig

nificantly↓sto

olenergy

Obese

microbiom

eisa

ssociated

with

increasedenergy

harvest

Caesar

etal.

2010

[12]

Swiss-W

ebste

rmice

(GF,CO

NV,

andE.

colimon

ocolon

ised

mice)

Whether

gutm

icrobiota

especiallyLP

Sprom

ote

inflammationin

white

adiposetissue

(WAT

)andim

pairglucose

metabolism

DEX

A,insulin,and

glucose

toler

ance.M

acroph

ageisolatio

n,im

mun

ohistochemistry,and

flow

cytometry

andim

mun

oblotin

WAT

,LPS

analysis,

andRT

-qPC

R

Mon

ocolon

isatio

nof

GFmicew

ithE.

coli

W3110

oriso

genics

trainMLK

1067

with

low

immun

ogenicLP

Shadim

paire

dglucose

tolerance.How

ever,onlyGFmicew

ithE.

coli

W3110,and

notM

LK1067,sho

wed↑

proinfl

ammatorymacroph

ageinfi

ltrationin

WAT

Macroph

agea

ccum

ulationis

microbiotad

ependent

butimpaire

dglucosetolerance

isno

t


Table3:Con

tinued.

Reference

Stud

ymod

elAim

ofthes

tudy

Stud

ydesig

nandou

tcom

esmeasures

Results

Con

clusio

n

Caric

illietal.

2011[41]

TLR2

knockout

mice

(TLR

2−/−)a

ndwild

-type

mice(𝑛=8

perg

roup

)

Influ

ence

ofgut

microbiotao

nmetabolic

parameters,glucose

intolerance,insulin

sensitivity,and

insulin

signalling

inTL

R2kn

ockout

mice

454pyrosequ

encing

↑Firm

icutes

(47.9

2%versus

13.95%

),Ba

cteroidetes(47.92%

versus

42.63%

),and↓

Proteobacteria(1.04%

versus

39.53

%)in

TLR2−/−.↑

LPSabsorptio

n,insulin

resis

tance,

impaire

dinsulin

signalling

,and

glucose

intoleranceinTL

R2−/−

comparedto

controls

Alteratio

nin

gutm

icrobiotain

non-germ

-free

cond

ition

slinks

geno

type

toph

enotype

Everardetal.

2013

[42]

C57B

L/6mice

(geneticallyob

ese,HF

fed,andtype-2

diabetic)

Toascertainther

oleo

fAk

kerm

ansia

muciniphilain

obesity

andtype-2

diabetes

Real-timeq

PCR,

MITCh

ipanalysis,

LTO-O

rbitrap

mass

spectro

meter,and

ELISAfor

insulin

andfaecalIgA

Akkerm

ansia

muciniphila↓ob

esity

andtype-2

diabetes

which

was

norm

alise

dby

oligofructose.Ad

ministratio

nof

A.muciniphila

reversed

markersof

metabolicdisorders.Th

ese

effectsneeded

viableA.

muciniphila

Thismicroorganism

couldbe

used

aspartof

apotentia

lstrategyforthe

treatmento

fobesity

Feiand

Zhao

2013

[43]

C57B

L/6J

GFmice

Endo

toxinprod

ucing

Enterobacterclo

acae

B29

isolatedfro

mob

ese

human

gutcou

ldindu

ceob

esity

andinsulin

resistanceinGFmice

16SrRNAgene

sequ

encing

for

bacteriaandlim

ulus

amebocyte

lysatetestfore

ndotoxin

measurement

Mon

ocolon

isatio

nof

GFmicew

ithE.

cloacae

indu

cedob

esity

andinsulin

resis

tanceo

nHF

dietwhileGFcontrolm

iceo

nlyon

HFdietdid

not.En

terobacter-colon

isedGFob

esem

iceh

ad↑plasmae

ndotoxin

levelsandinflammatory

markers

Gut

microbiota-prod

uced

endo

toxinmay

becausatively

relatedto

obesity

inhu

man

hosts

Geurtse

tal.

2011[20]

Leptin

resistant

db/db

mice

Toinvestigatethe

gut

microbiotac

ompo

sition

inob

esea

nddiabetic

leptin

resistant

mice

versus

lean

mice

Com

binedpyrosequ

encing

and

phylogeneticmicroarrayanalysis

of16SrRNAgene

↑Firm

icutes,P

roteob

acteria

,and

Fibrob

acteres

phylaindb/dbmicec

omparedto

lean

mice.

Odorib

acter,Prevotella,andRikenella

were

exclu

sively

presentindb/dbmicew

hile

Enterorhabdu

swas

identifi

edexclu

sively

inlean

mice.db/dbmiceh

ad↑tone

ofeC

Band↑

apelin

andAPJ

mRN

Alevels

Gut

microbiotav

arywith

geno

type

andplay

asignificantroleinthe

regu

lationof

eCBandapelin/A

PJmRN

Asyste

m

GF:

germ

-free

mice,CV

:con

ventionally

raise

dgerm

-free

mice,HF:

high

fatd

iet,LF

:low

fatd

iet,WD:W

estern

diet,P

CR:p

olym

erasechainreactio

n,FISH

:florescent

insituhybridization,

fiaf/a

ngptl4:fastin

gindu

cedadipocytefactor/ang

iopo

ietin

-like-protein

factor-4,N

F-𝜅B:

nucle

arfactor-kappaB,

CHO:carbo

hydrate,CA

RB-R:carbo

hydrate-redu

ceddiet,FAT

-R:FAT

-reduced,D

EXA

orDXA:d

ualenergyX-

ray

absorptio

metry,FT-ICR-MS:Fo

urier-transfo

rmioncyclo

tronresonancem

assspectrometry,O

TUs:op

erationaltaxon

omicun

its,LPS

:lipop

olysaccharide,DGGE:

denaturin

ggradient

gelelectroph

oresis,

GC:

gas

chromatograph

y,SC

FA:sho

rtchainfatty

acids,RE

LM-𝛽:resistin-like

molecule-𝛽,PUFA

:polyunsaturated

fatty

acids,MCP

1:mon

ocytec

hemoattractant

protein1,Alk-Pase:alkalin

epho

sphatase,T

LR4/MD2:To

ll-Like

Receptor

4/mito

gendetector-2,C

hREB

P:carboh

ydraterespon

seelem

entbinding

protein,SE

RBP-1:ste

rolrespo

nsee

lementbinding

protein-1,TG

:trig

lycerid

es,C

pt-1:carnitin

epalmito

yltransfe

rase-1,A

MPK

:adenosinem

onop

hosphatekinase-1,A

cc-p:acetylC

oAcarboxylase(ph

osph

orylated),WT:

wild

-type,G

C-MS:gasc

hrom

atograph

y-massspectrometry,and

eCB:

endo

cann

abinoidreceptor

syste

m.


FXR

BileCholic and

chenodeoxycholicacid

Deoxycholic andlithocholic acid

Enterohepaticcirculation

Cholesterol

Improved lipid andglucose metabolism

GLP-1

TGR5

Improved glucose metabolism

Improved insulin sensitivity

Gut lumen

Dys

bios

is

TGR5

FXR

↓ lipogenesis↓ VLDL export↓ TG turnover

+

+

+

−

Figure 1: Modulation of bile acid circulation by gut microbiotaand its effect on glucose metabolism. Concept adapted from [6–8]. TGR5: G protein coupled receptor 5, VLDL: very low densitylipoprotein, TG: triglycerides, GLP-1: glucagon like peptide-1, andFXR: farnesoid x receptor.

systemic transport of bile acids into the liver by enterohepaticcirculation (Figure 1). Study on germ-free and FXR deficientmice suggests that the expression of genes responsible for theuptake, transport, and export of bile acids into circulationafter ileocaecal resection is dependent on gutmicrobiota [19].Primary bile acids entering the large intestine are convertedto secondary bile acids (deoxycholic and lithocholic acids)by gut microbiota. Secondary bile acids are ligands for Gprotein coupled receptor 5 (TGR5) which helps in glucosehomeostasis by stimulating the expression of glucagon likepeptide-1 (GLP-1) and reduces serum and hepatic triglyceridelevels [7, 8]. Gut microbiota may therefore affect host hepaticadiposity by altering bile acid circulation via FXR and TGR5mechanisms. However, it is also suggested that bile acids mayreciprocally cause dysbiosis through their bactericidal activ-ity by damaging the microbial cell membrane phospholipid[77]. Furthermore, high saturated fat but not polyunsaturatedfat promotes the expansion of pathobionts such as Bilophilawadsworthia and activates proinflammatory markers such asIL-10 causing experimental colitis [78].

2.5. GutMicrobiota and Changes in Satiety (Gut-Neural Axis).The gut microbiota, through production of SCFA, may affecthost energy metabolism and development of obesity bychanging the hormonal milieu in the intestine and othervisceral organs (Figure 2). Glucagon like peptide-1 (GLP-1)plays a key role in regulating communication between thenutritional load in the gut lumen and peripheral organs such

as brain, liver, muscle, and adipose tissue by postprandialincreases in satiety, gut transit time, and incretin inducedinsulin secretion [79]. Secretion of GLP-1 is decreased inobesity secondary to weight gain which causes insulin resis-tance independent of circulating level of fatty acids [79]. Thegut microbiota regulate GLP-1 by influencing the expressionof its precursor, proglucagon, and increasing GLP-1 positiveenteroendocrine L-cell in the gut [80]. Dietary fibres (nondi-gestible and fermentable fibres), as well as SCFA, have beenshown to increase GLP-1 secretion in both human [81] andanimal studies [82].Mice lacking receptors for the attachmentof SCFA (GPR43 and GPR41 deficient mice) showed in vitroand in vivo reduced GLP-1 secretion and impaired glucosetolerance [83].

SCFA including acetate, propionate, and butyrate act asligands for the activation of G protein coupled receptors43 and 41 (GPR41 and GPR43) which are expressed by gutepithelial cells, endocrine cells, and adipocytes. GPR43 inwhite adipose tissue act as sensors of postprandial energyexcess and regulate energy expenditure and hence bodyenergy homeostasis. GPR43 and GPR41 enhance insulinsensitivity and activate the sympathetic nervous system atthe level of the ganglion to prevent excess energy depositionin adipose tissue and enhance energy expenditure in othertissues such as liver and muscles [22]. GPR43 deficient micehave metabolic abnormalities including excess fat accumu-lation. When treated with antibiotics or under germ-freeconditions, these metabolic abnormalities reverse suggestingthat the gut microbiota are key players in expression of thesereceptors [22]. Samuel et al. (2008) demonstrated that GFmice deficient in GPR41 genes remain lean compared withtheir wild type counterparts, although their body composi-tion was not different [84]. They also showed that GPR41stimulates the expression of the gut anorexigenic hormone,peptide YY (PYY), which in turn causes inhibition of gastricemptying, reduced intestinal transit time, increased energyharvest (in the form of caecal acetate and propionate), andincreased hepatic lipogenesis [84].

Bifidobacteria are inversely associated with the devel-opment of fat mass, glucose intolerance, and bacteriallipopolysaccharide (LPS) in the blood via SCFA-inducedstimulation of PYY and ghrelin. Intervention with prebi-otics such as dietary fructans or oligofructose stimulatesbifidobacterial growth [76] and reduces weight accompaniedby increased PYY and reduced ghrelin consistent with a lowerfood intake in the prebiotics group [85]. Intervention with16 g fructose/day or 16 g dextrin maltose/day for 2 weeks ina randomised control trial was associated with an increasein breath hydrogen (a marker of colonic fermentation) andincreased production of PYY and GLP-1 [86].

Overall, this evidence suggests that alteration in the gutmicrobiota may affect hormonal status via GLP-1 and Gprotein coupled receptors. These hormonal changes bring achange in satiety, food intake, and overall metabolic status ofan individual that could affect host adiposity. Whether thisrelationship is causal needs further investigation.

2.6. Gut Microbiota and Intestinal Permeability: Chronic Low-Grade Inflammation. Emerging evidence suggests close ties


SCFA

Fiaf/ANGPTL4

GPR43

GPR41

GLP-1

PYY

Induction of satiety

adipose tissue

Gut epithelium

+

+

−

Hypothalamus

↑ LPL

↑ Insulin sensitivity

↑ TG storage in

Figure 2: Proposedmechanism of the changes in gut hormonal axis by gut microbiota. TG: triglycerides, LPL: lipoprotein lipase, Fiaf: fastinginduced adipocyte factor, ANGPTL-4: angiopoitein-like protein-4, GLP-1: glucagon like peptide-1, GPR43 and GPR41: G protein coupledreceptors 43 and 41, PYY: peptide YY, and SCFA: short chain fatty acids. Minus sign indicates inhibitory effect; plus sign indicates stimulatoryeffect.

between metabolic and immune systems [11]. Obesity con-tributes to immune dysfunction by secretion of inflammatoryadipokines from adipose tissues such as TNF-𝛼, IL-6, andleptin [87]. Inflammatory adipokines induce carcinogenicmechanisms such as increased cellular proliferation and/ordedifferentiation that are potential risk factors for cancerssuch as colonic, oesophageal, and hepatocellular cancers. Anexample of this is the association of high levels of leptinwith hepatocellular carcinoma [87]. Intra-abdominal adiposetissue secretes adipokines with atherogenic properties (IL-1,IL-6, TNF-𝛼, and IFN-𝛼) which increase the risk of cardio-vascular diseases [88].These proinflammatory cytokines alsoactivate certain kinases, which in turn initiate the expressionof inflammatory and lipogenic genes, ultimately increasinginflammation and adipogenesis in a loop fashion (Figure 3).

2.6.1. Bacterial Lipopolysaccharide (LPS) and Inflammation.The gut microbiota may contribute to chronic low-gradeinflammation and obesity via the absorption of bacterial LPS,an outer membrane component of Gram negative bacteria,which is increasingly recognized as a player in chronic low-grade inflammation, a hallmark of obesity.

Cani et al. (2007) demonstrated the link between LPS andmetabolic disease by infusing bacterial LPS subcutaneouslyinto germ-free mice for 4 weeks which produced the samelevel of metabolic endotoxemia as by high fat diet [9].Furthermore, mice lacking functional LPS receptors wereresistant to these changes. Feeding high fat diet to mice withmucosal immune dysfunction (Toll-Like Receptor-4 knock-out mice) for 4 weeks resulted in two to three times increasedsystemic LPS levels in liver, adipose tissue and muscles, andhigher body fat mass, termed as “metabolic endotoxemia”[9]. This inflammatory status was associated with lower Bac-teroides, Bifidobacterium species, and Eubacterium rectale-C coccoides group [9]. Additionally, LPS stimulated markers

of inflammation (e.g., plasminogen activator inhibitor 1 andtumour necrosis factor alpha) and oxidative stress (e.g., lipidperoxidation) in visceral adipose tissue via the CD14 receptor.Absence of CD14 in CD14 deficient ob/ob (CD14 −/−) micehas been shown to protect against diet induced obesity andinflammation in mouse models [10].

2.6.2. Gut Barrier Integrity and Inflammation. Alterationin the gut microbiota is linked to changed gut barrierfunction [10] and may promote the release of bacterialendotoxins through damaged and leaky gut. Cani et al.(2007) showed a significant reduction in Bifidobacteria withhigh fat diet in male C57BL/6J mice [76]. Supplementationwith oligofructose was shown to restore the Bifidobacteriapopulation with improvement in gut barrier function evi-denced by the expression of precursors ofGLP-1, proglucagonmRNA, and decrease in endotoxemia [76]. No correlationwas found between endotoxemia and other bacteria (Lac-tobacilli/Enterococci, E. rectale/C. coccoides, Bacteroides, andsulphate reducing bacteria) [76]. GLP-1 helps in the differen-tiation of mucosal cells into enteroendocrine L-cells, whileGLP-2 helps in increased expression of mRNA for synthesisof tight junction proteins. These changes are associated withlower LPS in the blood suggesting increased integrity of thegut barrier function. In contrast treatment with antibioticsreduced inflammation by reducing the LPS-producing gutmicrobiota population, further elucidating the relationshipbetween gut microbiota, LPS levels, and inflammation [10].

2.6.3. High Fat Diet and Inflammation. The association ofhigh fat diet with subclinical or clinical inflammation inobesity has been investigated in several studies and thereis a clear evidence to suggest that consumption of highfat diet is associated with metabolic endotoxemia and 2-3-fold increase in bacterial LPS levels in the blood [9].


Altered microbiota (LPS abundance)

Reduced tight junction protein synthesis/reduced

GLP-1 and GLP-2

Increased LPS leakProinflammatory cytokine release

Macrophage infiltration

Hyperglycaemia and insulin resistance

Subcutaneous LPS

administration

High fat diet and obesity

Type-2 diabetes

Activation of JNK/IKK pathways

Activation of JNK/IKK pathways

Chronic low grade inflammationand obesity

MacrophageAdipose tissue

Activation of inflammatory and lipid metabolism

genes

Figure 3: Proposed model for the role of LPS in generating inflammation and its relationship with obesity. Concept adapted from [9–12]. Altered mucosal barrier function due to reduced expression of glucagon like peptides 1 and 2 (GLP-1 and GLP-2) leads to alteredmucosal function and reduced synthesis of tight junction proteins, Zonula Occludin-1 and Zonula Occludin-2 (ZO-1, ZO-2), increasing gutpermeability.This allows LPS to enter the systemic circulation inducing the release of proinflammatory cytokines. Proinflammatory cytokinesresult in activation of a family of kinases JNK and IKK (inhibitor of NFkB kinase) that increase the expression of inflammatory and lipidmetabolism genes. Subcutaneous administration of LPS, hyperglycaemia, and insulin resistance induces the same pathway by increasing theendoplasmic reticulum and mitochondrial stress. Type-2 diabetes, hyperglycaemia, and insulin resistance also cause macrophage infiltrationand inflammatory cytokine release leading to the same process. HF: high fat diet [9–12].

However, it is controversial whether this chronic low-gradeinflammation is dependent on the gut microbiota. Cani et al.(2007) found a dramatic change in gut microbiota (reducedLactobacillus, Bacteroides/Prevotella, and Bifidobacteria) ofobese ob/ob mice fed high fat diet [76]. This was associatedwith an increase in gut permeability indicated by a reducedexpression of Occludin and ZO-1 tight junction proteins.

In contrast, de la Serre et al. (2010) suggested that highfat diet induced intestinal inflammation in obese Sprague-Dawley rats may cause hyperphagia and obesity by impairingthe regulation of food intake. However, changes observedin the gut microbiota were independent of lean and obesephenotype [15]. High fat diet for 8 or 12 weeks in Sprague-Dawley rats revealed two genetically distinct groups, diet


induced obesity resistant (DIO-R) rats which were resistantto diet induced obesity and diet induced obesity prone (DIO-P) rats, which were prone to diet induced obesity on feedinghigh fat diet. DIO-P rats had significantly increased gutpermeability, increased LPS levels, lower intestinal alkalinephosphatase (iAP) levels (which detoxifies LPS), and systemicinflammation (high Toll-Like Receptor-4/Mitogen Detector-2 protein immunoreactivity) compared to DIO-R [15]. Acti-vation of TLR4 by LPS via MD-2 results in the production ofan inflammatory cascade (IL-6 and TNF alpha) [89] ensuingmetabolic endotoxemia. Mice with genetic deficiency ofTLR4 do not develop diet induced obesity [34]. This series ofchanges associated with high fat diet inducing inflammationmay alter food intake regulation and trigger hyperphagia, themechanism of which is yet to be fully understood.

2.7. Gut Microbiota and Endocannabinoid Receptor System.Cannabinoid receptors 1 and 2 (CB1 and CB2) are G proteinsactivated by the endocannabinoid (eCB) system. The eCBsystem is composed of endogenous lipids and plays animportant role in adipogenesis, as studied in geneticallyobese mice models. Two of the most widely studied lipidsin the eCB system are N-arachidonoylethanolamine and2-arachidonoylglycerol. The level of eCB components isinversely related to obesity and type-2 diabetes as both theconditions are associated with a higher tone of eCB system.Furthermore, the expression of CB1 and CB2 degradingenzymes (fatty acid amide hydrolase) is increased in adi-pose tissue of obese ob/ob mice compared with lean mice[10].

Bacterial LPS regulates the expression of cannabi-noid receptors via the LPS receptor signalling systemshown in both in vitro and in vivo studies [90]. Thisincreased tone is represented by higher levels of the pre-cursor enzymes N-acylphosphatidylethanolamine-selectivephospholipase-D, CB1 mRNA, and increased eCB compo-nents in plasma or adipose tissue [90]. Using CB1 receptorantagonists in ob/ob obese mice with disrupted gut barrierand metabolic endotoxemia improves gut permeability andreduces body weight, compared with lean littermates [90].The gut microbiota therefore regulate the activity of the eCBsystem and play an important role in host energy regulation.

A study by Geurts et al. (2011) in obese leptin resistantdb/db mice suggested that the abundance of Gram negativebacteria, higher Firmicutes and Proteobacteria, and lowerBacteroidetes were correlatedwith upregulation of apelin andAPJ expression. This was shown to be the result of directaction of bacterial LPS on the expression of apelin and APJmRNA in obese diabetic mice through chronic low-gradeinflammation [20]. These newly discovered adipokines arewidely expressed in mammalian tissues. Apelin is a ligandfor APJ, a G protein coupled receptor. Apelin/APJ systemplays a key role in the cardiovascular system by acting onheart contractility, blood pressure, fluid homeostasis, vesselformation, and cell proliferation. Apelin also affects glucosehomeostasis by acting through AMP kinase and nitric oxide(NO) dependent mechanisms [91]. Endocannabinoid systemdownregulates the expression of apelin and APJ mRNA inphysiological conditions. In contrast, higher levels of apelin

and APJ mRNA have been found in pathological conditionssuch as obesity and diabetes [20].

In summary, bacterial LPS increase the tone of eCBsystem and increase the expression of apelin/RPJ system inadipose tissue. However, how far gut microbiota populationcontribute to the actions of eCB and apelin/APJ and eCBin obesity is unknown. This has opened yet another area ofinterest in the role of gut microbiota in obesity.

3. Review of Animal Studies Relating GutMicrobiota with Obesity

The evidence from animal studies has thus far concen-trated on studies which looked at the interplay of diet,gut microbiota, and metabolic changes (in energy balance,lipoproteins, cholesterol, etc.) in animal models such as wild-type mice, leptin deficient ob/ob mice, and Sprague-Dawleyrats. Initial evidence suggesting a strong association of thegut microbiota with obesity was explored in a series ofstudies using germ-free and CONVmice. Components of gutmicrobiota acting as triggers in the development of obesity[40] and the emergence of diet induced obesity prone (DIO-P) mice and diet induced obesity resistant (DIO-R) mice fedon the same high fat diet [92] suggested that the peculiarcompositional differences alter the host response to prioritiseits metabolism towards increased energy harvest. Phylumlevel compositional differences in the relative proportions ofthe gut microbiota were therefore seen (Table 3) [1, 3, 4]and despite differences at species and genera level betweenstudies, there is a general agreement on reduced diversity andrichness of the gut microbiome in obese versus lean animals.

However the gut microbiota are located at the interfaceof environment and host. The effect of environmental factorsparticularly diet may therefore be highly significant andcontribute to changes in the gut microbiota compositionand function and ultimately their phenotype (obese or leanmicrobiome) [36]. Ingestion of high fat Western diets mayplay an important role in modifying the gut bacterial popu-lation which in turn alters the energy harvesting capability.This has been studied in various animal models such asGF/CONV mice and Sprague-Dawley rats [15, 17], leptindeficient ob/obmice models [31], and immune deficient micemodels (Toll-Like Receptor proteins deficient mice) [40]showing a tendency towards an increase in populations ofFirmicutes and reduction in Bacteroidetes after feeding withhigh fat Western diet.

Furthermore, observations from studies on GF/CONVmice and Sprague-Dawley rats suggest that a high fat diet,especially HF Western diet, is associated with increasedadiposity, reduced bacterial diversity [17], reduced numberof Bacteroides, a relative increase in favour of Firmicutes[17], and higher jejunal alkaline phosphatase activity [31].Moreover, high fat diet correlates with changes in inflamma-tory markers and oxidative stress [10] such as tumour necro-sis factor alpha (TNF-𝛼) and nuclear factor-kappaB (NF-kappaB), which play a major role in promoting inflammation[93], immune response, cellular proliferation, and apoptosis.In CONV mice, but not in germ-free mice, changes in the


expression of these inflammatory markers in the intestinepreceded weight changes and carried a strong positive cor-relation with high fat diet induced adiposity and markers ofinsulin resistance [32].This suggests an interaction of high fatdiet and enteric bacteria-promoting intestinal inflammationand insulin resistance prior to weight gain which is driven bythe high fat diet.

Studies in leptin deficient ob/ob mice, genetically proneto obesity, indicated that although the obese phenotype ischaracterised by a particular set of gut microbiota, changein caloric load and diet redistributes the equilibrium thatmay be independent of the genotype or phenotype (obese orlean) [16]. Changes in gut microbiota composition may beattributed to the high fat diet rather than genetic propensity toobesity. Furthermore, shift towards higher Firmicutes to Bac-teroidetes or the absence of gutmicrobiotamay not be associ-ated with the development of obesity [14]. The assertion thatgerm-free mice are protected from obesity was contradictedby Fleissner et al. (2010) where GF had a significantly higherbody weight gain than CONV mice on high fat diet despiteincreased Firmicutes (specifically, Erysipelotrichaceae) at theexpense of Bacteroidetes in CONV on a high fat diet andWestern diet [14].

Faecal transplantation studies support the causal roleof the gut microbiota in the aetiology of obesity. Trans-plantation of gut bacteria from obese human twins to leanmice caused not only obesity but also a higher numberof genes involved in detoxification and stress response,biosynthesis of cobalamin, essential and nonessential aminoacids, and gluconeogenic pathways. In contrast, animalswith lean-transplanted microbiota exhibited genes capableof fermenting plant polysaccharides and producing butyrateand propionate [94]. Additionally, the mere presence ofthe gut microbiota in conventionally raised mice has beenshown to result in higher levels of energy metabolites suchas pyruvic, citric, fumaric, and malic acid and higher rate ofclearance of cholesterol and triglycerides than in germ-freemice [95]. This suggests that the gut microbiota are essentialfor the characteristic pattern of metabolites in the gut of aspecies [96]. In postgastric bypass animals, gut microbiotatransplanted from a postgastric bypass animals who lostweight after surgery were associated with weight loss andother metabolic changes in recipient obese mice with nosurgery [97].

It is however interesting to observe that lean animalscohoused with obese cage mates are reported to developobesity and obesity related microbiota and metabolism insome studies [17] but not others [94] although themicrobiotaand metatranscriptome of obese animals became similarto the lean phenotype suggesting a “functional transforma-tion” [94]. As discussed above, the functional association ofmetabolic endotoxemia with gut microbiota was dependenton a high fat diet in the obese ob/ob animal model [10, 76].However, these effects were later shown to be independent ofobesity phenotype, as a high energy intake in lean C57BL/6Jmice fed a high fat diet produced a 2-3-fold increase inplasma LPS compared to normal chow diet. Furthermore, theincrease was blunted when the percentage intake of energycontributed by fat was reduced [98]. de Wit et al. (2012)

showed that a high fat diet composed of palm oil (withmore saturated fat) distinctly increased the Firmicutes toBacteroidetes ratio in the gut compared to a diet high in fat-olive oil, high fat-safflower oil, and low fat-palm oil [35]. Highfat-palm oil also stimulated expression of 69 genes related tolipidmetabolism in the distal intestine suggesting an overflowof lipids to the distal small intestine resulted in enhanced lipidmetabolism and changes in gut microbiota.

Several other studies suggested similar changes in gutmicrobiota and the presence of genes for lipid metabolism inanimal models using different dietary regimens [37, 38, 99](Table 2). Daniel et al. (2014) investigated composition andfunction of gut microbial ecology after 12 weeks of high fatdiet (HF) or high carbohydrate (CARB) diet [33]. Diets, andnot the gut microbiota, were shown to affect not only thedistribution of the gut microbiota communities (decreasein Ruminococcaceae and increase in Rikenellaceae with HFcompared to CARB) but also the metabolome and proteomeof the individual groups [33]. Although this study used twofunctional approaches to explore gutmicrobiota function, thenumbers were very low (𝑛 = 3) whichmight have contributedto variation within the groups.

3.1. Conclusion from Animal Studies. In conclusion, the rela-tionship of gut microbiota with diet and metabolic disordershas been studied in a variety of animal models. There iscontroversy as to whether these changes are attributable tothe diet itself or are caused by the gut microbiota. Studiesin germ-free mice suggest the gut microbiota are the criticalplayer in inflammation, development of immunity, and hostmetabolic regulation. However, diet is also considered a con-founding factor that determines a change in gut microbiotaand obesity because the diversity of gut microbiota has notbeen found to be different between wild-type and certaingenetic models of obese mice.

Discrepancies between and within studies could beattributed to the selection of animals (rats versus mice)and individual strains. A recent study by Walker et al.(2014) observed a distinct microbiome and metabolome intwo strains of C57BL/6J and C57BL/6N mice [100]. Somedifferences in the metabolome might also be attributedto gender [96] and described above in addition to othermethodological, host, and environmental differences. Theexactmechanismof how these changes lead to an obesity phe-notype is still not known. Large humans based interventionalstudies are therefore required to establish the true associationbetween diet and gut microbiota and obesity.

4. Review of Human Studies Relating GutMicrobiota with Obesity

Evidence linking the gut microbiota with obesity in humansis thus far inconclusive and controversial. This may bepartly due to marked interindividual variations in the gutmicrobiota and metabolic activity in humans with age, diet,use of antibiotics, genetics, and other environmental factors[101]. Apart from the interindividual variation in faecalmicrobiome and diversity, reanalysis of large datasets such asfrom the human microbiome project (HMP) and MetaHIT


Table 4: Association of gut microbial species/genera with obesity or leanness in human studies.

Bacteria Association∗ with obesity Group Level Other associations ReferenceLactobacillus reuteri +ve Firmicutes Species — [44, 45]Clostridium cluster XIVa +ve Firmicutes Group Anti-inflammatory [46]E. coli +ve Proteobacteria Species Nonalcoholic steatohepatitis (NASH) [46]Staphylococcus spp. +ve Firmicutes Genus Energy intake [47]Bacteroides −ve/+ve Bacteroidetes Genus Controversial [5]Akkermansia muciniphila −ve Verrucomicrobia Species Mucus degradation [42]Methanobrevibacter smithii −ve Archaea Species Increase in anorexia [48]Clostridium cluster IV; F. prausnitzii −ve Firmicutes Species Anti-inflammatory [49]Bifidobacteria −ve Actinobacteria Genus −ve association with allergy [44]∗Associations based on correlation or regression analysis or statistically significant differences between the lean and obese. +ve: positive association, −ve:negative association, and +ve/−ve: controversial.

has shown interstudy variability which was far greater thanthe actual differences between the lean and obese phenotypes[102]. Refined statistical modelling therefore led to loss ofsome correlations previously found, such as between BMIand Firmicutes to Bacteroides ratio [102]. Bridging these gapsin analysis and accounting for these technical and clinicalfactors is therefore important to elucidate differences betweennormal and altered host microbiome and metagenome.

The first evidence showing higher Firmicutes and lowerBacteroidetes in obese versus lean adults before the onsetof dietary intervention was presented by Ley et al. (2006)[5], followed by a number of studies reviewed in Table 6.Moreover, several gut microorganisms have been associatedwith obesity or leanness [44, 103] (Table 4). The type ofgut microbiota and their exact phylogenetic level at whichthey exhibit differences are still under investigation. Evidencesuggesting no phylum level differences between lean andobese gut microbiota [18, 65] may indicate that functionalityof bacteria may play a more important role than particularbacterial groups.

The energy harvesting capability of the gut microbiotain obese subjects is thought to be set at a higher thresholdthan in the lean with or without differences in the relativeabundance of the gut microbiota. Obese adults had higherindividual and total SCFA than lean adults in the absenceof any difference in the relative abundance of major gutbacterial phyla [29]. Moreover, no significant correlation ofthe gut microbiota with dietary factors in early [59] andlater childhood [47] and a positive correlation with BMISDS indicate that changes in the gut microbiota at thesedevelopmental stages may not depend on dietary factors.

On the other hand, evidence also suggests that dietplays an important role in altering the proportion of gutmicrobiota in individuals because the amount and typeof bacteria change significantly with diet [64, 67]. Thisvaries between individuals and may be due to the distinctmicrobiota colonisation during early life, altering the capacityfor energy harvest from the diet. Composition and caloriccontent of the diet significantly alter the relative abundanceof the gut microbiota [67]. An increased intake of resistantstarch was associated with an increase in Eubacterium rectale(a butyrate producing bacteria) to ∼10% and Ruminococcus

bromii (an acetate producer) to ∼17% compared with ∼4%in volunteers consuming nonstarch polysaccharides [67].These changes were reversed with weight loss diets alongwith a decrease in Collinsella aerofaciens, a member ofActinobacteria.This shows the substantial effect of diet on thegut microbiota and its energy harvesting capability [64, 67].Similarly, SCFA production is affected by nutrient load anddietary carbohydrate available for fermentation. Weight lossdiets usually have low carbohydrate and high protein contentand reduce the population of butyrate producing Roseburiaand Eubacterium rectale [66].

Long term changes in gut microbiota (such as lowercounts of Bifidobacteria and higher Bacteroides) have beenobserved in children who were exposed to antibiotics inearly childhood [104, 105]. Modulation of gut microbiotawith antibiotics (e.g., norfloxacin and ampicillin) altersthe expression of hepatic and intestinal genes involved ininflammation and metabolism thereby changing the hor-monal, inflammatory, and metabolic milieu of the host [106].These antibiotic induced changes may predispose children tooverweight and obesity by promoting “obesogenic-bacterial-growth” (Table 5). The development of gut microbiota ininfants and their tendency towards overweight and obesityin later childhood are linked to mother’s prepregnancy BMIand gutmicrobiota with significantly lower numbers of faecalBifidobacteria and Bacteroides and significantly higher E.coli and Staph. aureus in overweight and obese compared tonormal weight pregnant women [64].

In addition to compositional differences between leanversus obese subjects [5], functional differences in themetabolome of the obese and lean phenotype may be moreimportant. Calvani et al. (2010) in their preliminary studyof 15 morbidly obese and 10 age matched controls founddistinct gut microbial cometabolites in urine of obese versuslean participants, including lower levels of hippuric acid(benzoic acid derivative), trigonelline (niacin metabolite),and xanthine (purine metabolism) and higher levels of 2-hydroxybutyrate (metabolite of dietary protein) [49]. Themetabolic or functional representation of gut microbiotamight be proportional despite differences in the relativeabundance of the gut microbiota. Disturbance of this equi-librium is a hallmark of the obese phenotype as suggested


Table5:Po

pulationbasedstu

dies

toinvestigatethe

riskof

obesity

andoverweightinchild

renwho

wereg

iven

antib

iotic

sfor

treatmento

finfectio

nsin

early

infancy.

Stud

yreference

Designand

popu

latio

nAge

grou

pTo

ols

Prim

aryou

tcom

eFactorsc

onsid

ered

Find

ings

ISAAC

study

(International

Stud

yof

Asthmaa

ndAllergiesinCh

ildho

od)

[50]

𝑛=74,946

cross-sectional

5–8years

Questionn

aires/interviews,

measurements

Antibioticsu

sein

first12

mon

thso

flife

Ht.,Wt.,BM

I,age,gend

er,

antib

iotic

s,paracetamol,

breast-

feeding,maternal

smok

ing,grossn

ational

income,andasthma

Associationof

antib

iotic

suse

andBM

Iin

boys

(+0.107k

g/m2

𝑝<0.0001),no

tin

girls

even

after

adjustm

entfor

theo

ther

varia

bles

DNBC

study

(Danish

NationalB

irthCoh

ort)

[51]

𝑛=28,35

4Upto

7years

Questionn

aires/teleph

onic

interviewsb

ased

Antibioticsu

sein

<6mon

thso

flife

Socioecono

micstatus,

maternalage

andsm

oking,

gestationalw

eightg

ain,

parity,

deliverymod

e,breast-

feeding,

paternalBM

I,birthweight,

andagea

t7-yearfollow-up

Increasedris

kof

overweightinchild

ren

born

tono

rmalweightm

others(adjusted

OR:

1.54,95%CI

:1.09–

2.17)a

ndespeciallyin

boys

whenadjuste

dfor

maternalage,smok

ing,SE

status,birth

weight,andbreast-

feeding

ALSPA

Cstu

dy(Avon

Long

itudinalStudy

ofParentsa

ndCh

ildren)

[52]

𝑛=11,53

2long

itudinal

7years

Questionn

airesb

ased,

hospita

lrecords,and

objectivem

easurements

Antibiotic

expo

sure

at<6

mon

ths,6–

14mon

ths,and15–23

mon

thsa

ndBM

Iat

6weeks,10

mon

ths,20

mon

ths,38

mon

ths,and7

years

Maternalp

arity,socialclass,

education,

parentalBM

I,parentalsm

oking,

breast-

feeding,lifestyle,

and

dietarypatte

rns

Increasedris

kof

overweightat38mon

ths

(OR1.2

2,𝑝=0.029)b

utno

tat7

yearsin

child

renexpo

sedto

antib

iotic

s<6

mon

ths


Table6:Ev

idence

from

human

studies

abou

tthe

roleof

gutm

icrobiotainob

esity.

Reference

Stud

ymod

elAim

ofthes

tudy

Stud

ydesig

nandou

tcom

esmeasures

Results

Con

clusio

n

Stud

iessuggestin

gasso

ciatio

nofgutm

icrobiota

with

obesity

Kalliom

äkietal.

2008

[53]

Child

ren,

25ob

ese

and24

norm

alweight

at7yearso

fage

Toevaluatewhether

differences

ingut

microbiotaa

tanearly

agep

recede

the

developm

ento

fobesity

Subjectsexam

ined

at3,6,12,and

24mon

thsa

nd7years.Gut

microbiotac

ompo

sitionatageo

f6and12

mon

thsb

yFISH

,FISH

with

flowcytometry,and

qPCR

↑Bifid

obacteria

numbersand↓S.aureus

at6

and12

mon

thso

fage

inchild

renremaining

norm

alwt.↑Ba

cteroidesinob

esea

ndoverweightchildrendu

ring6and12

mon

ths

versus

norm

alwt.child

ren

↑nu

mbersof

Bifid

obacteria

and↓

numbersof

S.aureus

ininfancymay

providep

rotectionagainst

overweightand

obesity

developm

ent

Zhangetal.

2009

[23]

3Obese

(OB),3

norm

alwt.(N

W),

and3po

stgastric

bypass(G

B)patie

nts

Tocompare

theg

utmicrobialcommun

ityof

norm

alwt.,morbidly

obese,andpo

stgastric

bypasssurgerypatients

DNApyrosequ

encing

and

amplificatio

nby

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Documents

Review Article Role of Gut Microbiota in the Aetiology of Obesity: … · 2019. 7. 30. · Stomach, small and large intestine gut permeability and apelin and APJ mRNA expression Hormonal