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Review

The gut microbiota in energy homeostasis and

1 2 1

Michael Rosenbaum , Rob Knight , and Rudolph L. Leibel

1

Columbia University, Department of Pediatrics, Division of Molecular Genetics, New York, NY 10032, USA

2

Departments of Pediatrics and Computer Science and Engineering, University of California San Diego, La Jolla, CA 92093, USA

Numerous studies of rodents suggest that the gut micro- finer levels (including at the strain level) that is often

biota populations are sensitive to genetic and environ- important, some general trends emerge. The dominant

mental influences, and can produce or influence afferent phyla in the gut are Bacteroidetes (20–25%), Firmicutes

signals that directly or indirectly impinge on energy (60–65%), Proteobacteria (5–10%), and Actinobacteria

homeostatic systems affecting both energy balance (3%), which together constitute over 97% of the gut

( or loss) and energy stores. Fecal transplants microbe population (Table 1). The taxonomic variability

from obese and lean human, and from mouse donors to in the human gut is much higher than the functional

gnotobiotic mice, result in adoption of the donor so- variability, as measured by a variety of methods, suggest-

matotype by the formerly germ-free rodents. Thus, the ing that many different configurations of the microbiota

microbiota is certainly implicated in the development of lead to essentially the same functional result [5,6].

obesity, adiposity-related comorbidities, and the re-

sponse to interventions designed to achieve sustained Overview of rodent studies of obesity and the

weight reduction in mice. More studies are needed to microbiota

determine whether the microbiota plays a similarly po- In rodents, obesity is associated with an increase in the

tent role in human body-weight regulation and obesity. relative size of the Firmicutes versus Bacteroidetes popu-

lations in the gut [7], and a decrease in the diversity of the

Introduction microbiota that is due to both weight and composition.

The human gut microbiota (see Glossary) consists of up to The microbiota of diet-induced obese mice (DIO, induced by

100 trillion microbes that exist in largely symbiotic rela- an ad lib high-fat diet, HFD), which are weight-reduced by

tionship with their human hosts, and carry at least a calorie-restricted HFD, is more diverse compared to DIO

150 times more genes (the microbiome) than are present mice at maximal weight (similar diet, greater weight), and

in the entire human genome [1,2]. There is significant less diverse than control ad libitum fed mice on a chow diet

cross-sectional variability in the microbiota between indi- (different diet, similar weight) [7]. There is much greater

viduals and longitudinal variability within individuals. variability in human studies. Some studies report a similar

Both the abundance and the composition of the gut micro- increase in the ratio of Firmicutes/Bacteroides, as well as a

bial population are influenced by diet, medication, weight, decrease in the gut biodiversity in obese , but there

and overall metabolic state (energy balance, etc.,) of the are also numerous studies reporting contradictory findings

host. In turn, and based largely on animal studies, the in obese versus lean humans [8].

microbiota is capable of secreting or altering the produc- Studies of germ-free mice, or previously germ-free mice

tion of molecules that affect both energy balance (weight colonized with a defined microbial community (‘gnotobiotic

gain or loss) and energy stores (fat mass) [3,4]. The micro- mice’), have provided substantial evidence that the diver-

biota may, in this context, be regarded as a responsive sity, as well as the presence and relative proportion, of

entero-endocrine organ composed of more cells and genes different microbes in the gut play active roles in energy

than the host. This manuscript reviews what has been homeostasis. The overall importance of the microbiome is

done and what may be done to determine how translatable emphasized by the consequences of its absence and reple-

rodent data are to humans, and what the implications are tion. Germ-free mice are very inefficient at processing food,

for probiotic, prebiotic, and possibly targeted antibiotic and gain weight when colonized with almost any gut

approaches to the treatment or prevention of human obe- microbes. Inoculation of germ-free mice with ‘conventional’

sity. microbes results in weight gain to similar levels of fatness

to the donor mice. Surprisingly, this weight gain occurs

Composition of the microbiota despite an approximately 30% increase in energy expendi-

Although phylum-level and even family-level groupings of ture and 30% decline in energy intake, compared to the

microbes are very broad, and can conceal variability at mice who remain germ-free [9]. As discussed below, weight

gain, despite increased energy output and decreased in-

take, is consistent with a microbially mediated increase in

Corresponding author: Rosenbaum, M. ([email protected]).

energy harvest.

1043-2760/

ß 2015 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.tem.2015.07.002 The amount of weight gained by gnotobiotic rodents

differs substantially depending on which microbes are

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Glossary despite no significant differences between groups in daily

chow consumption [11].

Amylin: a hormone cosecreted with insulin from pancreatic b cells that slows

gastric emptying, inhibits glucagon release, and promotes satiety. The consequences of manipulation of the microbe pop-

Brain-derived neurotrophic factor (BDNF): a neurotrophic growth factor that ulation in rodents are also dependent on the environment

supports the survival of existing neurons and favors the growth of new

in which they are studied. In the twin study described

neurons and synapses in the central and peripheral nervous system.

Cocaine–amphetamine-related transcript (CART): an anorectic neuropeptide above [11], gnotobiotic mice developed the microbiota and

expressed in both the central and peripheral nervous systems as well as in

somatotype of their human donors. Cohousing of lean

multiple endocrine organs. ch ch ch

(Ln ) and obese (Ob ) mice results in acquisition by Ob

Ecological model: a framework for understanding the dynamic interrelations

among various personal (individual) and environmental factors (ranging from mice of the microbiota of lean animals, but no correspond-

family to cultural attitudes).

ing acquisition of the obese microbiome or somatotype in

Gastric inhibitory peptide (GIP): an incretin secreted by the jejunum and ileum ch ch

Ln . Ob mice fed a low saturated fat, high fruit and

that stimulates insulin release, increases lipoprotein lipase activity in

ch

adipocytes, and slows the rate at which nutrient moves through the intestines vegetable diet demonstrated greater invasion of the Ln

by decreasing motility and acid secretion.

microbiota. In addition, all mice on this diet with the lean

Genus: the second least inclusive taxonomic rank in the biological classifica- ch ch

microbiome (Ln/Ln, Ln , and Ob ) gained less fat mass,

tion system of organisms (Domain, Kingdom, Phylum, Class, Order, Family,

Genus, and Species). compared to mice fed a high saturated fat, low fruit, and

Germ-free: raised in a sterile environment resulting in no microorganisms ch

vegetable diet (Ln/Ln and Ln ), and to obese animals

living in or on the animal. ch

cohoused with a lean animal (Ob ) [11]. Similarly, studies

Glucagon-like peptide-1 (GLP-1): an incretin secreted by the intestines that

stimulates insulin secretion and sensitivity, promotes satiety, and inhibits of germ-free mice inoculated with microbiota from obesity-

secretion of glucagon.

prone (OP, higher Firmicutes/Bacteroidetes ratio) versus

Gnotobiotic: an animal in which only particular known strains of bacteria and

other microorganisms are present, usually a formerly germ-free animals that obesity-resistant (OR) rats, show that there is greater

has been intentionally colonized with a known microbial population. weight gain in the OP-treated group, but only in the

Technically, a germ-free animal is also gnotobiotic because its entire

setting of a HFD [12].

microbiome (which is none) is known.

Heritability: the fraction of the variability of a trait in a population that is In mice, there is some influence of host genotype (mouse

attributable to genes.

strain) on the microbiome [13], but the effect of the mouse

Lipoprotein lipase (LPL): a water-soluble enzyme that hydrolyzes triglycerides

environment is greater. Ericsson et al. [14] compared the

and includes hepatic lipase, pancreatic lipase, and endothelial lipase.

Microbiome: the collection of genomes of microbes in a system. microbiota of six different strains of purchased mice (some

Microbiota: the ecological community of commensal, symbiotic and patho-

strains duplicated) from three different laboratories and

genic microorganisms that live in our body; in other words, the collection of

organisms. from 3.5 weeks of age to 24 weeks of age, during which time

Neuropeptide Y (NPY): a neuropeptide produced in the hypothalamus and they were all housed and fed similarly and at a single site.

neurons of the sympathetic nervous system, and the most potent endogenous

The found roughly twice as many significant vendor effects

orexigen.

within populations of Bacteroidetes, Firmicutes, and Pro-

Peptide YY (PYY): an anorexigenic gut derived peptide secreted by the ileum

and colon in response to feeding. PYY binds to NPY receptors, increases water teobacteria compared to mouse strain effects. There was

absorption in the colon, and slows gastric emptying.

little change in these populations between 3.5 and 24 weeks

Phylum: the third most inclusive taxonomic rank in the biological classification

of age, suggesting the importance of early gut colonization

system of organisms (Domain, Kingdom, Phylum, Class, Order, Family, Genus,

and Species). and strain 3 vendor interactions. Parks et al. [15] reported

Prebiotic: selectively fermented nondigestible food ingredients or substances

significant heritability of weight gain and of gut microbial

specifically supporting the growth and/or activity of -promoting bacteria

that colonize the gastrointestinal tract. composition and plasticity (potential for change) in re-

Probiotic: a live microorganism that when administered in adequate amounts sponse to a high-fat/high-sugar diet, However, neither

confers a health benefit on the host.

the baseline enterotype nor the degree of plasticity

Pro-opiomelanocortin (POMC): a precursor polypeptide that is cleaved to form

adrenocorticotropic hormone (ACTH), the anorexiginic neuropeptide a-mela- (amount of change in response to diet) were independently

nocyte stimulating hormone (a-MSH), and the endogenous opioids b-

predictive of weight gain in mice. In humans, the predomi-

endorphin and [met]enkephalin.

nance of the environment in determining the composition

Resting energy expenditure: the energy expended in cardiorespiratory work

and in maintaining transmembrane ion gradients at rest. Generally it is about of the gut microbiome is also evident [16].

50–60% of 24 h energy expenditure.

Species: the least-inclusive taxonomic rank in the biological classification

Overview of the human gut microbiome

system of organisms (Domain, Kingdom, Phylum, Class, Order, Family, Genus,

and Species). The pediatric microbiome

UniFrac distance (unique fraction metric): a measure of the phylogenetic

In so-called ‘ecological models’, child adiposity is influ-

distance between sets of taxa.

enced by home and school environments, parental eating

behaviors, and food availability, in addition to adiposity

inoculated. Administration of microbes from leptin-defi- of parents [17]. Similarly, the establishment of the gut

ob

cient (Lep ) mice, that contain higher absolute and rela- microbial population in the neonate is a complex process

tive proportions of Firmicutes, similarly to other obese that may involve interactions between the maternal and

mice, results in greater weight and fat gain, lower energy fetal genes and environment (as exemplified by studies

expenditure, and increased energy harvest than does discussed above of the interactions of the microbiome

administration of microbiota from wild-type mice from genetically OP vs OR rats with diet type). These

[10]. Similar donor adiposity-related effects are noted interactions may begin even before birth and progress

following inoculation of germ-free mice with the micro- through multiple stages under the influence of various

biota from monozygotic (MZ) and dizygotic (DZ) human internal factors, such as the early decline in the abun-

twin pairs discordant for obesity. There was a progres- dance of oxygen in the gut that influences the balance of

sively greater increase in both fat mass and fat-free mass aerobes and anaerobes, and external factors such as diet

in animals receiving microbes from the obese twins, [18].

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Table 1. Overview of the composition of the human microbiome

Phylum % Microbiome Genus or species Relevant function Sources Refs

Major phyla (>1% of most individuals)

Firmicutes 60–65% Clostridium Some species ferment [5,6,9,28,81,82]

Eubacterium fiber into butyrate;

Faecalibacterium other functions range

Lactobacilli from symbionts to Roseburia pathogens

Ruminococcus Butyrate production

Bacteroidetes 20–25% Alistipes Polysaccharide Increased in protein- [5,6,8,9,16,81–84]

Bacteroides degradation rich, high-meat diets

Parabacteroides Some species ferment Increased in grain rich

Prophyromonas fiber into butyrate high-fiber diets

Prevotella

Proteobacteria 5–10% E. coli [5,6,9,81,82]

Actinobacteria 3% Bifidobacterium Vitamin biosynthesis Common probiotic [5,6,9,81,82]

colinsella

Minor phyla (<1% of most individuals)

Archaea <1% Methanobrevibacter Both convert hydrogen [85,86]

Methanosphaera gas to methane

(methanogens)

Deferribacteres <1% Degrade iron Increased in [8,16,83,84] gastrointestinal

bleeding

Fusobacteria <1% Fusobacterium Proinflammatory Increased in high-meat [16,87,88]

nucleatum colonic tumorigenic diets

factor

Melainabacteria <1% Synthesize vitamins B Increased in high-plant [16,28,89]

and K, ferment diets, present in

carbohydrate into groundwater

ethanol, lactate, and

formate

Spirochaetes <1% Predominantly More common in rural [16,30]

Treponema communities and in

high-fiber diets

Verrucomicrobia <1% Akkermansia Degrade mucin, [90]

muciniphila diminish inflammation,

and increase gut

butyrate and mucus

layer thickness

The vaginally delivered neonate is initially colonized that low-dose penicillin administered early in life increases

with the vaginal and distal gut bacteria of the mother, susceptibility to HFD-induced obesity, and that this sus-

while babies delivered by Caesarean section (C-section) are ceptibility can be transferred to gnotobiotic animals; in

initially colonized predominantly with the skin bacteria other words, it is the microbiome not the antibiotic per se

from the mother [19,20]. More specifically, vaginally deliv- that increases obesity susceptibility [27].

ered neonates have relatively and absolutely larger popu- While there are genetic influences on the human micro-

lations of Bacteroides and Bifidobacteria species than those biome, particularly in the first 3 years of life, these affects

born by C-section [21] that have been reported to persist for are small compared to those attributable to the environ-

months to years [18]. The observations that increased ment [6,28,29]. The UniFrac distance (unique fraction

Bacteroidetes populations are present in both obesity metric), a measure of the phylogenetic distance between

and children born by C-section [7] suggest that the infant sets of taxa (in this case, summarizing the evolutionary

microbiome may contribute to the subsequent 40% in- separation between the microbes present in different bio-

creased risk of obesity in children [22] and young adults logical specimens such as stool samples or gut biopsies), is

[23,24] delivered by C-section. This is apparent even when significantly greater in unrelated individuals than be-

corrected for pre- and post-gravid maternal body mass tween DZ or MZ twins or their mothers. There is substan-

index (BMI) and for gestational age. Similar- tial variation between fecal communities in different

ly, maternal exposure to antibiotics in the second and third geographic populations (Malawians, Amerindians, and

trimesters of pregnancy or early infancy is associated with other Americans) at all ages, and the within-community

decreased bacterial diversity of the first stool of the neo- diversity increases in all populations as age increases

nate, reduced abundance of lactobacilli and bifidobacteria [28]. Studies comparing industrialized societies such as

in the infant gut, and an approximately 80% increased risk the USA or with non-industrialized societies,

of [22,25,26]. The mechanisms for this such as the Hadza hunter-gatherers [30] or groups in

increase are not clear, but animal studies have suggested [31], indicate that industrialization

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is associated with lower diversity of fecal bacteria within there is a corresponding decrease in energy expenditure

individuals (a-diversity) and greater diversity between and vice versa [36,37]. Weight gain and clearly

individuals (b-diversity) [30,31]. In smaller studies of 54 represent an imbalance beyond the coupling capacity of

[6] and 63 [28] twin pairs of MZ versus DZ twin pairs, the energy intake and output. To induce or sustain changes in

UniFrac distance within human twin pairs was not signifi- body weight, microbiome-related mechanisms must some-

cantly different based on zygosity, suggesting that the how ‘disrupt’ this coupling.

shared environment contributes more than shared genes Briefly, the dynamic process of weight loss and the

to the gut microbe composition [6]. However, in a study of maintenance of a reduced body weight invoke similar

416 twin pairs in the TwinsUK population, there was a but not identical declines in energy expenditure (dispro-

significantly higher correlation within MZ twin pairs for portionate to changes in and weight) and

particular taxa, in particular the family Christensenella- increases in energy intake. The decline in energy expendi-

ceae. The abundance of Christensenellaceae species was ture is mainly due to increased chemomechanical efficiency

negatively associated with fatness in humans, and reduces of skeletal muscle, while the increases in energy intake

weight gain when transplanted into gnotobiotic mice [29], reflect increased food reward () and decreased food

indicating that physiologically important microbes may restraint (delayed satiation and decreased perception of

still be heritable at narrower taxonomic levels. how much has been eaten) [38–41]. These effects, which do

not abate over time [38–41], are largely mediated by the

The adult microbiota declines in circulating concentrations of the adipocyte-

In most [6,10], but not all [8], studies, both the diversity of derived hormone leptin, acting directly and via the auto-

the microbiota and the fractional proportion of Bacteroi- nomic nervous (ANS) and neuroendocrine systems. The

detes species relative to Firmicutes are decreased in obese ANS includes the sympathetic (SNS) and parasympathetic

versus lean individuals. These proportions are extremely (PNS) nervous systems. SNS tone modulates feeding be-

sensitive to caloric balance in subjects studied before, havior via effects on various gut peptides and transmission

during, and after weight loss while ingesting the same of nutrient-derived signals to the brainstem [42], and

liquid formula diet of identical macronutrient composition directly increases heart rate and secretion of thyroid hor-

[32]. Weight loss increased the relative proportion of Bac- mone [43,44], while increased PNS tone slows heart rate

teroidetes species and microbial diversity, thereby ‘revers- and decreased resting energy expenditure [38]. Thyroid

ing’ some of the microbial characteristics of obese versus hormone increases energy expenditure by increasing heart

lean individuals; this effect is augmented by a low-carbo- rate, blood pressure, muscle ATP consumption [largely by

hydrate versus a low-fat weight reduction diet [33]. Simi- stimulating production of muscle ATPase, and favoring

larly, studies of the microbiome have shown that gene expression of the less mechanically efficient myosin heavy

richness is diminished in obese versus lean individuals chain II (MHCII) isoform] [45,46]. The increase in PNS

[6] and is increased by dietary weight loss [34]. tone and the declines in SNS tone and circulating concen-

Studies of humans during weight loss and weight gain trations of leptin and bioactive thyroid hormones during

indicate that the slope of the line relating degree of over- and following weight loss effectively conspire to favor the

feeding and underfeeding to changes in the relative abun- regain of lost weight [47–52].

dance of Bacteroidetes and Firmicutes are almost identical Body weight regulation in mice is distinct from that of

– making it unclear whether these changes in the micro- humans in a number of key areas relevant to the energy

biome reflect nutrient balance or nutrient stores [35]. A key homeostatic systems described above (Table 2). For ex-

question is whether the positive association of the abun- ample, in addition to differences in natural diet compo-

dance of Firmicutes and negative association of the abun- sition (greater carbohydrate and less fat in murine diet),

dance of Bacteroidetes with changes in body weight biorhythms relevant to energy intake and expenditure

represent a direct effect of the degree and duration of (nocturnal vs diurnal), thermogenesis by brown (BAT)

negative or positive energy balance, or of changes in energy and beige (brite) accounts for over 50% of

stores as fat mass. Studies of changes in the microbiome adaptive thermogenesis in rodents versus <5% in adult

induced by , in other words reducing fat stores humans [53,54], In mice, both the amount and metabo-

without inducing a negative energy balance, might more lism of BAT are affected by diet composition and the gut

directly address this issue. microbiota [55,56]. Given the lower contribution of BAT

to human thermogenesis, it is questionable whether such

Overview of the human gut microbiome and energy manipulation has potential in the treatment or preven-

homeostasis tion of human obesity. It is clear that these anatomic,

Several mechanisms have been postulated by which the physiological, and behavioral differences must be consid-

microbiome might affect energy balance. These mecha- ered in attempts to extrapolate from mouse to human

nisms are poorly studied in humans. microbiota.

It is important to consider these possible mechanisms in There are few studies in humans examining the effects

the framework of what is known about human energy of alterations in the gut microbiome on energy expenditure

homeostasis. Briefly, the relative constancy of adult hu- and intake. As discussed below, there are many hypotheses

man body weight over time in a stable environment strong- regarding the mechanisms by which the human gut micro-

ly suggests that energy intake and output are coupled in a biota might affect these phenotypes, but these are largely

manner that ‘defends’ a relatively constant level of energy based on animal studies or indirect measurements of the

stores over time, such that if energy intake is decreased effects of experimental alterations of gut flora on other

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Table 2. Overview of major differences in mouse and human growth and energy homeostasis that may affect extrapolation of

a

rodent studies to humans

System Mouse Adult humans Refs

Somatic growth Lifelong Stops after adolescence [91]

Energy Intake Pattern Nocturnal Diurnal [92]

Exogenous Decreased energy intake before, Decreased energy intake after weight

Leptin Effects during, or after weight loss loss, less during

Energy expenditure Thermogenesis 20% of 24 h energy expenditure is Comparatively little basal [54,93,94]

devoted to maintaining body thermogenesis and very little brown

temperature and is largely dependent adipose tissue (BAT) thermogenesis

on brown adipose tissue (BAT) at rest

thermogenesis

Autonomic No a-adrenoreceptors (most strains) a- and b-adrenoreceptors

Nervous System

Thyroid "T3, T4, TSH with weight gain leading "T3 during weight gain, little brown

to "BAT thermogenesis adipose tissue (BAT) or beige adipose

tissue (brite) thermogenesis

a

Abbreviations: ", increased; T3, triiodothyronine; T4, thyroxine; TSH, thyroid stimulating hormone.

molecules known to affect energy output and intake, with- Bifidobacterium, Lactobacillus, Roseburia, and Faecalibac-

out direct measures of these variables. terium species [61]. In a double-blind placebo-controlled

It has been observed that probiotics (defined as live study of 16 adults, Cani et al. [62] administered an inulin-

microorganisms which when administered in adequate like prebiotic fiber versus a similar-tasting placebo (dex-

amounts confer a health benefit on the host [57]) – such trin/maltose), and found that prebiotic administration was

as particular species of Lactobacillus – reduce weight and associated with a significant decrease in hunger and post-

body fat in DIO mice without changing energy intake [58], prandial glucose excursions, and a significantly greater

in contrast to the transfer of intestinal flora from lean mice satiation after a meal, as well as increases in plasma

to gnotobiotic mice that results in weight (fat) gain despite glucagon-like peptide 1 (GLP-1) and peptide YY (PYY),

decreased energy intake and increased energy expenditure both of which are gut-derived peptides known to promote

[9]. This conjunction of phenotypes suggests increased satiation and glucose homeostasis [63].

efficiency in extraction of calories from the diet. Gnotobiotic mice that have been colonized with conven-

In humans, Kocelak et al. [59] examined resting energy tional mouse gut biota [9,10,33] gain weight (increase

expenditure (REE), body composition, and the gut micro- energy stores) despite decreasing energy intake and in-

bial population in 50 obese and 30 lean healthy weight- creasing energy expenditure. Rather than violating the

stable subjects. They reported that the obese subjects had a first law of thermodynamics, this observation raises the

significantly greater total microbial count without signifi- possibility that the microbiota affect nutrient absorption

cant differences in the ratio of Bacteroidetes/Firmicutes, (‘energy harvest’). Gut microbiota can facilitate extraction

both of which had been reported to be lower in obese of calories from ingested dietary substances. Turnbaugh

individuals in other studies [6,10]. Over the entire group et al. [10] reported that the obese (ob/ob) mouse microbiota

of subjects, the size of the population of Firmicutes was have an increased capacity to harvest energy from the diet

positively correlated with fat mass and negatively corre- regardless of whether it is present in ob/ob or gnotobiotic

lated with REE and the maximal oxygen consumption mice. More specifically, transplantation of the caecal

(VO2max). Total bacterial count was significantly positively microbiota of an obese mouse to gnotobiotic mice results

correlated with VO2 and negatively correlated with VCO2 in increased energy harvesting, including increased ab-

(rate of carbon dioxide production in this case during sorption of monosaccharides from the gut. This in turn

). However, in multiple regression analysis includ- leads to increased hepatic lipogenesis and stimulation of

ing fat mass, none of these correlations between the micro- both hepatic lipoprotein lipase (LPL) and sterol regulatory

biota and energy expenditure remained significant [59]. element-binding proteins (SREBPs). The approximately

The observation that administration of conventional 10% decrease in kcal lost per gram of stool in gnotobiotic

mouse bacteria to germ-free mice results in decreased mice colonized with the ob/ob microbiota suggests that

energy intake suggests a direct or indirect role of the associated decreases in energy harvest would be a poten-

microbiome in appetite regulation and the efficiency of tial means of reducing energy stores by switching to a

energy harvest from food [9,60]. Prebiotics, defined as leaner microbiome. However, this does not address issues

selectively fermented nondigestible food ingredients or such as ‘if one absorbs less food energy why doesn’t one just

substances specifically supporting the growth and/or activ- eat more or burn less to compensate, as occurs after

ity of health-promoting bacteria that colonize the gastro- induction of decreased energy harvest by pharmacological

intestinal tract [57], are valuable tools for modulating the agents such as orlistat?’ [64].

human gut microflora [61]. Most relevant, addition of small Similar studies examining energy harvest after manip-

amounts of dietary inulin-type fructo-oligosaccharides giv- ulating the gut microbial population have not, to our

en to humans stimulate the growth of health-promoting knowledge, been done in humans. Jumpertz et al. [35]

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examined the effects of underfeeding (2400 kcal/day) and mice produce almost no SCFAs [69]) and have distinct

overfeeding (3400 kcal/day) on the human microbiota of actions relevant to energy homeostasis in addition to serv-

12 lean and nine obese individuals. They reported that the ing as an energy source for the colonic epithelia (butyrate),

differences between caloric intake and weight mainte- liver (propionate), and peripheral tissues (acetate)

nance calories were positively correlated with the relative [70]. The major microbiotic phyla with species affecting

abundance of Firmicutes species and negatively correlated SCFA production in the gut are Firmicutes and Bacter-

with the relative abundance of Bacteroidetes species in oidetes, as well as the minor phyla Melainabacteria

lean and obese humans. This suggests that the microbiota (Table 1).

is responsive to energy balance (degree of overfeeding or SCFAs bind to two orphan G protein-coupled receptors

underfeeding) as well as actual adiposity. An approximate- (GPCRs); GPR41 (also termed free fatty acid receptor 3 or

ly 20% increase in abundance of Firmicutes and a corre- FFAR3) primarily activates Gi/o protein and is expressed in

sponding decrease in Bacteroidetes abundance (which is the gut, adipocytes, and the peripheral nervous system,

what would occurring during weight gain) was associated and GPR43 (also termed free fatty acid receptor 2 or

with a 150 kcal/day (approximately 5% of ingested calories) FFAR2) primarily activates Gi/o and Gq proteins in the

increase in daily energy harvest [35]. gut and adipose tissue. In addition, activation of GPR43 in

The mechanisms by which the microbiome affects ener- the intestines by SCFAs has been shown to decrease

gy harvest are not clear. Nutrient absorption is clearly release of inflammatory cytokines [69], which may also

influenced by the length of exposure to the gut mucosa. increase hypothalamic sensitivity to leptin [71,72]; GPR41

Kashyap et al. [65] reported that colonization of germ-free activation promotes growth and activation of the sympa-

mice with human or rodent bacteria was associated with a thetic nervous system. The density of Bacteroidetes in the

decrease in gastrointestinal (GI) transit time, and that gut microbiota has been positively associated with fecal

shortening transit time or increasing GI transit time concentrations of propionate, butyrate, acetate, and

resulted respectively in increased abundances of Bacter- SCFAs [73]; in other words, the same bacterial species

oidaceae and Porphyromonadaceae. This type of study is that are diminished in obese subjects and increased by

clearly replicable in humans. weight loss are also correlated with changes in SCFAs

whose increase would favor weight loss and the mainte-

Leptin signaling nance of reduced weight.

Introduction of conventional gut bacteria into gnotobiotic SCFAs affect energy homeostasis by discrete mecha-

mice increases fatness but does not appear to affect the nisms. GPR41 is activated equally by propionate and

anticipated increases in the anorexigenic hormones lep- butyrate [74,75], whereas GPR43 is more responsive to

tin, and hypothalamic pro-opiomelanocortin (POMC) and propionate and acetate than to butyrate [74,75]]. Butyrate

cocaine–amphetamine-related transcript (CART), and has been shown to inhibit histone deacetylases (HDACs),

decreased expression of the orexigenic peptides agouti- thereby inducing histone hypermethylation with subse-

related peptide (AGRP) and neuropeptide Y (NPY) quent changes in gene transcription affecting genes and

[66,67]. This suggests that the changes in the adipose– pathways involved in fatty acid oxidation, epithelial integ-

leptin–hypothalamic pathway are induced by increases rity, and apoptosis [76–78].

in adipose tissue mass rather than by a primary central- Similarly, there are distinct effects of each SCFA on

ly mediated effect. However, the colonization of germ- various gut peptides (incretins and hormones) and energy

free mice with conventional microbiota significantly homeostasis. Oral administration of butyrate significantly

blunts both the weight loss and the decline in Agrp increases plasma levels of gastric inhibitory peptide (GIP),

and Npy expression following leptin administration GLP-1, PYY, insulin, and amylin which would have a net

[66,67], and prebiotic manipulation of the microbiota effect of slowing digestion and nutrient intestinal transit,

to decrease Firmicutes and increase Bacteroidetes phyla promoting satiety, and increasing plasma insulin. There is

in leptin-deficient mice resulted in increased leptin-sen- a more modest, but still significant, effect of propionate

sitivity [68], suggesting that the gut microbiota centrally administration on GIP, insulin, and amylin, and no effect

affect leptin signaling. Germ-free mice have significantly of acetate administration on any of these hormones. Ace-

increased brainstem expression of proglucagon (Gcg) tate is reported to increase leptin release by fat cells;

that encodes the precursor for the incretin and anti- butyric acid and propionate increase G-protein mediated

obesity molecule glucagon-like peptide 1 (GLP-1). Reduc- secretion of PYY and GLP-1 in the gut, and rates of

tions are also seen in the anorexigenic brain-derived lipolysis and lipogenesis in fat cells [74,75].

neurotrophic factor (Bdnf) and leptin resistance-associ- Butyrate and acetate have been reported to protect

ated suppressor of cytokine signaling 3 (Socs3) expres- against diet-induced obesity without hypophagia, while

sion in both the brainstem and hypothalamus, compared propionate has been reported to reduce food intake and

to conventionally raised mice. Microbe-induced suppres- increase locomotor activity [70]. Collectively, these data

sion of any or all of these molecules might promote would suggest that acetate, because it has little effect on

weight gain [51,52]. activity or energy intake, exerts its actions primarily via

effects on energy harvest and/or basal thermogenesis (as

Short-chain fatty acids evidenced by increased oxygen consumption in rats given

Short-chain fatty acids (SCFAs), predominantly butyrate, oral acetate [79]), whereas butyrate and propionate have

acetate, and propionate, are the end-products of fermenta- more direct effects on feeding behavior and physical

tion of dietary fiber by various gut microbes (germ-free activity.

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Concluding remarks and future perspectives nature and have failed to distinguish whether alterations

This review focuses on the role of the human gut microbiota of the microbiota are a cause or consequence of changes in

in systems regulating energy homeostasis. It should be fat mass.

recognized that the effects of the intestinal microbial pop- Some of the same methodologies used in rodent stud-

ulation extend to multiple other systems, including glucose ies are clearly applicable to humans. Manipulation of the

and lipid homeostasis, blood pressure, inflammation microbiota by transplantation, diet, or prebiotics in dif-

(Box 1), and other adiposity-related comorbidities [80]. ferent metabolic states (obese, formerly-obese, and nev-

Most systems regulating energy intake are altered fol- er-obese) could be used to isolate the possible role(s) of

lowing weight loss in a manner that facilitates weight the microbiota in the regulation of energy homeostasis.

regain. By contrast, weight reduction is associated with In rodent studies discussed above there is substantial

a change in the gut microbiota towards that of a non-obese variability in the responses of the animals to fecal

individual. If the obese microbiota facilitates weight gain transplantation based on diet and even the source (ven-

in humans, then the shift in the microbiota as a result of dor) of the rodents. Exactly as there is large interindi-

weight reduction should assist rather than oppose sus- vidual variation in human responses to diet, exercise,

tained weight reduction. pharmacological or other weight-loss interventions,

A review of the mouse literature suggests that the there are likely to be differences between individuals

microbiome affects energy intake and expenditure such in the salience of the roles of microbiota in energy

that specific alterations of the gut microbiota might con- homeostasis. Data from larger-scale studies could iden-

stitute a potential therapeutic intervention to prevent tify specific behavioral, microbiotic, and metabolic phe-

obesity and/or to promote and sustain weight loss in notypes that might be predictive of response to different

humans (Box 2). Before such applications are considered types of behavioral, pharmacological, pro-/pre-biotic, or

in humans, further studies are clearly necessary, using surgical therapies.

manipulations involving prebiotics, probiotics, and diet

composition. In contrast to mechanistic work done in the Acknowledgments

mouse, studies of the role of human microbiota and energy Work by the authors and cited in this manuscript has been supported by

the US National Institutes of Health (DK30292, DK078669, DK70977,

homeostasis have been predominantly epidemiological in

DK64774, HG4872, and UL1TR000040).

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