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Abstract and Figures

The gut microbiome consists of trillions of bacteria which play an important role in human metabolism. Animal and human studies have implicated distortion of the normal microbial balance in obesity and metabolic syndrome. Bacteria causing weight gain are thought to induce the expression of genes related to lipid and carbohydrate metabolism thereby leading to greater energy harvest from the diet. There is a large body of evidence demonstrating that alteration in the proportion of Bacteroidetes and Firmicutes leads to the development of obesity, but this has been recently challenged. It is likely that the influence of gut microbiome on obesity is much more complex than simply an imbalance in the proportion of these phyla of bacteria. Modulation of the gut microbiome through diet, pre- and probiotics, antibiotics, surgery, and fecal transplantation has the potential to majorly impact the obesity epidemic.
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INTEGRATIVE CARE (C LAMMERSFELD, SECTION EDITOR)
The Gut Microbiome and Obesity
George Kunnackal John
1
&Gerard E. Mullin
1
Published online: 2 June 2016
#Springer Science+Business Media New York 2016
Abstract The gut microbiome consists of trillions of bacteria
which play an important role in human metabolism. Animal
and human studies have implicated distortion of the normal
microbial balance in obesity and metabolic syndrome.
Bacteria causing weight gain are thought to induce the expres-
sion of genes related to lipid and carbohydrate metabolism
thereby leading to greater energy harvest from the diet.
There is a large body of evidence demonstrating that alteration
in the proportion of Bacteroidetes and Firmicutes leads to the
development of obesity, but this has been recently challenged.
It is likely that the influence of gut microbiome on obesity is
much more complex than simply an imbalance in the propor-
tion of these phyla of bacteria. Modulation of the gut
microbiome through diet, pre- and probiotics, antibiotics, sur-
gery, and fecal transplantation has the potential to majorly
impact the obesity epidemic.
Keywords Microbiome .Obesity .Microbiota .Diet .
Prebiotic .Probiotic .Microbial balance .Bacteroidetes .
Firmicutes
Introduction
The human microbiome encompasses several trillion mi-
crobes residing in the gut and the genes that are encoded by
them [1,2]. The majority of these microbes reside in the colon,
where they are present in a concentration of 10
9
10
12
CFU/
mL [3]. There is clear evidence from animal and human stud-
ies that the gut microbiome plays a crucial role in the func-
tioning of the digestive tract and in harvesting energy from the
diet [4,5].
The microbiome maintains the integrity of the intestinal
epithelial barrier thereby offering protection from pathogenic
bacterial colonization [6,7]. In addition, the microbiome is
essential for metabolizing indigestible polysaccharides and
in the absorption of short-chain fatty acids produced by bac-
terial fermentation [8]. It also plays a key role in the regulation
of intestinal transit, thereby affecting the amount of energy
absorbed from the diet [9]. These and other key functions
elucidate the crucial role of the microbiome in weight gain
and metabolism and are reviewed in more detail [10,11].
Current data estimates that approximately 600 million peo-
ple around the world are obese, with an additional 1.9 billion
people being overweight [12]. One of the most cited
microbiome related factors differentiating obese and healthy
individuals has been the shift in the proportion of bacterial
flora belonging to the Firmicutes and Bacteroidetes phyla
which together comprise about 90 % of the microbiota of
the adult gut [13]. The Firmicutes phylum comprises gram
positive organisms from greater than 200 different genera in-
cluding Catenibacterium,Clostridium,Eubacterium,Dorea,
Faecalibacterium,Lactobacillus,Roseburia,Ruminococcus,
and Veillonella while the Bacteroidetes phylum consists of
gram negative bacteria from approximately 20 genera includ-
ing Bacteroides,Odoribacter,Prevotella,andTannerella [14].
Studies using 16S rRNA gene sequencing of the distal gut
microbiota of ob/ob mice show that there is significant reduc-
tion in the abundance of Bacteroidetes and a similar increase
in the Firmicutes phyla in obese mice [8]. However, subse-
quent studies have shown discrepancies in the proportion of
Bacteroidetes/Firmicutes and its relation to obesity and it is
This article is part of the Topical Collection on Integrative Care
*Gerard E. Mullin
gmullin1@jhmi.edu
1
Division of Gastroenterology and Hepatology, Johns Hopkins
Hospital, 600 N. Wolfe Street, Baltimore, MD 21287, USA
Curr Oncol Rep (2016) 18: 45
DOI 10.1007/s11912-016-0528-7
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
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The gut microbiota helps balance key vital functions for the host, including immunity and nutritional status. Studies have also linked the microbiome to human mood and behavior, as well as many gut disorders, eczema, and a number of systemic disorders (Azad et al., CMAJ 185:385–394, 2013). Changes in the gut microbiota composition and/or activity may be implicated in the control of inflammation, fat storage, and altered glucose response in obese patients. Dietary short-chain fatty acids appear to be “indirect nutrients” produced by the gut microbiota that can modulate adiposity and immunity as well as send signals to the gut to produce hormones that regulate appetite, permeability, and inflammation. Numerous data have been published regarding differences in the composition of the gut microbiota in obesity. Taken together, the data currently published suggest that specific changes in the gut microbiota occur in overweight or obese patients and are either positively or negatively linked with adiposity, inflammation, and glucose or lipid homeostasis. Manipulation of the microbiota though diet can promote healthy weight loss by altering gut function and metabolism. Probiotics and prebiotics are interesting research tools to assess the relevance of specific bacteria in obesity. Prebiotics may lessen obesity and related metabolic stress by modulating gut peptides involved in the control of appetite and gut barrier function.