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can carry up to 2 kg of microbes in our gut. Within the tens of trillions of
micro-organisms that live there are at least 1,000 species of bacteria
consisting of over 3 million genes. What is more, two thirds of the gut
microbiome - the population of microbes in the intestine - is unique to each
individual. But do you know how your gut microbiota could be influencing your
health?
The bacteria in our gut are estimated to weight up to 2 kg.
Most of us are aware that the bacteria in our gut play an important role
in digestion. When the stomach and small intestine are unable to digest certain
foods we eat, gut microbes jump in to offer a helping hand, ensuring we get the
nutrients we need.
In addition, gut bacteria are known to aid the production of certain vitamins - such as vitamins B and K - and play a major
role in immune function.
But increasingly, researchers are working to find out more about how gut
bacteria - particularly the bacteria that is unique to us individually -
influence our health and risk of disease.
Perhaps most studied is how gut microbiota affects an individual's risk of obesity and
other metabolic conditions. In November 2014, for example, Medical News Today reported on a study claiming our
genetic makeup shapes what type of bacteria reside in our gut, which may affect our weight.
In this Spotlight, we take a look at obesity and some of the other -
perhaps surprising - health conditions that may be driven by our gut
microbiota.
The development of gut microbiota
Belief has long held that the development of gut microbiota does not start
until birth, with the gastrointestinal tract of a fetus considered to be a
sterile environment.
According to Gut
Microbiota Worldwatch - an information service created by the
Gut Microbiota and Health Section of the European Society for
Neurogastroenterology & Motility, a member of the United European
Gastroenterology (UEG) - the digestive tract of a newborn is rapidly colonized
with micro-organisms from the mother and the surrounding environment.
An infant's gut microbiota, for example, can be influenced by
breastfeeding. Gut Microbiota Worldwatch explain that the gut of breastfed
babies primarily consists of Bifidobacteria -
considered a "friendly" bacteria that benefits the gut - while
formula fed babies are likely to have less of these bacteria.
However, some studies have challenged the belief
that the fetus is a sterile environment, suggesting that the development of gut
microbiota begins before birth.
A 2008 study published
in the journal Research in Microbiology identified
bacteria, including Enterococcus and Staphylococcus, in the early feces of baby mice - known as
the meconium - indicating the bacteria were transferred to the fetus from the
mother's gut during pregnancy.
In this study, a group of pregnant mice was also inoculated with the
bacterium Enterococcus fecium, which was isolated
from human breast milk. The baby mice were delivered by Cesarean section 1 day
before the predicted labor date, and their meconium was tested. The researchers
identified E. fecium in their feces,
but no trace was found in the meconium of a control group.
"Based on the sum of evidence, it is time to overturn the sterile
womb paradigm and recognize the unborn child is first colonized in the
womb," Seth Bordenstein, a biologist at Vanderbilt University in
Nashville, TN, told The Scientist last
year.
The more diverse our gut bacteria, the better
While the debate over whether infants are born with gut bacteria
continues, it seems scientists are in agreement about one thing: from birth
until old age, our gut bacteria are constantly evolving.
As mentioned previously, two thirds of the gut microbiome is unique to
each person, and what makes this unique is the food we eat, the air we breathe
and other environmental factors. Some studies have even suggested the makeup of
the gut microbiome is influenced by genes.
But how does this unique gut bacteria affect our health? This is a
question that researchers have become increasingly interested in answering.
Past research has suggested that a broader diversity of bacteria in gut is
better for human health. A recent study reported by MNT, for example,
found that infants with less diverse gut bacteria at the age of 3 months were more likely to be sensitized to specific foods -
including egg, milk and peanut - by the age of 1 year, indicating that lack of
gut bacteria diversity in early life may be a driver for food allergies.
But the implications of a low-diversity gut microbiome do not stop there.
You may be surprised to learn how lack of or overpopulation of specific
bacteria may impact your health.
Obesity
More and more studies are looking at the association between the gut
microbiome and weight gain, with some scientists suggesting the makeup of
bacteria in the gut may influence an individual's susceptibility to weight
gain.
Earlier in the article, we mentioned a 2014 study that claims our genes
may determine what bacteria live in our gut, and that these bacteria may
influence how heavy we are.
One study identified a specific strain of gut bacteria that may influence our weight.
In detail, the study - conducted by researchers from Cornell University in
Ithaca, NY, and King's College London in the UK - found that a certain strain
of bacteria - Christensenellaceae minuta -
was more common in people with a low body weight,
and that the presence of this particular strain is highly influenced by genes.
What is more, introducing this bacteria to the guts of mice caused the
animals to gain less weight, indicating the bacteria may reduce or prevent
obesity.
"Our findings show that specific groups of microbes living in our gut
could be protective against obesity - and that their abundance is influenced by
our genes," said study author Prof. Tim Spector of King's College London.
"The human microbiome represents an exciting new target for dietary
changes and treatments aimed at combating obesity."
In 2012, another study published in the Journal of Proteome
Research suggested that a lack of bacteria in the large
intestine may drive obesity by slowing down the activity of brown fat, which
protects against weight gain when stimulated by burning calories and white fat.
A more recent study provides further evidence that gut bacteria may
influence weight gain. A case report published in the journal Open Forum Infectious Diseases revealed how a women
who underwent fecal microbiota transplantation (FMT) using an overweight donor rapidly became obese herself following the procedure.
While the jury is still out on whether gut bacteria are directly
associated with obesity, it is an area that certainly warrants further
investigation.
Cancer
In recent years, scientists have increasingly investigated the link
between gut bacteria and cancer.
In a 2013 study published in The Journal of Cancer
Research, US researchers claimed to discover specific bacteria in
the intestines - Lactobacillus johnsonii -
that may play a role in the development of
lymphoma, a cancer of the white blood cells.
Another 2013 study conducted by UK researchers found that a common gut
bacteria called Helicobacter pylori may cause stomach cancer and duodenal ulcers by deactivating
a part of the immune system involved in regulating inflammation.
And in 2014, MNT reported
on research from the Icahn School of Medicine at Mount Sinai, New York, NY, in
which investigators associated a specific combination of gut bacteria with the development of colorectal cancer.
For this study, the team gave antibiotics to mice that possessed gene mutations known
to cause colorectal polyps, which can develop into cancer. The antibiotics were
administered to interfere with the gut bacteria of the mice. The researchers
found that these mice did not develop polyps, suggesting that gut microbes may
be involved in their development.
But as well as being linked to cancer development,
research has found that gut bacteria may be important for improving the
effectiveness of cancer treatment.
In 2013, a study by researchers from the National Cancer Institute found
that immunotherapy and chemotherapy were less effective in mice lacking gut
bacteria, with such treatments working significantly better in mice with a
normal gut microbiome.
Similar results were found in another 2013 study by French researchers. An
antitumor drug - cyclophosphamide - was found to be much less effective in mice
with limited gut bacteria, compared with mice with normal gut bacteria.
"Both studies are very exciting in that they show initial links
between gut microbiota and [response to] therapies," Harvard University's
Peter Turnbaugh told The Scientist.
"The papers underscore the importance [of] microbes in shaping not just
our initial predisposition to disease, but also our recovery from it."
Mental Health
Not many of us are likely to think about how gut bacteria affect the
mental state, but they actually play a very important role.
According to the American Psychological Association
(APA), gut bacteria produce an array of neurochemicals that the brain uses for
the regulation of physiological and mental processes, including memory,
learning and mood. In fact, 95% of the body's supply of serotonin is produced
by gut bacteria, according to the APA.
Since gut bacteria produce many of the neurochemicals responsible for regulating mental processes, it is no surprise that researchers have linked gut bacteria to mental health.
With this in mind, it is perhaps unsurprising that gut bacteria have been
associated with a number of mental health problems, including anxiety disorders and depression.
In 2014, for example, a study published in the journal Psychopharmacology found that prebiotics - carbohydrates that boost healthy bacteria in the gut -
may be effective for
reducing stress and anxiety.
For the study, 45 healthy adults were randomized to receive the prebiotic
or a placebo once a day
for 3 weeks. All participants were then exposed to both negative and positive
stimuli.
The team found that the participants who received the prebiotic were less
likely to pay attention to the negative stimuli than those who received the
placebo - suggesting lower anxiety in negative situations. They also had lower
levels of the "stress hormone" cortisol.
"Time and time again, we hear from patients that they never felt
depressed or anxious until they started experiencing problems with their
gut," said lead study author Dr. Kirsten Tillisch, the study's lead
author. "Our study shows that the gut-brain connection is a two-way
street."
Autism
Autism is
estimated to affect 1 in 68 children in the US. While studies have associated
environmental factors - such as pollution - and genetics as potential causes of
the disorder, researchers are increasingly looking at the role of gut bacteria
in its development.
In 2013, a study by researchers from Arizona State University found that
children with autism possessed
lower levels of three types of gut bacteria - Prevotella, Coprococcus and Veillonellaceae - compared with children free of the
condition.
A more recent study from the team found that concentrations of specific
chemicals produced by gut bacteria - called metabolites - in fecal samples of
children with autism differed to the concentrations found in the fecal samples
of children without the disorder.
This led the researchers to hypothesize that gut
microbes alter the metabolites associated with communication between the gut
and the brain, which interferes with brain function.
Further strengthening the association between gut bacteria and autism is a
2013 study published in Cell that
found the bacterium Bacteroides fragilis reduced
autism-like symptoms in mice.
Can we alter our gut bacteria?
Since the gut microbiome is influenced by the food we eat and the
environment around us, it makes sense that there are ways to make it healthier.
According to the Mayo Clinic, a healthy diet can encourage the presence of
good gut bacteria. They note that consuming fermented foods - such as miso and
sauerkraut - increases the level of fermenting bacteria in the gut. In
addition, fruits and vegetables contain fibers and sugars that can boost the
health of gut bacteria.
Exercise may also be key to improving gut bacteria diversity, according to
a study reported by MNTin June
2014.
The study, published in the journal Gut, compared
the gut bacteria of 40 professional rugby players with that of two control
groups. They found that the rugby players had much higher levels of Akkermansiaceae in their gut - a bacterium that has
been associated with reduced risk of obesity.
In an editorial linked to the study, Dr. Georgina Hold, of the Institute
of Medical Sciences at Aberdeen University in Scotland, noted that a better understanding
of ways to improve health via gut bacteria is crucial:
"As life expectancy continues to increase,
it is important that we understand how best to maintain good health. Never has
this been more relevant than in respect of our resident microbiota.
Understanding the complex relationship among what we choose to eat, activity
levels and gut microbiota richness is essential.
Developing new ways to manipulate the beneficial
properties of our microbiota by finding ways to integrate health-promoting properties
into modern living should be the goal.
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