You and the Parasites in Your Body
When you eat, there will be trillions of microbes,
including fungi and bacteria, sharing your meal.
They are essential to your good health. These
microscopic organisms break down toxins in the body,
make essential amino acids and vitamins, and help the
immune system prepare against non-beneficial
microorganisms. And, they may help control weight.
The body is
home to a huge but
ever-changing community of
microbes which mostly live in the intestines; the bulk of the
rest inhabit the mouth,
esophagus, stomach, upper
airway, skin and vagina. No one knows
how many different species coexist
inside the human intestinal tract. In one study at Stanford University,
scientists
quit counting after they hit
395 different species in three healthy subjects. The number would almost certainly number in the
thousands.
Curiously;
we don't start life with such a microbial partnership. A
developing baby floats in
sterile amniotic, fluid, protected
from bumps—and bugs. That isolation
ends during the baby's trip through the birth canal, which is a haven
for bacteria. The baby picks up microbes on his or here skin; some get into the mouth.
From
then on, helpful
microbes somehow convince the immune system that they mean no harm. They settle down in
hospitable regions, and crowd
out those that can't
compete.
At
first, the microbes in a baby's body resemble those in the mother. Over
time, the community takes on
its own identity, nudged this way and that by the child's genes, the
environment and the unceasing flow of new microbes from food, beverages
and unwashed hands. Eventually, an individual's intestinal microbes becomes as unique as a fingerprint.
The
gut is composed of the small and large intestine. Stretched out, it's
as long as a school bus.
Flatten out the millions of
fingerlike projections that line its sides and it would easily cover a
tennis court. The small intestine is
where much of the food you
eat is broken down into
simple sugars, fats and amino acids. These are all small enough to be
shuttled across the lining
of the intestine and into nearby blood vessels. Fiber from fruits, vegetables
and, grains, along with
other indigestible material,
Passes largely unchanged into
the large intestine, also known as the colon.
What's
indigestible to you is a seven- course meal to your gut microbiota.
The conditions in the colon—dark, moist and five of oxygen--are just what your gut microbiota
needs to ferment indigestible material
passed on from the small intestine and produce simple sugars and short,
chain-free fatty acids. They
do this for their own good,
but they also share some of
these energy-rich substances with their host-us. Some people get up to 10
% of their daily
calories from substances produced
by their microorganisms.
It may be that the gut microbes in some people are
more efficient at extracting
energy than those in other
people. This could partly explain
why
some individuals gain more weight on
the same diet that allows others
to stay lean. Gordon's
hypothesis is supported by a series of elegant experiments.
First, Dr. Jeffrey Gordon's team, at Washington University, raised generations
of mice in sterile conditions.
These
microbe-free mice
downed almost one
third
more food each day than their ordinary
counterparts—yet had 40 percent less body fat When the researchers
took samples of gut
microbes from ordinary mice and transplanted them into
germ-free
mice, the newly
inoculated rodents began to gain weight even though they weren't
eating any extra food. The
team took the work a
step further with help from a strain
>of genetically obese mice. Transplanting
gut microbes from these fat mice into
lean, germ-free mice led to greater gains in
body fat than transplanting gut
microbes from normal mice.
lb
see if fat mice had different gut bugs
than lean mice, the Washington
University
team took
genetic snapshots using high-tech DNA sequencers. In ordinary
normal-weight mice,
bacteria belonging to the
'group
known as Finnicutes accounted for about two thirds of the
gut's bacterial
community. Members of the Bacteroidetes group made up most of the rest.
In contrast, genetically
obese mice had even
more
Firmicutes and many fewer Bacteroidetes.
By analyzing the sequence of genes extracted from various microbiota
samples, Gordon's team
discovered that the
bacterial community in obese mice had more genes for breaking down
complex
starches and
fiber. In other words, microbes
from obese mice were better at re
leasing
calories from the gut's contents
than were the microbes from lean
mice.
Think of it this war the gut community
in obese mice is like a fuel-efficient car,
extracting more energy from food
and passing more along to
its host than its gas-
guzzler counterpart in lean mice.
Mice are mice. Does any of this apply to
humans? 'lb find out, the
Washington University team
asked a dozen obese men
and women to follow a low-fat or low-
carbohydrate diet for a year. Before starting
these
diets, these obese volunteers had
more Firmicutes and fewer
Bacteroidetes in their guts than did several lean volunteers
acting as controls—just as was
seen
in obese and lean mice, As
the volunteers lost weight, their microbial communities
underwent a remarkable shift,
with an in
crease in the
gas guzzlers (Bacteroidetes) and a decrease in the efficient
energy ex
tractors
(Firmicutes). The type of diet
didn't matter; only significant
weight loss
sparked the
shift,
One implication of this
work is that the energy content of food isn't a
fixed quantity. Consider the 110
calories per cup listed on
a box of Cheerios. Some people may get
that much, others may get less,
depending
on their gut
microbiota. A difference of just 25 calories a day—that's half a
rice cake or one
chocolate kiss—between what
you take in and what you burn
could mean a gain or loss
of more than two pounds in a year and 20 pounds over
a decade.
On a more practical note,
this work suggests that
somehow altering the microbial
populations in the gut could be one way to modify weight. If
Gordon's work continues
to pan out, it may be possible
someday to use probiotics
—dietary supplements
containing potentially beneficial microbes—or other
microbe-manipulating strategies to aid weight loss by nudging
the gut microbiota to be less efficient at extracting energy.
Probiotics are already on
the market, most of them
containing some form of
Lactobacillus,
best known for its yogurt-
making abilities. They're
used to fight allergies,
diarrhea and a variety of other conditions, although the evidence
for their use <
remains spotty. Major companies such as General Mills and
food-ingredient supplier
Danisco are exploring links between probiotics and weight control.
It's a bit early to do such
microbial gardening for
weight loss, cautions Randy
Seeley, associate director of
the Obesity
Research Center at the University' of
Cincinnati College of Medicine.
'Though he's impressed
with the work Go is team
has done, he isn't sure the results make sense from an
evolutionary perspective,
As the obese volunteers lost weight, their gut microbes shifted toward
a community that would
extract less energy from
its food supply. That doesn't make sense from a survival point of
view: "If I saw myself getting leaner, I'd want my body to say to my microbes,
'Guys, help me
out here,
and make the extra more calories,
not fewer," says Seeley.
Gordon is the first to acknowledge that there's a lot of work to be
done before anyone can point to gut microbes as a cause of obesity or start manipulating them as a way to lose weight Even if this line of research
doesn't pan out, the convergence of microbiology, molecular biology
and a host of other disciplines will shine new light on how we process what we
eat and what causes obesity.
While scientists have known for more than a century that we humans
live with a huge
community of permanent tiny neighbors,
it is only recently that research like Gordon's has suggested that these
neighbors may have
unexpected effects on our
health. In response, the National Institutes
of Health has launched the Human Microbiome Project, in order to
learn more about our gut
bugs—starting with their
genes. Sequencing of bacterial genes could also help researchers
prospect for hitherto
unknown chemicals made by our microbes that protect our health.
What is making the Human Microbiome Project feasible is the recent
development of superfast
gene sequencing technologies.
Taking a census is important for several reasons. "We need to know who's
there, especially the
good bugs that make up the majority of the microbial
community, so we can
minimize any harm to them when we go after the bad guys," says
George Weinstock, a
professor of molecular and human genetics at Baylor
College of Medicine who
is working on the project.
While obesity gets most of the attention,
Gordon has his eyes on another prize: fighting malnutrition.
Making microbes in the
gut more efficient could
be one way to help the millions of people around the globe who don't get
enough to eat each
day or those who involuntarily
lose weight while battling cancer or heart failure.
Your small and large intestines
are home to countless microbes that some scientists think may play a
major role in determining how fat or skinny you are.
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