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The Food Fight in Your Gut: Why Bacteria Will Change the
Way You Think about Calories

By Ferris Jabr
September 12, 2012
http://blogs.scientificamerican.com/brainwaves/2012/09/12/the-food-fight-in-your-guts-why-bacteria-will-change-the-way-you-think-about-calories/

There’s a food fight in your guts. Not the Tater-Tot-
chucking, spoonful-of-mashed potato-flinging, melee-in-
the-cafeteria type of food fight. Rather, your
intestines are the site of an ancient and complex war
between your own cells and trillions of bacteria—a war
over what happens to your food as it moves through your
body. Some of the bacteria form genuine alliances with
your intestinal cells, breaking down tough plant fibers
that your cells cannot handle on their own, or chopping
up lengthy caterpillar molecules into more digestible
packages, in exchange for a portion of the day’s
calories. Other bacteria lurk and loiter, sipping the
nutrient-rich broth sloshing in your intestines as they
wait for their chance to overrun your guts at the
expense of your health. Every day, these microorganisms
squabble amongst themselves for greater access to
available nutrients. And sometimes your cells fight
back, working extra hard to digest the food you eat
before those persistent microbes help themselves to a
disproportionately large serving. Studies suggest that
the diversity of bacterial species in our guts partially
determines how efficiently our cells process and store
food and that, in a feedback loop, what we eat alters
the demographics of the bacteria in our intestines.
Commonly prescribed antibiotics are responsible for
unintended microbial casualties, further changing how
our resident population of microorganisms responds to
our diet. Although scientists are still figuring out the
rules of this intricate food fight, it’s evident by now
that our guts are not entirely our own—they are
composite organs, part-human, part-microbe, which
evolved, and continue to function, as communities whose
many minute members are sometimes cooperative, sometimes
combative and always hungry.

A study published this week adds nuance to scientists’
evolving understanding of how gut bacteria change the
way animals digest food. Ivana Semova and John Rawls of
the University of North Carolina at Chapel Hill, along
with their colleagues, studied the absorption of
fluorescent fatty acids in the intestines of tiny
translucent zebrafish (Danio rerio). Compared to
zebrafish raised in germ-free environments, zebrafish
whose guts were colonized by bacteria absorbed more fat
from their diets. And the more the fish ate, the larger
the population of bacteria in their guts. In particular,
eating encouraged the growth of a tribe of bacteria
known as Firmicutes, which in turn increased the number
of energy-rich fat bubbles stored within the fish’s
intestinal cells for later use. Studies with people and
mice have also shown that high-calorie diets stimulate
the growth of Firmicutes in the gut, hinting that this
particular group of bacteria may respond to its host’s
diet in similar ways across many different species. What
remains unclear is whether Firmicutes helps animals
absorb more calories from their food in a mutually
beneficial partnership or if the relationship is more
complex—and sometimes less than benevolent.

Bacteria constitute between 40 and 60 percent of the dry
weight of human feces, with trillions of cells in every
gram. Zebrafish intestines are not home to the exact
same species of bacteria that live in our own guts,
but—if you take a broad enough view of the
communities—they have a surprising amount of overlap.
Both communities are dominated by the phyla
Proteobacteria, Firmicutes, and Bacteroidetes (phylum is
the taxonomic level below kingdom). Young zebrafish are
also particularly convenient for scientists who want an
inside look on the digestive process because day-old
zebrafish are transparent—you you can see everything
that is happening in their intestines under a microscope
without the need for a damaging and disruptive
dissection.

Semova and Rawls chemically bonded fluorescent molecules
to two common fatty acids, palmitic acid pentanoic acid,
and mixed the glowing fats into the egg yolk of
embryonic zebrafish. The intestinal cells of zebrafish
that were exposed to bacteria as they developed glowed
more brightly than the intestinal cells of zebrafish
that were raised in sterile environments, indicating
that zebrafish guts squirming with bacteria absorbed
more fat. The intestinal cells of zebrafish with healthy
populations of gut bacteria, collectively known as gut
microbiota, also contained larger lipid droplets—bubbles
of fat stored as expedient sources of energy.

The number of lipid droplets in the fish’s intestinal
cells depended on their diet. Fish with bacteria in
their guts and a steady source of food had much higher
numbers of lipid droplets in their intestinal cells than
fish that were denied food for a few days. Eating
specifically promoted the growth of bacterial species in
the phylum Firmicutes and this increase was not
reflected by changes in the numbers of bacteria in the
surrounding water. Eating changes a fish’s internal
ecosystem. The more a zebrafish eats, the more
Firmicutes in its guts. And the more Firmicutes in a
zebrafish’s guts, the more efficiently its intestinal
cells absorb fat.

To investigate how Firmicutes stimulates fat absorption,
Semova and Rawls grew different strains of bacteria in
different liquid media, which you can think of as a kind
of broth. After filtering out the bacteria, they exposed
baby zebrafish to the different media. Only media from
Firmicutes significantly increased the number of lipid
droplets in the fish’s intestinal cells, suggesting that
whatever proteins or molecules those bacteria secreted
into the media somehow enhanced fatty acid absorption.
The results were published September 13 in Gut Host &
Microbe.

These findings mirror the conclusions of many previous
studies, which have shown, for example, that starving
mice for a single day reduces the population of
Firmicutes in their guts and that transplanting
Firmicutes from obese mice into the germ-free intestines
of lean mice makes the thin rodents plump. When obese
people begin a low-fat or low-carb diet, Bacteroidetes
proliferates and Firmicutes dwindles. Clearly,
Firmicutes is happiest when we are eating a lot. One
pertinent and unanswered question is whether we should
share that happiness. Are Firmicutes graciously helping
us extract more calories from our food, taking only a
modest cut for themselves? Are they selfishly increasing
their own numbers when the eating is good, forcing our
cells to sweat to get the most out of our food? Are they
in fact making digestion too easy, liberating so many
calories from our food that we absorb far more than we
need? Perhaps there is truth in all these scenarios.

“We are in the midst of a revolution of our ability to
describe the composition and physiological potential of
these bacterial communities,” Rawls says. “What we can
begin to speculate on, though, are the different types
of relationships that might be taking place. We know gut
microbiota enhance our ability to extract calories from
complex carbohydrates, which is clearly a mutually
beneficial relationship. But it’s thought that all
vertebrates have the capacity to digest and absorb other
types of nutrients, such as lipids, proteins and simple
carbohydrates, so it’s not readily clear how we could
enter into a mutually beneficial relationship with
bacteria with regard to those nutrients. When we see
fatty acid absorption increased in zebrafish, that may
be selfish or defensive response. Perhaps the fish
recognizes the presence of more bacteria and increases
its own fatty acid absorption. It may not always be such
a friendly arrangement.”

About the Author: Ferris Jabr is an associate editor
focusing on neuroscience and psychology. Follow on
Twitter @ferrisjabr.

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