What's living in your gut may be responsible
University of Pittsburgh
"We
all have those urges -- like if you ever you just feel like you need to eat a
salad or you really need to eat meat," said Kevin Kohl, an assistant
professor in the Department of Biology in the Kenneth P. Dietrich School of
Arts and Sciences. "Our work shows that animals with different
compositions of gut microbes choose different kinds of diets."
Despite
decades of speculation by scientists about whether microbes could influence our
preferred diets, the idea has never been directly tested in animals bigger than
a fruit fly. To explore the question, Kohl and his postdoc Brian Trevelline
(A&S '08), now at Cornell University, gave 30 mice that lacked gut microbes
a cocktail of microorganisms from three species of wild rodents with very
different natural diets.
The duo found that mice in each group chose food rich in different nutrients, showing that their microbiome changed their preferred diet. The researchers published their work today in the Proceedings of the National Academy of Sciences.
While
the idea of the microbiome affecting your behavior may sound far-fetched, it's
no surprise for scientists. Your gut and your brain are in constant
conversation, with certain kinds of molecules acting as go-betweens. These
byproducts of digestion signal that you've eaten enough food or maybe that you
need certain kinds of nutrients. But microbes in the gut can produce some of
those same molecules, potentially hijacking that line of communication and
changing the meaning of the message to benefit themselves.
One
such messenger will be familiar to anyone who's had to take a nap after a
turkey dinner: tryptophan.
"Tryptophan
is an essential amino acid that's common in turkey but is also produced by gut
microbes. When it makes its way to the brain, it's transformed into serotonin,
which is a signal that's important for feeling satiated after a meal,"
Trevelline said. "Eventually that gets converted into melatonin, and then
you feel sleepy."
In
their study, Trevelline and Kohl also showed that mice with different
microbiomes had different levels of tryptophan in their blood, even before they
were given the option to choose different diets -- and those with more of the
molecule in their blood also had more bacteria that can produce it in their
gut.
It's
a convincing smoking gun, but tryptophan is just one thread of a complicated
web of chemical communication, according to Trevelline. "There are likely
dozens of signals that are influencing feeding behavior on a day-to-day basis.
Tryptophan produced by microbes could just be one aspect of that," he
said. It does, however, establish a plausible way that microscopic organisms
could alter what we want to eat -- it's one of just a few rigorous experiments
to show such a link between the gut and the brain despite years of theorizing
by scientists.
There's
still more science to do before you should start distrusting your food
cravings, though. Along with not having a way to test the idea in humans, the
team didn't measure the importance of microbes in determining diet compared to
anything else.
"It
could be that what you've eaten the day before is more important than just the
microbes you have," Kohl said. "Humans have way more going on that we
ignore in our experiment. But it's an interesting idea to think about."
And
it's just one behavior that microbes could be tweaking without our knowledge.
It's a young field, Kohl points out, and there's still lots to learn.
"I'm just constantly amazed at all of the roles we're finding that microbes play in human and animal biology," Kohl said.