The next time you can’t figure out why you’re suddenly craving a huge slab of ultra-decadent chocolate cake—consider the possibility that it’s not just you doing the craving. New research suggests that the armies of bacteria living in our guts can pull the strings in our brains to get what they want.
We’ve all heard about the bacterial universe within our bodies, but what’s less well known is just how vast this universe is in comparison to the rest of us: bacteria outnumber all of the cells in our body 100 to 1. And just like us, certain bacteria have a taste for certain nutrients, and they’ve developed ways of influencing their hosts to deliver more of their preferred vittles to the dinner table.
"Bacteria within the gut are manipulative," said Carlo Maley, PhD, director of the UC San Francisco Center for Evolution and Cancer and study co-author. "There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals, and others not."
How this happens is still an unfolding story, but researchers think bacteria release chemical signals that are carried along the vagus nerve—the nervous system superhighway that runs from the digestive system all the way to the base of the brain. These signals may affect our moods and appetites, and influence us to get more of what the bacteria crave into our mouths.
"Our diets have a huge impact on microbial populations in the gut," Maley said. "It's a whole ecosystem, and it's evolving on the time scale of minutes."
A strain of bacteria found only in the guts of Japanese people, for instance, has evolved to specifically digest seaweed, a normal part of Japanese diets. In the same way, certain bacteria subsist on fats and sugars in diets more heavily laden with those items.
It’s theorized that if bacteria want more sugar, they use a chemical carrot and stick approach; certain chemicals cause us to feel bad until we ingest the sugar, and others perk up our mood as a reward for delivering the goods.
"Microbes have the capacity to manipulate behavior and mood through altering the neural signals in the vagus nerve, changing taste receptors, producing toxins to make us feel bad, and releasing chemical rewards to make us feel good," said study co-author Athena Aktipis, PhD.
The good news, the researchers tell us, is that we can influence changes in our gut dwellers through dietary choices.
"Because microbiota are easily manipulatable by prebiotics, probiotics, antibiotics…and dietary changes, altering our microbiota offers a tractable approach to otherwise intractable problems of obesity and unhealthy eating," the researchers wrote.
There's a growing base of scientific literature supporting the claim. A few studies have shown that probiotics can decrease anxiety levels in mice, and last year researchers from UC Los Angeles published results showing that the brains of people ingesting a probiotic for four weeks had less activity in brain areas associated with excessive anxiety. It's theorized that the probiotic altered the bacterial landscape in the gut, with the effect of changing chemical signals sent to the brain.
This is, of course, just the beginning of a lengthy investigation into the relationship between our intestinal microbiota and the brain, but so far it seems quite plausible that not all of our appetites are our own.
The study was published in the journal BioEssays.
You can find David DiSalvo on Twitter @neuronarrative and at his website The Daily Brain. His latest book is Brain Changer: How Harnessing Your Brain’s Power To Adapt Can Change Your Life.
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