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How Your Gut Affects Your Mood, by FiveThirtyEight

Stashed in: Brain, Awesome, Scatology, Nate Silver, Microbiome, Mental Health, Mental Health, Probiotics!

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First time I've heard the term psychobiotics. 

At any given moment, you have somewhere between 10 trillion and 100 trillionmicroorganisms inhabiting your gut — that’s more microbes in your bowels than there are cells in your body. If that isn’t impressive enough, consider that collectively these microbes have about 150 times as many genes as your own genome. Scientists are still trying to figure out exactly which microbes make up the human microbiome, but it’s estimated to contain more than 1,000 species and 7,000 distinct strains of bacteria. Your gut is never alone.

It’s also not working in isolation. What’s becoming more and more clear is that the microbes in the gut are crucial for the brain and mental health. Ted Dinan is an expert in this field, and he became so almost by accident. It was the early 2000s, and he’d recently taken a position at University College Cork, a place that he said was “known for its heavy-hitting microbiologists.” Some of these microbiologists were talking about a type of bacteria they described as “probiotic” — conferring some kind of health benefit. As a psychiatrist, Dinan thought it would be interesting to see what happened when he fed these probiotics to some rats he was studying in an experimental model of mental health. Lo and behold, rats given the probiotics expressed fewer signs of anxiety and depression. Dinan and his colleagues would go on to coin the term “psychobiotics” for microbes that can benefit the brain or behavior.

It’s the fourth (and final!) day of this week’s series on gut science. We’ve written about whether probiotics workwhether gut science is biased and why we’re so obsessed with constipation, and we’ve made a video about what poop can tell us about our health.

The gut-brain axis is a complicated system that is difficult to study. 

There’s little doubt that changing your diet changes the microbiome in your gut. “What’s debatable is whether or not you can guide your microbiome in a direction that will have benefits,” Eisen said. Research has shown that people with certain health characteristics or disease have particular microbes, but that doesn’t tell us with any certainty that those microbes have any causative relationship with those traits. As author Ed Yong wrote a few years back, there is no “normal” or “healthy” microbiome that one should aim for. “The microbiome is complex, varied, ever changing and context-dependent.”

“Too many large-scale, spurious claims are being made,” Dinan said, mostly by people with something to sell. Although we have “very good data” from animal studies, he said, there’s nothing that could be described as solid clinical data linking the gut microbiome to depression or anxiety in people. This hasn’t stopped people from making outrageous microbiome-related promises. Eisen regularly calls out unscientific claims on his blog by awarding them “Overselling the Microbiome” awards. One award debunked the idea that antibiotics are “extinguishing” our microbiomes, and another called out the claim that changing your gut bacteria can prevent a stroke.

Eisen probably hasn’t given out his last “Overselling” award. Microbiome research is hot right now — the National Institute of Mental Health has awarded $3.7 million in grants for 2015 and 2016 to study the microbiome’s role in mental health, the U.S. Office of Naval Research is also funding research on the issue, and a European project called MyNewGut is investigating the gut-brain connection. It’s clear that scientists are on to something interesting, but some patience is in order.

Biomedicine, after all, has a history of grandiose expectations about what new lines of study might yield. When the Human Genome Project was launched more than 25 years ago, it was widely hoped that it would lead to cures for many genetic diseases. That hasn’t happened, at least not on a large scale. After the genome, the epigenome — the chemical modifications that influence how genes function — became the hot new thing. Now the microbiome has taken on the same aura of expectation. But before we assume it will explain everything, we’d be wise to remember that biology is rarely as simple as we want it to be.

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