A Lot to Digest

What with gut feelings and gut instincts, you have heard of the stomach being cast as a second brain. Diego Bohórquez, assistant professor of medicine at the Duke School of Medicine, thinks that may have the order backward.

“Very simple organisms do not have a brain,” he says, smiling. “But they have a gut.”

He has a point. Our relationship with food is primary. Organisms have to eat to live, and organisms were eating for around a billion years before they were even breathing, much less thinking. And, indeed, you are what you eat. “We talk about the self,” Bohórquez says. “Most of the time we talk about the conscious self, but a huge portion of us isinside. We eat three times a day. That is modulating who we are.”

He expressed it succinctly in September when he gave a research seminar at the medical school. “At the core of who we are,” he said, “we are food.”

Okay, so big deal, right? That’s obvious: You eat, you build yourself, you move along. What used to be food is now you. Let’s not even mention your gut biome. But Bohórquez is talking about something far more complex and surprising. Information can get from your gut, it now turns out, in the blink of an eye. There are cells in your

ears that turn perception of sounds into electrical signals and so interact directly with your nervous system. Likewise, cells in your intestines can send information to your brain in milliseconds.

Gut feeling, indeed.

Bohórquez, who describes himself as a gutbrain neuroscientist, recently published a paper on the mechanism by which your gut and your brain communicate. The days of talking about feeling full only twenty minutes after you ate, of getting sleepy after tryptophan kicks off a hormonal reaction, are history. Scientists have long understood that the long vagus nerve connects the brain with, among other things, the intestines, carrying information from the gut to the brain. And in 2015, Bohórquez and his team discovered sensory cells in intestinal lining that ended in synapses capable ofconnecting with the nervous system. Called neuropods, those cells, like similar cells in the nose and tongue, end in synapses—nerve endings—that could connect with the nervous system.

In his most recent publication, Bohórquez and his team put rabies virus, specially labeled with a fluorescent tag, into the stomachs of mice. The tag enabled them to watch the virus makes its way directly up the vagus nerve and into the brain. Petri-dish work demonstrated that vagal neurons liked to connect with neuropod nerves and that the neurotransmitter glutamate might be the messenger. At least in mice—and likely in humans— the gut talks directly to the brain.

The gut, that is, is a sensory organ, transmitting to the brain way more than whether you feel full or not.

This falls in line with other things we know about the gut and our behavior, Bohórquez notes. People who have gastric bypass surgery, for example, lose weight in six months, but within two days diabetes symptoms improve—and some patients develop alcoholism. “The gut clearly affects the brain and behavior.” Bohórquez was studying choanocytes, very early bacteria that have a food vacuole with primitive receptors that communicated whether the vacuole was stretched or warm. So “I started to wonder, how is it we learn to eat? Both evolutionarily and developmentally?”

That’s when he began to realize that studying the gut “would get to the crux of how it is that we’re built, how it is we evolved around food.” The gut, he says, is underrated, “judged by what he produces, not by what he contributes.” It clearly contributes far more than scientists have been able to demonstrate, though Bohórquez’s recent publication of his research in Science moves knowledge along. “It’s a constant iteration of the process,” he says: You get a little knowledge, you digest it, then along comes the next course.

Kind of like a nice meal. —Scott Huler