A new study, published in the journal Science, has found that the process through which the gut communicates with the brain is much quicker than previously believed, relying on synapses more than it does on hormones.

Over the past 2 decades, the gut-brain axis has been thoroughly documented.

This started with a study in the early 1990s that showed that oral antibiotics can successfully treat a brain disorder called hepatic encephalopathy.

Fast forward to 2013, when research revealed that the bacteria in our guts influence anxiety and depression.

Even more recently, a review that was published only last month made it clear that gut bacteria can influence mood and emotions, highlighting their connection with a range of psychiatric disorders.

Medical News Today have also reported that changing the composition in our gut microbiota could enhance our resilience to stress, and that eating fiber promotes a more diverse range of gut bacteria, which, in turn, keeps our brains healthy and young for longer.

These studies are unraveling, bit by bit, the gut's vast influence on the brain, but the exact process through which this "second brain" influences our mental states and behavior remains unclear.

Some scientists believe that the main way in which the gut communicates with the brain is through hormones that are released into the bloodstream. However, a new study challenges this claim.

Researchers led by Diego Bohórquez, PhD, an assistant professor of medicine at Duke University School of Medicine in Durham, NC, suggest that the "conversation" between the gut and the brain occurs much more quickly and is more direct than previously believed.

The biological basis for a sixth sense

Bohórquez and colleagues set out to examine the process through which the gut tells the brain that it is full, curbing the appetite.

The scientists built on their previous research, in which they showed that the sensory cells in the gut lining have nerve endings resembling synapses. At the time, the findings suggested to the researchers that these cells could be part of a larger neural network.

So, in the new study, the researchers wanted to map this neural circuit. To this end, they modified a rabies virus so that it would become fluorescent and thus detectable. The researchers administered the virus to mice.

Bohórquez and his colleagues were able to trace the virus and watch it traverse the vagus nerve to reach the brainstem. Then, the researchers grew laboratory cultures of sensory gut cells together with vagal neurons.

Their experiment revealed that neurons move toward the gut cells in an attempt to connect and fire signals.

Finally, the team added sugar into the Petri dish, which accelerated the neuronal firing rate to the point of milliseconds. The results suggested to the researchers that glutamate could serve as a messenger that conveys the information from the gut to the brain.

"Scientists talk about appetite in terms of minutes to hours. Here we are talking about seconds," says Bohórquez, highlighting the contribution of the study.

Given the rapidity with which the information is sent from the gut to the brain, explain the authors, we can speak of a "gut sense" in the same way that we talk about the sense of touch or smell.

"We think these findings are going to be the biological basis of a new sense [...] One that serves as the entry point for how the brain knows when the stomach is full of food and calories. It brings legitimacy to [the] idea of the 'gut feeling' as a sixth sense."

–Diego Bohórquez

The findings have "profound implications for our understanding of appetite," continues Bohórquez.

"Many of the appetite suppressants that have been developed," he notes, "target slow-acting hormones, not fast-acting synapses. And that's probably why most of them have failed."